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X-WR-CALNAME;VALUE=TEXT:Past Talks
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BEGIN:VEVENT
UID:news-1023@dmi.unibas.ch
DTSTAMP:20200220T145956
DTSTART;TZID=Europe/Zurich:20200225T103000
DTEND;TZID=Europe/Zurich:20200225T120000
SUMMARY:Seminar Algebra and Geometry: Anne Lonjou (Universität Basel)
LOCATION:Seminarraum 05.002
DESCRIPTION:A key tool to study the plane Cremona group is its action on a
hyperbolic space. Saddly, in higher rank such an action is not
available. Recently in geometric group theory, actions on CAT(0
) cube complexes turned out to be a powerfull tool to study a l
arge class of groups. \r\nIn this talk, based on a common
work with Christian Urech, we will construct such complexes on
which Cremona groups of rank n act. We will then see which kin
d of results on these groups we can obtain.
END:VEVENT
BEGIN:VEVENT
UID:news-987@dmi.unibas.ch
DTSTAMP:20191206T022316
DTSTART;TZID=Europe/Zurich:20191209T150000
DTEND;TZID=Europe/Zurich:20191209T163000
SUMMARY:Seminar Algebra and Geometry: Michel van Garrel (Warwick)
LOCATION:Spiegelgasse 5, 05.002
DESCRIPTION:In this joint work with Christian Böhning and Hans-Christian
von Bothmer we apply Voisin's criterion of existence of a deco
mposition of the diagonal to semistable degenerations. In doing
so, we obtain partial results towards proving that very genera
l cubic threefolds are stably irrational.
END:VEVENT
BEGIN:VEVENT
UID:news-922@dmi.unibas.ch
DTSTAMP:20191127T114131
DTSTART;TZID=Europe/Zurich:20191203T103000
DTEND;TZID=Europe/Zurich:20191203T120000
SUMMARY:Seminar Algebra and Geometry: Erik Paemurru (University of Loughborough)
LOCATION:Seminarraum 00.003
DESCRIPTION:It is known that quasismooth 3-fold Fano hypersurfaces with ind
ex 1 in weighted projective spaces over ℂ are birationally ri
gid (not birational to any other Fano 3-folds, conic bundles or
del Pezzo fibrations). But very little is known when they carr
y non-orbifold singularities. I consider sextic double solids,
one of the simplest such 3-folds, which have an isolated cA_n s
ingularity. I have shown that n is at most 8, and that rigidity
fails for n > 3. In this talk, I will illustrate this phenomen
on by giving some examples.
END:VEVENT
BEGIN:VEVENT
UID:news-924@dmi.unibas.ch
DTSTAMP:20191112T123824
DTSTART;TZID=Europe/Zurich:20191126T103000
DTEND;TZID=Europe/Zurich:20191126T120000
SUMMARY:Seminar Algebra and Geometry: Carlos Amendola (University of Munich)
LOCATION:Seminarraum 00.003
DESCRIPTION:We study the maximum likelihood estimation problem for several
classes of toric Fano models. We start by exploring the maximum
likelihood degree for all 2-dimensional Gorenstein toric Fano
varieties. We show that the ML degree is equal to the degree of
the surface in every case except for the quintic del Pezzo sur
face with two singular points of type A1 and provide explicit e
xpressions that allow to compute the maximum likelihood estimat
e in closed form whenever the ML degree is less than 5. We then
explore the reasons for the ML degree drop using A-discriminan
ts and intersection theory. Based on joint work with Dimitra K
osta and Kaie Kubjas.
END:VEVENT
BEGIN:VEVENT
UID:news-927@dmi.unibas.ch
DTSTAMP:20191104T113358
DTSTART;TZID=Europe/Zurich:20191112T103000
DTEND;TZID=Europe/Zurich:20191112T120000
SUMMARY:Seminar Algebra and Geometry: Enrica Mazzon (Imperial College)
LOCATION:Seminarraum 00.003
DESCRIPTION:To a degeneration of varieties, we can associate the dual inter
section complex, a topological space that encodes the combinato
ric of the central fiber and reflects the geometry of the gener
ic fiber. The points of the dual complex can be identified to v
aluations on the function field of the variety, hence the dual
complex can be embedded in the Berkovich space of the variety.I
n this talk I will explain how this interpretation gives an ins
ight in the study of the dual complexes. I will focus on some d
egenerations of hyper-Kähler varieties and show that we are ab
le to determine the homeomorphism type of their dual complex us
ing techniques of Berkovich geometry. The results are in accord
ance with the predictions of mirror symmetry, and the recent wo
rk about the rational homology of dual complexes of degeneratio
ns of hyper-Kähler varieties, due to Kollár, Laza, Saccà and
Voisin.This is joint work with Morgan Brown.
END:VEVENT
BEGIN:VEVENT
UID:news-925@dmi.unibas.ch
DTSTAMP:20191020T194626
DTSTART;TZID=Europe/Zurich:20191105T103000
DTEND;TZID=Europe/Zurich:20191105T120000
SUMMARY:Seminar Algebra and Geometry: Shengyuan Zhao (University of Rennes)
LOCATION:Seminarraum 00.003
DESCRIPTION:Let Y be a smooth complex projective surface. Let U be a connec
ted Euclidean open set of Y. Let G be a subgroup of Bir(Y) whic
h acts by holomorphic diffeomorphisms on U (i.e. preserves U an
d without indeterminacy points in U), in a free, properly disco
ntinuous and cocompact way, so that the quotient X=U/G is a com
pact complex surface. Such a birational transformation group G,
or more precisely such a quadruple (Y,U,G,X), will be called a
birational Kleinian group. Once we have a birational Kleinian
group, the quotient surface is equipped with a birational struc
ture, i.e. an atlas of local charts with rational changes of co
ordinates. I will present some basic properties and subtleties
of birational structures, compared to the classical geometric s
tructures. Then I will begin by studying birational structures
on a special type of non-algebraic surfaces, the Inoue surfaces
, to reveal some of the general strategy. Using classification
of solvable and abelian groups of the Cremona group, and by rel
ating the foliations on Inoue surfaces with some birational dyn
amical systems via Ahlfors-Nevanlinna currents, I will show tha
t the Inoue surfaces have one unique birational structure. Then
I will move on to the general study of birational Kleinian gro
ups with the additional hypothesis that the quotient surface is
projective. I will explain how to use powerful results from Cr
emona groups, holomorphic foliations and non-abelian Hodge theo
ry to get an almost complete classification of such birational
Kleinian groups.
END:VEVENT
BEGIN:VEVENT
UID:news-923@dmi.unibas.ch
DTSTAMP:20191004T195327
DTSTART;TZID=Europe/Zurich:20191029T103000
DTEND;TZID=Europe/Zurich:20191029T120000
SUMMARY:Seminar Algebra and Geometry: Stavros Papadakis (University of Ioannina)
LOCATION:Seminarraum 00.003
DESCRIPTION:Un projection theory, initiated by Miles Reid, aims to construc
t and analyze complicated commutative rings in terms of simpler
ones. It can also be considered as an algebraic language for b
irational geometry. \r\n\r\nThe main purpose of the talk is to
give a short introduction to the theory and describe some of it
s applications.
END:VEVENT
BEGIN:VEVENT
UID:news-934@dmi.unibas.ch
DTSTAMP:20191004T195339
DTSTART;TZID=Europe/Zurich:20191015T103000
DTEND;TZID=Europe/Zurich:20191015T120000
SUMMARY:Seminar Algebra and Geometry: Tomasz Pełka (University of Bern)
LOCATION:Seminarraum 00.003
DESCRIPTION:ℚ-homology planes satisfying the Negativity Conjecture
END:VEVENT
BEGIN:VEVENT
UID:news-933@dmi.unibas.ch
DTSTAMP:20191004T195349
DTSTART;TZID=Europe/Zurich:20191008T103000
DTEND;TZID=Europe/Zurich:20191008T120000
SUMMARY:Seminar Algebra and Geometry: Christian Urech (EPFL)
LOCATION:Seminarraum 00.003
DESCRIPTION:Julie Déserti showed that every automorphism of the plane Crem
ona group is inner up to a field automorphism of the base-field
. In this talk we generalize this result to Cremona groups of a
rbitrary rank, however, only under the additional restriction t
hat the automorphisms are also homeomorphisms with respect to t
he Zariski or the Euclidean topology on the Cremona group. We w
ill consider similar questions for groups of polynomial automor
phisms and groups of birational diffeomorphisms. This is joint
work with Susanna Zimmermann.
END:VEVENT
BEGIN:VEVENT
UID:news-898@dmi.unibas.ch
DTSTAMP:20190619T152715
DTSTART;TZID=Europe/Zurich:20190703T103000
DTEND;TZID=Europe/Zurich:20190703T120000
SUMMARY:Seminar Algebra and Geometry: Stéphane Lamy (Université de Toulouse)
LOCATION:Spiegelgasse 5 - Seminarraum 05.002
DESCRIPTION:The group of tame polynomial automorphisms of the n-dimensional
complex affine space is the group generated by linear maps and
polynomial transvections. I will describe an action of this gr
oup on a metric space whose construction is inspired from the t
heory of affine Bruhat-Tits buildings. In dimension n = 3, we s
how that this space is simply connected with non-positive curva
ture. This allows to get a description of the finite subgroups
of the 3-dimensional tame group. (Joint work with P. Przytycki)
END:VEVENT
BEGIN:VEVENT
UID:news-883@dmi.unibas.ch
DTSTAMP:20190517T143050
DTSTART;TZID=Europe/Zurich:20190521T103000
DTEND;TZID=Europe/Zurich:20190521T120000
SUMMARY:Seminar Algebra and Geometry: Prof. Niko Beerenwinkel (Dept. of Biosystems Science and Engineering, ETHZ)
LOCATION:
DESCRIPTION:TBA
END:VEVENT
BEGIN:VEVENT
UID:news-852@dmi.unibas.ch
DTSTAMP:20190423T201701
DTSTART;TZID=Europe/Zurich:20190514T103000
DTEND;TZID=Europe/Zurich:20190514T120000
SUMMARY:Seminar Algebra and Geometry: Urban Jezernik (University of the Basque Country)
LOCATION:Seminar Room 05.002
DESCRIPTION:The rationality problem in algebraic geometry asks whether a gi
ven variety is birational to a projective space. We will gently
introduce the problem and take a look at some recent advances,
principally in the direction of negative examples constructed
via cohomological obstructions. Special focus will be set on qu
otient varieties by linear group actions.
END:VEVENT
BEGIN:VEVENT
UID:news-846@dmi.unibas.ch
DTSTAMP:20190430T141302
DTSTART;TZID=Europe/Zurich:20190507T103000
DTEND;TZID=Europe/Zurich:20190507T120000
SUMMARY:Seminar Algebra and Geometry: Yuri Prokhorov (Steklov Mathematical Institute, Moscow)
LOCATION:Seminar room: 05.002
DESCRIPTION:A conic bundle f: X\to S is a flat morphism of smooth varieties
whose fibers are plane conics. In this talk, I will first
discuss application of Sarkisov program to the rationality p
roblem of algebraic varieties having conic bundle structures.
Then I concentrate on some special, so-called tetragonal ty
pes of conic bundles, which lie on the "boundary" between birat
ionally rigid and non-rigid ones and are especially interestin
g for this reason. The second part of the talk is based on t
he joint work in progress with V. Shokurov.
END:VEVENT
BEGIN:VEVENT
UID:news-845@dmi.unibas.ch
DTSTAMP:20190327T160809
DTSTART;TZID=Europe/Zurich:20190416T103000
DTEND;TZID=Europe/Zurich:20190416T120000
SUMMARY:Seminar Algebra and Geometry: Matilde Manzaroli (École Polytechnique and Université de Nantes)
LOCATION:Seminar room 05.002
DESCRIPTION:The study of the topology of real algebraic varieties dates bac
k to the work of Harnack, Klein and Hilbert in the 19th century
; in particular, the isotopy type classification of real algebr
aic curves with a fixed degree in the real projective plane is
a classical subject that has undergone considerable evolution.
On the other hand, apart from studies concerning Hirzebruch sur
faces and at most degree 3 surfaces in the real projective spac
e, not much is known for more general ambient surfaces. In part
icular, this is because varieties constructed using the patchwo
rking method are hypersurfaces of toric varieties. However, the
re are many other real algebraic surfaces. Among these are the
real rational surfaces, and more particularly the real minimal
surfaces. In this talk, we present some results about the class
ification of topological types realized by real algebraic curve
s in real minimal del Pezzo surfaces of degree.
END:VEVENT
BEGIN:VEVENT
UID:news-637@dmi.unibas.ch
DTSTAMP:20190314T120420
DTSTART;TZID=Europe/Zurich:20190319T103000
DTEND;TZID=Europe/Zurich:20190319T120000
SUMMARY:Seminar Algebra and Geometry: Mirko Mauri (Imperial College London)
LOCATION:Seminar Room 05.002
DESCRIPTION:The geometric P=W conjecture is a conjectural description of th
e asymptotic behavior of a celebrated correspondence in non-abe
lian Hodge theory. In particular, it is expected that the dua
l boundary complex of the compactification of character variet
ies is a sphere. In a joint work with Enrica Mazzon and Matthe
w Stevenson, we manage to compute the first non-trivial examp
les of dual complexes in the compact case. This requires to dev
elop a new theory of essential skeletons over a trivially-value
d field. As a byproduct, inspired by these constructions, we s
how that certain character varieties appear in degenerations
of compact hyper-Kähler manifolds. In this talk we will exp
lain how these new non-archimedean techniques can shed new li
ght into classical algebraic geometry problems.
END:VEVENT
BEGIN:VEVENT
UID:news-821@dmi.unibas.ch
DTSTAMP:20190222T141344
DTSTART;TZID=Europe/Zurich:20190305T103000
DTEND;TZID=Europe/Zurich:20190305T120000
SUMMARY:Seminar Algebra and Geometry: Federico Lo Bianco (Université de Marseille)
LOCATION:Seminar room 00.003
DESCRIPTION:We consider a holomorphic (singular) foliation F on a projectiv
e manifold X and a group G of birational transformations of X w
hich preserve F (i.e. it permutes the set of leaves). We say th
at the transverse action of G is finite if some finite index su
bgroup of G fixes each leaf of F. \r\n\r\nI will briefly recall
a criterion for the finiteness of the transverse action in the
case of algebraically integrable foliations (i.e. foliations w
hose leaves coincide with the fibres of a fibration). Then I wi
ll explain how the presence of certain transverse structures on
the foliation allow to recover the same result; in this case,
one can study the monodromy of such a structure (which is defin
ed in an analogous way as that of a more familiar (G,X)-structu
re) and apply factorization results in order to reduce the prob
lem to subvarieties of quotients of the product of unit discs,
whose geometry is now quite well understood.
END:VEVENT
BEGIN:VEVENT
UID:news-410@dmi.unibas.ch
DTSTAMP:20181204T082628
DTSTART;TZID=Europe/Zurich:20181211T103000
DTEND;TZID=Europe/Zurich:20181211T120000
SUMMARY:Seminar Algebra and Geometry: Luca Studer (University of Bern)
LOCATION:Seminar Room 00.003
DESCRIPTION:In the talk we discuss how Oka theory helps to solve systems of
equations with complex analytic entries. A classical example i
s the fact that for every pair of complex analytic functions a,
b: C^n -> C with no common zero there are complex analytic fun
ctions x, y: C^n -> C satisfying the Bézout identity ax+by=1.
A more recent example is Leiterer's work, where the solvability
of xax^{-1}=b for complex analytic matrix-valued maps a, b: C^
n -> Mat(n x n, C) is investigated. Both examples are brought i
nto the context of the speakers research.
END:VEVENT
BEGIN:VEVENT
UID:news-348@dmi.unibas.ch
DTSTAMP:20181204T083213
DTSTART;TZID=Europe/Zurich:20181204T103000
DTEND;TZID=Europe/Zurich:20181204T120000
SUMMARY:Seminar Algebra and Geometry: Bruno Laurent (Grenoble)
LOCATION:Seminar room 00.003
DESCRIPTION:The varieties which are homogeneous under the action of an alge
braic group are very symmetric objects. More generally, we get
a much wider class of objects, having a very rich geometry, by
allowing the varieties to have not a unique orbit, but a dense
orbit. Such varieties are said to be almost homogeneous; this i
ncludes the case of toric varities, when the group is an algebr
aic torus.In this talk, I will explain how to classify the pair
s (X,G) where X is a curve or a surface and G is a smooth a
nd connected algebraic group acting on X with a dense orbit.F
or curves, I will mainly focus on the regular ones, defined ove
r an arbitrary field. Over an algebraically closed field, the "
natural" notion of non-singularity is "smoothness". However, ov
er an arbitrary field, the weaker notion of "regularity" is mor
e suitable. I will recall the difference between those two noti
ons and show that there exist regular homogeneous curves which
are not smooth.For surfaces, I will restrict to the smooth ones
, defined over an algebraically closed field. The situation is
more complicated than for curves. Moreover, new phenomena and s
everal difficulties appear in positive characteristic, and I wi
ll highlight them.
END:VEVENT
BEGIN:VEVENT
UID:news-341@dmi.unibas.ch
DTSTAMP:20181204T082628
DTSTART;TZID=Europe/Zurich:20181120T103000
DTEND;TZID=Europe/Zurich:20181120T120000
SUMMARY:Seminar Algebra and Geometry: Nikon Kurnosov (University of Georgia, Athens, US)
LOCATION:Seminar Room 00.003
DESCRIPTION:Hyperkähler manifolds are higher-dimensional generalizations o
f K3 surfaces. The Beauville conjecture predicts that the numbe
r of deformation types of compact irreducible hyperkähler mani
folds is finite in any dimension. In this talk I will briefly d
iscuss some basic notions of the theory, explain why hyperkähl
er manifolds play a very important role in classification of co
mplex manifolds, and then explain what are the evidences for Be
auville's conjecture.
END:VEVENT
BEGIN:VEVENT
UID:news-361@dmi.unibas.ch
DTSTAMP:20181204T082628
DTSTART;TZID=Europe/Zurich:20181113T103000
DTEND;TZID=Europe/Zurich:20181113T120000
SUMMARY:Seminar Algebra and Geometry: Lukas Lewark (University of Bern)
LOCATION:Seminar Room 00.003
DESCRIPTION:This talk will explore different notions of positivity of knots
, how to encode such knots as graphs, and how to unknot them. J
oint results with Baader/Liechti and Feller/Lobb will make appe
arances. No prerequisites in knot theory will be necessary.
END:VEVENT
BEGIN:VEVENT
UID:news-340@dmi.unibas.ch
DTSTAMP:20181204T082628
DTSTART;TZID=Europe/Zurich:20181106T103000
DTEND;TZID=Europe/Zurich:20181106T120000
SUMMARY:Seminar Algebra and Geometry: Juliette Bavard (University of Rennes)
LOCATION:Seminar Room 00.003
DESCRIPTION:The plane minus a Cantor set and its mapping class group appear
in many dynamical contexts, including group actions on surfa
ces, the study of groups of homeomorphisms on a Cantor set, and
complex dynamics. In this talk, I will motivate the study of
this 'big mapping class groups'. I will then present the 'ray
graph', which is a Gromov-hyperbolic graph on which this gro
up acts by isometries.
END:VEVENT
BEGIN:VEVENT
UID:news-347@dmi.unibas.ch
DTSTAMP:20181204T082628
DTSTART;TZID=Europe/Zurich:20181016T103000
DTEND;TZID=Europe/Zurich:20181016T120000
SUMMARY:Seminar Algebra and Geometry: Pierre-Marie Poloni and Richard Griffon (University of Basel)
LOCATION:Seminar room 00.003
DESCRIPTION:TBA
END:VEVENT
BEGIN:VEVENT
UID:news-324@dmi.unibas.ch
DTSTAMP:20181204T082628
DTSTART;TZID=Europe/Zurich:20181002T103000
DTEND;TZID=Europe/Zurich:20181002T120000
SUMMARY:Seminar Algebra and Geometry: Immanuel van Santen, Anne Lonjou and Jeremy Blanc (University of Basel)
LOCATION:Seminar Room 00.003
DESCRIPTION: \r\nImmanuel van Santen will speak about „Embeddings and ta
me automorphisms in affine geometry", \r\n\r\nAnne Lonjou will
explain some link between "Cremona group and geometric group t
heory", and \r\n\r\nJérémy Blanc will present us "Birational
geometry of surfaces and threefolds".
END:VEVENT
BEGIN:VEVENT
UID:news-322@dmi.unibas.ch
DTSTAMP:20181204T082628
DTSTART;TZID=Europe/Zurich:20180925T103000
DTEND;TZID=Europe/Zurich:20180925T120000
SUMMARY:Seminar Algebra and Geometry: Julia Schneider, Egor Yasinsky and Philipp Mekler (University of Basel)
LOCATION:Seminar Room 00.003
DESCRIPTION: \r\nJulia will speak about "A_k-singularities of plane curves
of fixed bidegree".\r\n\r\nEgor will answer to "What transform
ation groups in algebraic, differential and metric geometry hav
e in common?". \r\n\r\nPhilipp will speak about "Algebraic Sta
tistics: Gaussian Mixtures and Beyond".
END:VEVENT
BEGIN:VEVENT
UID:news-644@dmi.unibas.ch
DTSTAMP:20190125T172712
DTSTART;TZID=Europe/Zurich:20180529T103000
DTEND;TZID=Europe/Zurich:20180529T120000
SUMMARY:Seminar Algebra and Geometry: Jérémy Blanc (Basel)
LOCATION:Seminar room 00.003
DESCRIPTION:I will describe a joint work with Stéphane Lamy and Susanna Zi
mmermann which provides abelian quotients of the Cremona group
s in high dimension.
END:VEVENT
BEGIN:VEVENT
UID:news-643@dmi.unibas.ch
DTSTAMP:20190125T172506
DTSTART;TZID=Europe/Zurich:20180522T103000
DTEND;TZID=Europe/Zurich:20180522T120000
SUMMARY:Seminar Algebra and Geometry: Andriy Regeta (Köln)
LOCATION:Seminar room 00.003
DESCRIPTION:In this talk we are going to discuss the following question: t
o which extent a so-called Danielewski surface is determined by
its automorphism group seen as an abstract group or as an ind
-group?
END:VEVENT
BEGIN:VEVENT
UID:news-642@dmi.unibas.ch
DTSTAMP:20190125T172321
DTSTART;TZID=Europe/Zurich:20180515T103000
DTEND;TZID=Europe/Zurich:20180515T120000
SUMMARY:Seminar Algebra and Geometry: Peter Feller (ETHZ)
LOCATION:Seminar room 00.003
DESCRIPTION:We start by recalling that a smooth algebraic curve of degree d
in CP2 is a genus (d-1)(d-2)/2 surface (read `smooth 2-manifo
ld'). The `Thom Conjecture', proven by Kronheimer and Mrowka, a
sserts that such algebraic curves have a surprising minimizing
property. We derive consequences of the Thom Conjecture for tr
ansversal intersections of algebraic curves with round spheres
, describe the knots one finds as such intersections following
Rudolph, and give precise instances of the sentiment that the
se intersections constitute very special elements in the so-ca
lled smooth concordance group. In contrast, in the topological
category, we prove that all knots are topological concordant
to such an intersection. Based on joint work with Maciej Borodz
ik. No knowledge about knot theory and concordance theory---the
study of 1-manifolds in the 3-dimensional sphere and surfaces
in 4-dimensional ball bounding them---will be assumed.
END:VEVENT
BEGIN:VEVENT
UID:news-641@dmi.unibas.ch
DTSTAMP:20190125T172012
DTSTART;TZID=Europe/Zurich:20180508T103000
DTEND;TZID=Europe/Zurich:20180508T120000
SUMMARY:Seminar Algebra and Geometry: Arthur Bik (Bern)
LOCATION:Seminar room 00.003
DESCRIPTION:Finite-dimensional vector spaces are Noetherian, i.e. every des
cending chain of Zariski-closed subsets stabilizes. For infini
te-dimensional spaces this is not true. However what can be tr
ue is that for some group G acting on the space every descendi
ng chain of G-stable closed subsets stablizes. We call spaces
for which this holds G-Noetherian. In this talk, we will go ov
er some known examples and non-examples of spaces that are Noe
therian up to a group action and introduce some new ones.
END:VEVENT
BEGIN:VEVENT
UID:news-640@dmi.unibas.ch
DTSTAMP:20190125T171800
DTSTART;TZID=Europe/Zurich:20180424T103000
DTEND;TZID=Europe/Zurich:20180424T120000
SUMMARY:Seminar Algebra and Geometry: Mattias Hemmig (Basel)
LOCATION:Seminar room 00.003
DESCRIPTION:In 2012 Costa constructed a family of unicuspidal curves of deg
ree 9 in the projective plane that are pairwise non-equivalent
but have isomorphic complements. We show that a family as the
one of Costa cannot contain a unicuspidal curve C that admits
a line intersecting C only in the singular point. To state the
result more precisely, if D is any plane curve and there exist
s an isomorphism between the complements of C and D, then the
two curves are projectively equivalent, even though the isomor
phism is not necessarily linear. The proof works over an algeb
raically closed field of any characteristic and generalizes a
result of Yoshihara (1984) who proved the claim over the comple
x numbers. We then use this result to show that any two irredu
cible curves of degree at most 8 have isomorphic complements i
f and only if they are projectively equivalent.
END:VEVENT
BEGIN:VEVENT
UID:news-634@dmi.unibas.ch
DTSTAMP:20190121T162647
DTSTART;TZID=Europe/Zurich:20180417T103000
DTEND;TZID=Europe/Zurich:20180417T120000
SUMMARY:Seminar Algebra and Geometry: Andrea Fanelli (Düsseldorf)
LOCATION:Seminar room 00.003
DESCRIPTION:Even if the precise notion of rationally simply connected varie
ty is still not clear in general, the recent works by de Jong,
He and Starr produced great interest and new research directi
ons. In the current joint project with Laurent Gruson and Nicol
as Perrin, we study some examples of Fano varieties in low dime
nsion via explicit birational methods.
END:VEVENT
BEGIN:VEVENT
UID:news-633@dmi.unibas.ch
DTSTAMP:20190125T172302
DTSTART;TZID=Europe/Zurich:20180403T103000
DTEND;TZID=Europe/Zurich:20180403T120000
SUMMARY:Seminar Algebra and Geometry: Pierre-Marie Poloni (Bern)
LOCATION:Seminar room 00.003
DESCRIPTION:An $\mathbb{A}^2$-fibration is a flat morphism between complex
affine varieties whose fibers are isomorphic to the complex af
fine plane. In this talk, we study explicit families $f:\mathb
b{A}^4\to\mathbb{A}^2$ of $\mathbb{A}^2$-fibrations over the a
ffine plane. The famous Dolgachev-Weisfeiler conjecture predict
s that such fibrations are in fact isomorphic to the trivial b
undle. We will show that this holds true in some particular ex
amples. For instance, we will recover a result of Drew Lewis w
hich states that the $\mathbb{A}^2$-fibration induced by the s
econd Vénéreau polynomial is trivial. Our proof is inspired b
y a previous work of Kaliman and Zaidenberg and consists in fi
rst showing that the considered fibrations have a fiber bundle
structure when restricted over the punctured affine plane. Thi
s is a joint work in progress with Jérémy Blanc.
END:VEVENT
BEGIN:VEVENT
UID:news-632@dmi.unibas.ch
DTSTAMP:20190121T142447
DTSTART;TZID=Europe/Zurich:20180313T103000
DTEND;TZID=Europe/Zurich:20180313T120000
SUMMARY:Seminar Algebra and Geometry: Konstantin Shramov (HSE Moscow)
LOCATION:Seminar room 00.003
DESCRIPTION:I will speak about finite groups acting by birational automorph
isms of surfaces over algebraically non-closed fields, mostly
function fields. One of important observations here is thata s
mooth geometrically rational surface S is either birational to
a product of a projective line and a conic (in particular, S
is rational provided that it has a point), or finite subgroups
of its birational automorphism group are bounded. We will als
o discuss some particular types of surfaces with interesting au
tomorphism groups, including Severi-Brauer surfaces.
END:VEVENT
BEGIN:VEVENT
UID:news-631@dmi.unibas.ch
DTSTAMP:20190121T141926
DTSTART;TZID=Europe/Zurich:20180306T103000
DTEND;TZID=Europe/Zurich:20180306T120000
SUMMARY:Seminar Algebra and Geometry: Hans-Christian Graf v. Bothmer (Hamburg)
LOCATION:Seminar room 00.003
DESCRIPTION:I will review some of the history and recent developments of th
e rationality question for conic bundles over projective space
s. I will then explain our contribution to this question in th
e case of conic bundles over IP^3 (joint work with Asher Auel,
Christian Boehning and Alena Pirutka).
END:VEVENT
BEGIN:VEVENT
UID:news-652@dmi.unibas.ch
DTSTAMP:20190126T193831
DTSTART;TZID=Europe/Zurich:20171212T103000
DTEND;TZID=Europe/Zurich:20171212T120000
SUMMARY:Seminar Algebra and Geometry: Sara Durighetto (Ferrara)
LOCATION:Seminar room 00.003
DESCRIPTION:Let C, D be two birational subvarieties of the projective space
Pn. I am interested in understanding when there exists a Cre
mona modification f: Pn --->Pn such that f(C) = D.
For the sake of this talk I will restrict to a configuration of
lines in P2. In this special case I'll suggest a classifica
tion of configurations of lines that are contractible to a bun
ch of points. In doing this I will propose an unexpected confi
guration that seems to violate an established conjecture.
END:VEVENT
BEGIN:VEVENT
UID:news-651@dmi.unibas.ch
DTSTAMP:20190126T192636
DTSTART;TZID=Europe/Zurich:20171121T103000
DTEND;TZID=Europe/Zurich:20171121T120000
SUMMARY:Seminar Algebra and Geometry: Susanna Zimmermann (Angers)
LOCATION:Seminar room 00.003
DESCRIPTION:The Noether-Castelnuovo theorem implies that over algebraically
closed fields there is no non-trivial homomorphism from the C
remona group of the plane to a finite group. Over non-closed f
ields there are many, and I would like to explain some example
s.
END:VEVENT
BEGIN:VEVENT
UID:news-650@dmi.unibas.ch
DTSTAMP:20190126T192314
DTSTART;TZID=Europe/Zurich:20171107T103000
DTEND;TZID=Europe/Zurich:20171107T120000
SUMMARY:Seminar Algebra and Geometry: Andrea Fanelli (Düsseldorf)
LOCATION:Seminar room 00.003
DESCRIPTION:Fibre-like Fano manifolds naturally appear in the context of th
e minimal model program. In this talk I will discuss some exam
ples, with special focus on: toric varieties, manifolds with h
igh index and manifolds with high Picard rank. These have been
obtained in recent joint works with Casagrande-Codogni and Co
dogni-Svaldi-Tasin.
END:VEVENT
BEGIN:VEVENT
UID:news-649@dmi.unibas.ch
DTSTAMP:20190125T181307
DTSTART;TZID=Europe/Zurich:20171024T103000
DTEND;TZID=Europe/Zurich:20171024T120000
SUMMARY:Seminar Algebra and Geometry: Immanuel van Santen (Basel)
LOCATION:Seminar room 00.003
DESCRIPTION:This is joint work with Hanspeter Kraft (University of Basel) a
nd Andriy Regeta (University of Cologne). The main problem we
address in this talk is the characterization of the affine spa
ce An by its automorphism group Aut(An). More precisely, we as
k, whether the existence of an abstract group isomorphism Aut(
X) \simeq Aut(An) implies the existence of an isomorphism of a
lgebraic varieties X \simeq An. The following is our main resu
lt. Main Theorem. Let X be a quasi-affine irreducible variety s
uch that Aut(X) \simeq Aut(An). Then X \simeq An if one of the
following conditions holds. (1) X is a Q-acyclic open subset o
f a smooth affine rational variety, and dim(X) is a most equal
to n; (2) X is toric and dim(X) is at least equal to n. After g
iving a brief history on some related results that concern the
characterisation of geometric objects via their automorphisms,
we give the key ideas of the proof of our main result.
END:VEVENT
BEGIN:VEVENT
UID:news-648@dmi.unibas.ch
DTSTAMP:20190126T201746
DTSTART;TZID=Europe/Zurich:20171010T103000
DTEND;TZID=Europe/Zurich:20171010T120000
SUMMARY:Seminar Algebra and Geometry: Frederic Han (Paris 7)
LOCATION:Seminar room 00.003
DESCRIPTION:After a short introduction to birational transformations of a p
rojective space, we will focus on the case of P3. Few is known
about birational transformations of P3 and the case of degree
3 is already difficult. To explain how things differ in large
r degree it is natural to look at transformations of bidegree
(4,4). In this talk we detail families of examples in degree 3
and bidegree (4,4) to illustrate this opposition. (Joint work
with J. Déserti)
END:VEVENT
BEGIN:VEVENT
UID:news-647@dmi.unibas.ch
DTSTAMP:20190125T175652
DTSTART;TZID=Europe/Zurich:20171003T103000
DTEND;TZID=Europe/Zurich:20171003T120000
SUMMARY:Seminar Algebra and Geometry: Yves de Cornulier (Lyon 1)
LOCATION:Seminar room 00.003
DESCRIPTION:Given a group G, a G-action on a set D commensurates a subset M
if M differs from each of its G-translates by finitely many e
lements. Commensurating actions naturally induce actions on CAT
(0) cube complexes. For every irreducible variety X, we define
a set of (virtual) hypersurfaces, which contains the set of h
ypersurfaces of X and on which the group Bir(X) of birational
self-transformations of X acts, extending its partial action o
n hypersurfaces. This action commensurates the set of hypersur
faces of X. This construction thus provides information about
the structure of Bir(X) and its subgroups. (Joint work with Se
rge Cantat)
END:VEVENT
BEGIN:VEVENT
UID:news-646@dmi.unibas.ch
DTSTAMP:20190125T174546
DTSTART;TZID=Europe/Zurich:20170926T103000
DTEND;TZID=Europe/Zurich:20170926T120000
SUMMARY:Seminar Algebra and Geometry: Hanspeter Kraft (Basel)
LOCATION:Seminar room 00.003
DESCRIPTION:Let G be a semisimple algebraic group acting on an affine varie
ty X. An orbit O⊂X is called minimal if it is G-isomorphic
to the orbit of highest weight vectors in an irreducible repres
entation of G. These orbits have many interesting properties.
E.g. the closure of a minimal orbit in any affine G-variety X
is of the form $\bar O = O \cup \{x_0\}$ where x0 ∈ X is a
fixed point, and they are even characterised by this property.
An affine G-variety X is called small if all non-trivial orbi
ts in X are minimal. It turns out that these varieties have man
y remarkable properties. The most interesting one is that the
coordinate ring is a graded G-algebra. This allows a classif
ication. In fact, there is an equivalence of categories of sma
ll G-varieties with so-called fix-pointed k*-varieties, a clas
s of well-understood objects which have been studied very care
fully in different contexts. A striking consequence is the foll
owing result. Theorem. Let n > 4. Then a smooth $\SL_n$-variety
of dimension d < 2n-2 is an $\SL_n$-vector bundle over a smoo
th variety of dimension d-n. There are also interesting applic
ations to actions of the affine group $\Aff_n$. This was the s
tarting point of this joint work with with Andriy Regeta and S
usanna Zimmermann.
END:VEVENT
BEGIN:VEVENT
UID:news-645@dmi.unibas.ch
DTSTAMP:20190125T173135
DTSTART;TZID=Europe/Zurich:20170919T103000
DTEND;TZID=Europe/Zurich:20170919T120000
SUMMARY:Seminar Algebra and Geometry: Anne Lonjou (Basel)
LOCATION:Seminar room 00.003
DESCRIPTION:The Cremona group is the group of birational transformations of
the projective plane. It acts on a hyperbolic space which is
an infinite dimensional version of the hyperboloid model of Hn
. This action is the main recent tool to study the Cremona gro
up. After defining it, we will study its Voronoï tesselation,
and describe some graphs naturally associated with this const
ruction. Finally we will discuss which of these graphs are Gro
mov-hyperbolic.
END:VEVENT
BEGIN:VEVENT
UID:news-660@dmi.unibas.ch
DTSTAMP:20190212T221333
DTSTART;TZID=Europe/Zurich:20170609T103000
DTEND;TZID=Europe/Zurich:20170609T120000
SUMMARY:Seminar Algebra and Geometry: Andrea Fanelli (Düseldorf)
LOCATION:Seminar room 00.003
DESCRIPTION:Del Pezzo fibrations are possible outputs of a 3-fold MMP also
in positive characterisitic. A natural question is whether geo
metrically non-normal del Pezzo surfaces can appear as generic
fibre of such a fibration. This talk is based on a joint work
with S. Schröer.
END:VEVENT
BEGIN:VEVENT
UID:news-659@dmi.unibas.ch
DTSTAMP:20190212T221404
DTSTART;TZID=Europe/Zurich:20170602T103000
DTEND;TZID=Europe/Zurich:20170602T120000
SUMMARY:Seminar Algebra and Geometry: Youssef Fares (Amiens)
LOCATION:Seminar room 00.003
DESCRIPTION:
END:VEVENT
BEGIN:VEVENT
UID:news-658@dmi.unibas.ch
DTSTAMP:20190212T224851
DTSTART;TZID=Europe/Zurich:20170505T103000
DTEND;TZID=Europe/Zurich:20170505T120000
SUMMARY:Seminar Algebra and Geometry: Ivan Cheltsov (Edinburgh)
LOCATION:Seminar room 00.003
DESCRIPTION:The automorphism group of Igusa quartic is the symmetric group
of degree 6. There are other quartic threefolds that admits a
faithful action of this group. One of them is the famous Burkh
ardt quartic threefold. Together they form a pencil that conta
ins all $\mathbb{S}_6$-symmetric quartic threefolds.Arnaud Bea
uville proved that all but four of these quartic threeffolds a
re irrational. Later Cheltsov and Shramov proved that the rema
ining threefolds in this pencil are rational. In this talk, I
will give an alternative prove of both these results. To do th
is, I will describe Q-factorizations of the double cover of the
four-dimensional projective space branched over the Igusa qua
rtic, which is known as Coble fourfold. Using this, I will sho
w that $\mathbb{S}_6$-symmetric quartic threefolds are biratio
nal to conic bundles over quintic del Pezzo surfaces whose deg
eneration curves are contained in the pencil studied by Wiman a
nd Edge.This is a joint work with Alexander Kuznetsov and Cons
tantin Shramov (Moscow).
END:VEVENT
BEGIN:VEVENT
UID:news-657@dmi.unibas.ch
DTSTAMP:20190212T221448
DTSTART;TZID=Europe/Zurich:20170428T103000
DTEND;TZID=Europe/Zurich:20170428T120000
SUMMARY:Seminar Algebra and Geometry: Marcello Bernardara (Toulouse)
LOCATION:Seminar room 00.003
DESCRIPTION:Let X be a smooth projective variety over a field k, and assume
that weak factorization holds (e.g., k has characteristic zero
).I will introduce the Grothendieck ring of triangulated categ
ories, and show how, using Bondal-Larsen-Lunts motivic measure
, a subgroup of such ring will define a subgroup of the group
Bir(X) of birational self-maps of X. A main example is given b
y the filtration via the motivic dimension, which induces a fi
ltration on Bir(X). As a consequence, in the case X=Pn, we can
show that the group generated by the standard Cremona transfo
rmation and PGL(n+1) is strictly contained in the group contra
cting rational varieties, as soon as n > 4. Another example al
lows to reconstruct Frumkin's genus filtration of Bir(X) in the
cases where X is a uniruled threefold.
END:VEVENT
BEGIN:VEVENT
UID:news-656@dmi.unibas.ch
DTSTAMP:20190212T221502
DTSTART;TZID=Europe/Zurich:20170421T103000
DTEND;TZID=Europe/Zurich:20170421T120000
SUMMARY:Seminar Algebra and Geometry: Nestor Fernandez Vargas (Montpellier/Rennes)
LOCATION:Seminar room 00.003
DESCRIPTION:We are interested in rank 2 parabolic vector bundles over a 2-p
unctured elliptic curve C. We will describe the moduli space a
ssociated to these objects, and state a Torelli theorem.The fo
rmer moduli space is itself related to the moduli space of rank
2 parabolic bundles over a 5-punctured P1. We will explain thi
s link, recovering at the same time the beautiful geometry of d
el Pezzo surfaces of degree 4.
END:VEVENT
BEGIN:VEVENT
UID:news-655@dmi.unibas.ch
DTSTAMP:20190212T221514
DTSTART;TZID=Europe/Zurich:20170331T103000
DTEND;TZID=Europe/Zurich:20170331T120000
SUMMARY:Seminar Algebra and Geometry: Giosuè Muratore (Roma-Strasbourg)
LOCATION:Seminar room 00.003
DESCRIPTION:The 2-Fano varieties, defined by De Jong and Starr, satisfy som
e higher dimensional analogous properties of Fano varieties. W
e propose a definition of (weak) k-Fano variety and conjecture
the polyhedrality of the cone of pseudoeffective k-cycles for
those varieties in analogy with the case k=1. Then, we calcul
ate some Betti numbers of a large class of k-Fano varieties to
prove some special case of the conjecture. In particular, the
conjecture is true for all 2-Fano varieties of index ≥n-2,
and also we complete the classification of weak 2-Fano varietie
s of Araujo and Castravet.
END:VEVENT
BEGIN:VEVENT
UID:news-654@dmi.unibas.ch
DTSTAMP:20190212T224919
DTSTART;TZID=Europe/Zurich:20170317T103000
DTEND;TZID=Europe/Zurich:20170317T120000
SUMMARY:Seminar Algebra and Geometry: Yohan Brunebarbe (Zurich)
LOCATION:Seminar room 00.003
DESCRIPTION:For any positive integers g and n, let A_g(n) be the moduli spa
ce of principally polarized abelian varieties with a level-n s
tructure (it is a smooth quasi-projective variety for n>2). Bu
ilding on works of Nadel and Noguchi, Hwang and To have shown
that the minimal genus of a curve contained in A_g(n) grows wi
th n. We will explain a generalization of this result dealing
with subvarieties of any dimension. In particular, we show tha
t all subvarieties of A_g(n) are of general type when n > 6g.
Similar results are true more generally for quotients of bounde
d symmetric domains by lattices.
END:VEVENT
BEGIN:VEVENT
UID:news-653@dmi.unibas.ch
DTSTAMP:20190212T224932
DTSTART;TZID=Europe/Zurich:20170303T103000
DTEND;TZID=Europe/Zurich:20170303T120000
SUMMARY:Seminar Algebra and Geometry: Jan Draisma (Bern)
LOCATION:Seminar room 00.003
DESCRIPTION:Every real or complex matrix admits a singular value decomposit
ion, in which the terms are pairwise orthogonal in a strong se
nse. Higher-order tensors typically do not admit such an ortho
gonal decomposition. Those that do have attracted attention fr
om theoretical computer science and scientific computing. Comp
lementing this existing literature, I will present an algebro-
geometric analysis of the set of orthogonally decomposable ten
sors. This analysis features a surprising connection between o
rthogonally decomposable tensors and semisimple algebras---ass
ociative in the case of ordinary or symmetric tensors, and of
compact Lie type in the case of alternating tensors. (Joint wor
k with Ada Boralevi, Emil Horobet, and Elina Robeva.)
END:VEVENT
BEGIN:VEVENT
UID:news-670@dmi.unibas.ch
DTSTAMP:20190212T225047
DTSTART;TZID=Europe/Zurich:20161216T140000
DTEND;TZID=Europe/Zurich:20161216T153000
SUMMARY:Seminar Algebra and Geometry: Eleonora Di Nezza (Imperial College London)
LOCATION:Seminar room 00.003
DESCRIPTION:The geometry and topology of the space of Kähler metrics on a
compact Kähler manifold is a classical subject, first systema
tically studied by Calabi in relation with the existence of ex
tremal Kähler metrics. Then, Mabuchi proposed a Riemannian st
ructure on the space of Kähler metrics under which it (formal
ly) becomes a non-positive curved infinite dimensional space.
Chen later proved that this is a metric space of non-positive
curvature in the sense of Alexandrov and its metric completion
was characterized only recently by Darvas. In this talk we wi
ll talk about the extension of such a theory to the setting whe
re the compact Kähler manifold is replaced by a compact singu
lar normal Kähler space. As one application we give an analyt
ical criterion for the existence of Kähler-Einstein metrics o
n certain mildly singular Fano varieties, an analogous to a cr
iterion in the smooth case due to Darvas and Rubinstein.This i
s based on a joint work with Vincent Guedj.
END:VEVENT
BEGIN:VEVENT
UID:news-669@dmi.unibas.ch
DTSTAMP:20190212T225059
DTSTART;TZID=Europe/Zurich:20161216T103000
DTEND;TZID=Europe/Zurich:20161216T120000
SUMMARY:Seminar Algebra and Geometry: Zsolt Patakfalvi (EPFL)
LOCATION:Seminar room 00.003
DESCRIPTION:Stable varieties are higher dimensional generalizations of stab
le curves. Their moduli space contains an open locus parametri
zing varieties of general type up to birational equivalence, j
ust as the space of stable curves contains the space of smooth
curves in dimension one. Furthermore, also similarly to the o
ne dimensional picture, it provides a compactification of the
above locus, which is known in characteristic zero but it is o
nly conjectural in positive characteristic in dimension at lea
st two. I will present a work in progress aiming to prove the
projectivity of every proper subspace of the moduli space of s
table surfaces in characteristic greater than 5.
END:VEVENT
BEGIN:VEVENT
UID:news-668@dmi.unibas.ch
DTSTAMP:20190212T225134
DTSTART;TZID=Europe/Zurich:20161209T103000
DTEND;TZID=Europe/Zurich:20161209T120000
SUMMARY:Seminar Algebra and Geometry: Peter Feller (Bonn)
LOCATION:Seminar room 00.003
DESCRIPTION:After a brief introduction to plane curve singularities, we rec
ast some of their classical invariants in terms of modern knot
theory. Then we will see connections between purely algebraic
questions about deformations of singularities and purely knot
theoretic questions about cobordisms between knots, and answe
r some of them. This talk aims to be elementary and will not a
ssume familiarity with knot theory or singularity theory.
END:VEVENT
BEGIN:VEVENT
UID:news-667@dmi.unibas.ch
DTSTAMP:20190212T225148
DTSTART;TZID=Europe/Zurich:20161111T103000
DTEND;TZID=Europe/Zurich:20161111T120000
SUMMARY:Seminar Algebra and Geometry: Ruth Kellerhals (Fribourg)
LOCATION:Seminar room 00.003
DESCRIPTION:This talk starts with a survey about hyperbolic volume in three
dimensions, dilogarithms and zeta values.The main part deals
with similar features and questions about hyperbolic volume of
(non-)arithmetic Coxeter orbifolds of higher dimensions. We d
iscuss the volume problem for certain Coxeter pyramids. This fa
mily gives rise to interesting (non-)arithmetic reflection gro
ups whose commensurability classification has recently been pe
rformed (joint work with R. Guglielmetti and M. Jacquemet).Thi
s is a report about ongoing work.
END:VEVENT
BEGIN:VEVENT
UID:news-666@dmi.unibas.ch
DTSTAMP:20190212T225159
DTSTART;TZID=Europe/Zurich:20161104T103000
DTEND;TZID=Europe/Zurich:20161104T120000
SUMMARY:Seminar Algebra and Geometry: Diletta Martinelli (University of Edinburgh)
LOCATION:Seminar room 00.003
DESCRIPTION:Finding minimal models is the first step in the birational cla
ssification of smooth projective varieties. After it is establi
shed that a minimal model exists, some natural question arise,
such as: is the minimal model unique? If not, how many are th
ey? After recalling all the necessary notions of the Minimal M
odel Program, I will explain that varieties of general type ad
mit a finite number of minimal models. I will talk about a rec
ent joint project with Stefan Schreieder and Luca Tasin, where
we prove that in the case of threefold this number is bounded
by a constant depending only on the Betti numbers. I will also
show that in some cases it is possible to compute this consta
nt explicitly.
END:VEVENT
BEGIN:VEVENT
UID:news-665@dmi.unibas.ch
DTSTAMP:20190212T225215
DTSTART;TZID=Europe/Zurich:20161021T103000
DTEND;TZID=Europe/Zurich:20161021T120000
SUMMARY:Seminar Algebra and Geometry: Jesus Martinez (MPIM Bonn)
LOCATION:Seminar room 00.003
DESCRIPTION:We study variations of GIT quotients of log pairs (X,D) where X
is a hypersurface of some fixed degree and D is a hyperplane
section. GIT is known to provide a finite number of possible c
ompactifications of such pairs, depending on one parameter. An
y two such compactifications are related by birational transfo
rmations. We describe an algorithm to study the stability of t
he Hilbert scheme of these pairs, and apply our algorithm to t
he case of cubic surfaces. Finally, we relate these compactifi
cations to the (conjectural) moduli space of logK-semistable p
airs showing that any log K-stable pair is an element of our mo
duli and that there is a canonically defined CM line bundle th
at polarizes our moduli. This is a joint work with Patricio Ga
llardo (University of Georgia) and Cristiano Spotti (Aarhus Un
iversity).
END:VEVENT
BEGIN:VEVENT
UID:news-664@dmi.unibas.ch
DTSTAMP:20190212T225229
DTSTART;TZID=Europe/Zurich:20161014T103000
DTEND;TZID=Europe/Zurich:20161014T120000
SUMMARY:Seminar Algebra and Geometry: Stefano Urbinati (Padova)
LOCATION:Seminar room 00.003
DESCRIPTION:Given a Mori Dream Space X we construct via tropicalization a m
odel dominating all the small Q-factorial modifications. Via t
his construction we recover a Minkowski bases for the Newton-O
kunkov bodies on X and hence the movable cone for X.This is a
work in progress with Elisa Postinghel.
END:VEVENT
BEGIN:VEVENT
UID:news-663@dmi.unibas.ch
DTSTAMP:20190212T225243
DTSTART;TZID=Europe/Zurich:20161007T103000
DTEND;TZID=Europe/Zurich:20161007T120000
SUMMARY:Seminar Algebra and Geometry: Hannah Bergner (Freiburg)
LOCATION:Seminar room 00.003
DESCRIPTION:Let J be a (complex) semi-simple Jordan algebra, and consider t
he diagonal action of its automorphism group on the n-fold pro
duct of J. In this talk, geometric properties of this action ar
e studied. In particular, a characterization of the closed orb
its is given, which is similar to the description in the case
where a reductive linear algebraic group acts on (the n-fold p
roduct of) its Lie algebra.
END:VEVENT
BEGIN:VEVENT
UID:news-662@dmi.unibas.ch
DTSTAMP:20190212T225255
DTSTART;TZID=Europe/Zurich:20160930T103000
DTEND;TZID=Europe/Zurich:20160930T120000
SUMMARY:Seminar Algebra and Geometry: Hanspeter Kraft (Basel)
LOCATION:Seminar room 00.003
DESCRIPTION:The aim is to give a modern proof of the famous Theorem of A. W
eil showing that for every rational action of an algebraic gr
oup G on a variety X there is a regular action of G on a vari
ety Y and a G-equivariant birational map X → Y. As a guidel
ine we use an approach given by D. Zaitsev (J. Lie Theory 5,
1995).
END:VEVENT
BEGIN:VEVENT
UID:news-661@dmi.unibas.ch
DTSTAMP:20190212T225309
DTSTART;TZID=Europe/Zurich:20160923T103000
DTEND;TZID=Europe/Zurich:20160923T120000
SUMMARY:Seminar Algebra and Geometry: Hanspeter Kraft (Basel)
LOCATION:Seminar room 00.003
DESCRIPTION:The aim is to give a modern proof of the famous Theorem of A. W
eil showing that for every rational action of an algebraic gro
up G on a variety X there is a regular action of G on a variet
y Y and a G-equivariant birational map X → Y. As a guideline
we use an approach given by D. Zaitsev (J. Lie Theory 5, 1995
).
END:VEVENT
BEGIN:VEVENT
UID:news-683@dmi.unibas.ch
DTSTAMP:20190212T225424
DTSTART;TZID=Europe/Zurich:20160602T103000
DTEND;TZID=Europe/Zurich:20160602T120000
SUMMARY:Seminar Algebra and Geometry: Anne Lonjou (Toulouse)
LOCATION:Seminar room 00.003
DESCRIPTION:In this talk, we will study a 2-dimensional simplicial complex
on which the Cremona group acts. After defining this complex i
ntroduced by Wright in 1992, we will see some of its propertie
s. Then, we will discuss about some work in progress aimed at
showing that this complex is Gromov hyperbolic.
END:VEVENT
BEGIN:VEVENT
UID:news-682@dmi.unibas.ch
DTSTAMP:20190212T225437
DTSTART;TZID=Europe/Zurich:20160527T103000
DTEND;TZID=Europe/Zurich:20160527T120000
SUMMARY:Seminar Algebra and Geometry: Tatiana Bandman (Bar Ilan)
LOCATION:Seminar room 00.003
DESCRIPTION:A group G is called Jordan if there is a positive integer J
= JG such that every nite subgroup B of G contains a
commutative subgroup A c B such that A is normal in B an
d the index [B : A ]<= J . There is no example of an algebra
ic variety with the non-Jordan automorphism group. It is known
that the group of birational automorphisms Bir(X ) of a project
ive variety X is Jordan if it is not uniruled and is not Jord
an if X is birational to the direct product of a projective s
pace with an abelian variety. I will give an introduction to th
e topic and discuss the case when variety X is a conic bundle
over a non-uniruled variety Y and is not birational to Y x
P^1: This is a joint work with Y. Zarhin.
END:VEVENT
BEGIN:VEVENT
UID:news-681@dmi.unibas.ch
DTSTAMP:20190212T225453
DTSTART;TZID=Europe/Zurich:20160513T103000
DTEND;TZID=Europe/Zurich:20160513T120000
SUMMARY:Seminar Algebra and Geometry: Vladimir Lazic (Bonn)
LOCATION:Seminar room 00.003
DESCRIPTION:Given an algebraic variety, finding non-trivial morphisms to o
ther varieties is one of the fundamental problems in algebraic
geometry. Mori and others solved this problem in the 1980s fo
r varieties whose canonical class is not non-negative on all c
urves. The question remains open for other varieties. However,
in this talk I will report on a recent progress on this probl
em in a joint work with Thomas Peternell. I will concentrate o
n varieties with trivial canonical class and on Calabi-Yau man
ifolds in particular.
END:VEVENT
BEGIN:VEVENT
UID:news-680@dmi.unibas.ch
DTSTAMP:20190212T225509
DTSTART;TZID=Europe/Zurich:20160429T103000
DTEND;TZID=Europe/Zurich:20160429T120000
SUMMARY:Seminar Algebra and Geometry: Arnaud Girand (Rennes)
LOCATION:Seminar room 00.003
DESCRIPTION:In this talk we will discuss how various methods from biration
al and algebraic geometry may be used to classify and explicit
ly construct a specific family of isomonodromic deformations o
ver the five punctured sphere. These objects (which we will in
troduce during the course of the talk) have been linked to the
study of algebraic solutions of certain partial differential
equations and offer some interesting ways of bridging some alg
ebraic, geometric and analytical topics.
END:VEVENT
BEGIN:VEVENT
UID:news-679@dmi.unibas.ch
DTSTAMP:20190212T225522
DTSTART;TZID=Europe/Zurich:20160408T103000
DTEND;TZID=Europe/Zurich:20160408T120000
SUMMARY:Seminar Algebra and Geometry: Luca Tasin (Bonn)
LOCATION:Seminar room 00.003
DESCRIPTION:Generalising a question of Hirzebruch, Kotschick asked the foll
owing: which Chern numbers are determined up to finite ambigui
ty by the underlying smooth manifold? Together with S. Schreie
der, I treated this question in dimension higher than 3. After
explaining such results, I will talk about the 3 dimensional
case, where it is believed that Chern numbers are bounded. Res
ults in this direction have been obtained in a recent preprint
with P. Cascini, where tools from the Minimal Model Program h
ave been used, combined with topology's and arithmetic's techn
iques.
END:VEVENT
BEGIN:VEVENT
UID:news-678@dmi.unibas.ch
DTSTAMP:20190212T225533
DTSTART;TZID=Europe/Zurich:20160401T103000
DTEND;TZID=Europe/Zurich:20160401T120000
SUMMARY:Seminar Algebra and Geometry: Daniel Panazzolo (Mulhouse)
LOCATION:Seminar room 00.003
DESCRIPTION:PSL(2,C), the exponential and some new free groups
END:VEVENT
BEGIN:VEVENT
UID:news-677@dmi.unibas.ch
DTSTAMP:20190212T225547
DTSTART;TZID=Europe/Zurich:20160318T103000
DTEND;TZID=Europe/Zurich:20160318T120000
SUMMARY:Seminar Algebra and Geometry: Giulio Codogni (Roma Tre)
LOCATION:Seminar room 00.003
DESCRIPTION:We prove (using algebro-geometric methods) two results that all
ow to test the positivity of the Donaldson-Futaki weights of a
rbitrary polarised varieties via test-configurations which are
equivariant with respect to a maximal torus in the automorphi
sm group. It follows in particular that there is a purely alge
bro-geometric proof of the K-stability of projective spaces (o
r more generally of smooth toric Fanos with vanishing Futaki c
haracter, as well as of the examples of non-toric Kahler-Einst
ein Fano threefolds due to Ilten and Suss) and that K-stabilit
y for toric polarised manifolds can be tested via toric test-c
onfigurations. A further application is a proof of the K-stabil
ity of constant scalar curvature polarised manifolds with cont
inuous automorphisms. Our approach is based on the method of f
iltrations introduced by Wytt Nystrom and Szekelyhidi. This is
a joint work with J. Stoppa.
END:VEVENT
BEGIN:VEVENT
UID:news-676@dmi.unibas.ch
DTSTAMP:20190212T225558
DTSTART;TZID=Europe/Zurich:20160311T103000
DTEND;TZID=Europe/Zurich:20160311T120000
SUMMARY:Seminar Algebra and Geometry: Jean Fasel (Grenoble)
LOCATION:Seminar room 00.003
DESCRIPTION:In this talk, which is a joint work with Adrien Dubouloz, we wi
ll explain that the unstable homotopy category is unable to di
stinguish between the affine space of dimension 3 and the Kora
s-Russell threefolds of the first kind. We will first make som
e historical remarks and then spend some time to explain the b
asic features of the unstable homotopy category, in particular
how to distinguish isomorphisms in some special cases. We wil
l then proceed with the proof of the above claim.
END:VEVENT
BEGIN:VEVENT
UID:news-675@dmi.unibas.ch
DTSTAMP:20190212T225615
DTSTART;TZID=Europe/Zurich:20160310T110000
DTEND;TZID=Europe/Zurich:20160310T120000
SUMMARY:Seminar Algebra and Geometry: Alexander Perepechko (Moscow)
LOCATION:Seminar room 00.003
DESCRIPTION:It is well known that A1-fibrations are of particular interest
in the study of automorphism groups of affine surfaces. That i
s, description of automorphism groups is based on subgroups pr
eserving A1-fibrations, except for surfaces without such fibra
tions. We provide a method to directly compute these subgroups
in terms of a boundary divisor of an SNC-completion. We also
use the concepts of arc spaces and formal neighbourhoods. This
allows us to establish the following structure of a subgroup
preserving a A1-fibration. Up to a finite index, it is a semid
irect product of an abelian unipotent subgroup acting by trans
lations on fibers and of a finite-dimensional subgroup that fix
es a certain section.In particular, we derive an example of a
surface with infinite discrete automorphism group.
END:VEVENT
BEGIN:VEVENT
UID:news-674@dmi.unibas.ch
DTSTAMP:20190212T225629
DTSTART;TZID=Europe/Zurich:20160308T140000
DTEND;TZID=Europe/Zurich:20160308T150000
SUMMARY:Seminar Algebra and Geometry: De-Qi Zhang (Singapore)
LOCATION:Seminar room 00.003
DESCRIPTION:We will report our recent results on solvable groups G of autom
orphisms of normal projective varieties X. One machinery we us
e is the equivariant minimal model program. One purpose is to
know more geometry of X from the existence of such G acting on
X.
END:VEVENT
BEGIN:VEVENT
UID:news-673@dmi.unibas.ch
DTSTAMP:20190212T225652
DTSTART;TZID=Europe/Zurich:20160303T103000
DTEND;TZID=Europe/Zurich:20160303T120000
SUMMARY:Seminar Algebra and Geometry: Hamid Ahmadinezhad (Bristol)
LOCATION:Seminar room 00.003
DESCRIPTION:I will give an overview of the geometry of Fano 3-folds after M
ori theory. After discussing past approaches to the classifica
tion, I will highlight why such attempts seem hopeless. Buildi
ng on recent advances in the geometry of Fanos, I introduce a
new viewpoint on the classification problem. A main emphasis w
ill be given to the unpredicted behaviour of the first example
s of non-complete intersection Fanos, discovered in a joint wo
rk with Takuzo Okada.
END:VEVENT
BEGIN:VEVENT
UID:news-672@dmi.unibas.ch
DTSTAMP:20190212T225702
DTSTART;TZID=Europe/Zurich:20160226T103000
DTEND;TZID=Europe/Zurich:20160226T120000
SUMMARY:Seminar Algebra and Geometry: Alvaro Liendo (Talca)
LOCATION:Seminar room 00.003
DESCRIPTION:This is a joint work with A. Dubouloz.In this talk we present s
ome recent results about additive group actions on non-necessa
rily affine algebraic varieties that generalize the usual desc
ription of additive group actions on affine varieties via loca
lly nilpotent derivations. In particular, we provide a charact
erization of additive group actions on a wide class of algebra
ic varieties in terms of a certain type of integrable vector f
ields.
END:VEVENT
BEGIN:VEVENT
UID:news-671@dmi.unibas.ch
DTSTAMP:20190212T225715
DTSTART;TZID=Europe/Zurich:20160205T093000
DTEND;TZID=Europe/Zurich:20160205T110000
SUMMARY:Seminar Algebra and Geometry: François Dumas (Clermont-Ferrand)
LOCATION:Seminar room 00.003
DESCRIPTION:The natural algebraic question of extending a group action on a
commutative algebra A of "functions" to a noncommutative alge
bra of differential (or rational differential, or pseudodiffer
ential) "operators" with coefficients in A is closely related
to the study of deformations of A.In the particular case of ho
mography groups, we will give in this talk an overview of some
results about this topic involving transvectants, modular forms
and Rankin-Cohen brackets.
END:VEVENT
BEGIN:VEVENT
UID:news-694@dmi.unibas.ch
DTSTAMP:20190212T230240
DTSTART;TZID=Europe/Zurich:20151211T103000
DTEND;TZID=Europe/Zurich:20151211T120000
SUMMARY:Seminar Algebra and Geometry: Federico Lo Bianco (Rennes)
LOCATION:Seminar room 00.003
DESCRIPTION:An automorphism f acting on a complex projective (or, more gen
erally, compact Kaehler) manifold X induces by pull-back a lin
ear isomorphism f* of the cohomology of X. The study of the pos
sible isomorphisms that can be realized this way is interestin
g per se and also has important applications in the study of t
he dynamics of f: for example, the topological entropy of f, m
easuring the chaos created by repeatedly applying f, can be re
covered from the eigenvalues of f*. If X is a surface the situ
ation is well understood: if f* is not of finite order, then e
ither it is virtually unipotent with only one non-trivial Jord
an block of dimension 3, or semi-simple with only two eigenval
ues with module different than 1. I will present the similar r
esults I obtained with Cantat for threefolds and show their opt
imality with examples on complex tori.
END:VEVENT
BEGIN:VEVENT
UID:news-693@dmi.unibas.ch
DTSTAMP:20190212T230253
DTSTART;TZID=Europe/Zurich:20151204T103000
DTEND;TZID=Europe/Zurich:20151204T120000
SUMMARY:Seminar Algebra and Geometry: Jung-Kyu Canci (Basel)
LOCATION:Seminar room 00.003
DESCRIPTION:In a recent work in collaboration with Salomon Vishkautasan, we
provide a complete classification of possible graphs of ratio
nal preperiodic points of endomorphisms of the projective lin
e of degree 2 defined over the rationals with a rational peri
odic critical point of period 2, under the assumption that the
se maps have no periodic points of period at least 5. We expl
ain how this extends results of Poonen on quadratic polynomia
ls. We show that there are 13 possible graphs, and that such m
aps have at most 9 rational preperiodic points.
END:VEVENT
BEGIN:VEVENT
UID:news-692@dmi.unibas.ch
DTSTAMP:20190212T230306
DTSTART;TZID=Europe/Zurich:20151113T103000
DTEND;TZID=Europe/Zurich:20151113T120000
SUMMARY:Seminar Algebra and Geometry: Emilie Dufresne (Durham)
LOCATION:Seminar room 00.003
DESCRIPTION:The study of separating invariants is a new trend in Invariant
Theory and a return to its roots: invariants as a classificati
on tool. For a finite group acting linearly on a vector space,
a separating set is simply a set of invariants whose elements
separate the orbits o the action. Such a set need not generat
e the ring of invariants. In this talk, we give lower bounds o
n the size of separating sets based on the geometry of the act
ion. These results are obtained via the study of the local coh
omology with support at an arrangement of linear subspaces nat
urally arising from the action.(Joint with Jack Jeffries)
END:VEVENT
BEGIN:VEVENT
UID:news-691@dmi.unibas.ch
DTSTAMP:20190212T230316
DTSTART;TZID=Europe/Zurich:20151106T103000
DTEND;TZID=Europe/Zurich:20151106T120000
SUMMARY:Seminar Algebra and Geometry: Elisa Gorla (Neuchâtel)
LOCATION:Seminar room 00.003
DESCRIPTION:In this talk I will discuss a new idea for studying initial id
eals and generic initial ideals of ideals in a polynomial ring
graded over Zm. \r\n\r\nThese ideas will then be applied to t
he study of universal Groebner bases of ideals of minors, whic
h admit such a grading. The main tools and concepts are Zm gra
dings, multigraded Hilbert series, universal Groebner bases, i
nitial and generic initial ideals. I will introduce and study
two families of multigraded ideals with the property that any t
wo ideals in the same family that have the same Hilbert series
also have the same generic initial ideal. I will show that id
eals of minors belong to one of these families, and derive som
e result about their universal Groebner bases.
END:VEVENT
BEGIN:VEVENT
UID:news-690@dmi.unibas.ch
DTSTAMP:20190212T230333
DTSTART;TZID=Europe/Zurich:20151030T103000
DTEND;TZID=Europe/Zurich:20151030T120000
SUMMARY:Seminar Algebra and Geometry: Enrica Floris (MPI-Bonn)
LOCATION:Seminar room 00.003
DESCRIPTION:Let X be a smooth algebraic surface. A foliation F on X is, rou
ghly speaking, a subline bundle TF of the tangent bundle of X.
The dual of TF is called the canonical bundle of the foliatio
n KF. In the last few years birational methods have been succes
sfully used in order to study foliations. More precisely, geome
tric properties of the foliation are translated into properties
of the canonical bundle of the foliation. One of the most imp
ortant invariants describing the properties of a line bundle L
is its Kodaira dimension kod(L), which measures the growth of
the global sections of L and its tensor powers. The Kodaira di
mension of a foliation F is defined as the Kodaira dimension of
its canonical bundle kod(KF). In their fundamental works, Bru
nella and McQuillan give a classfication of foliations on surf
aces on the model of Enriques-Kodaira classification of surfac
es. The next step is the study of the behaviour of families of
foliations. Brunella proves that, for a family of foliations (
Xt, Ft) of dimension one on surfaces, satisfying certain hypot
heses of regularity, the Kodaira dimension of the foliation do
es not depend on t. By analogy with Siu's Invariance of Plurig
enera, it is natural to ask whether for a family of foliations
(Xt, Ft) the dimensions of global sections of the canonical b
undle and its powers depend on t. In this talk we will discuss
to which extent an Invariance of Plurigenera for foliations is
true and under which hypotheses on the family of foliations i
t holds.
END:VEVENT
BEGIN:VEVENT
UID:news-689@dmi.unibas.ch
DTSTAMP:20190212T230411
DTSTART;TZID=Europe/Zurich:20151023T103000
DTEND;TZID=Europe/Zurich:20151023T120000
SUMMARY:Seminar Algebra and Geometry: Jakub Witaszek (Imperial College London)
LOCATION:Seminar room 00.003
DESCRIPTION:The famous Fujita conjecture says that KX + (d+2)A is very ampl
e, where X is a smooth projective variety of dimension d, and
A is an ample divisor. Despite its simple statement, the conje
cture is only known for d < 3 in characteristic zero. In posit
ive characteristic, the conjecture hasn't been even proved for
surfaces, but thanks to the recent progress due to Di Cerbo a
nd Fanelli, we know that when X is a projective smooth surface
there exist constants a, b ∈ ℕ such that aKX + bA is very
ample. In this talk, we will discuss similar bounds in the ca
se when X is singular.
END:VEVENT
BEGIN:VEVENT
UID:news-688@dmi.unibas.ch
DTSTAMP:20190212T230425
DTSTART;TZID=Europe/Zurich:20151016T103000
DTEND;TZID=Europe/Zurich:20151016T120000
SUMMARY:Seminar Algebra and Geometry: Arthur Renaudineau (Genève)
LOCATION:Seminar room 00.003
DESCRIPTION:After recalling general problems in the study of topology of r
eal algebraic varieties and Viro's method to construct such var
ieties, we will present a construction of real algebraic surfa
ces with many handles in (CP1)3.
END:VEVENT
BEGIN:VEVENT
UID:news-687@dmi.unibas.ch
DTSTAMP:20190212T230438
DTSTART;TZID=Europe/Zurich:20151009T103000
DTEND;TZID=Europe/Zurich:20151009T120000
SUMMARY:Seminar Algebra and Geometry: Jean-Philippe Furter (Basel)
LOCATION:Seminar room 00.003
DESCRIPTION:The groups GL2(C[X1,...,Xm]) and Aut(An) can naturally be c
onsidered as ind-groups (algebraic groups of infinite dimension
). As such, they are endowed with the Zariski topology. We wi
ll describe several topological properties of these two groups
. In particular, we will give examples of closed subgroups.
END:VEVENT
BEGIN:VEVENT
UID:news-686@dmi.unibas.ch
DTSTAMP:20190212T230452
DTSTART;TZID=Europe/Zurich:20151002T103000
DTEND;TZID=Europe/Zurich:20151002T120000
SUMMARY:Seminar Algebra and Geometry: Andrea Fanelli (Basel)
LOCATION:Seminar room 00.003
DESCRIPTION:In this talk I will give a fast introduction to the problem of
birational classification of algebraic varieties, where Mori f
ibre spaces naturally appear. I will then discuss a classifica
tion project of the Fano varieties which can appear as a gener
al fibre of a Mori fibre space (fibre-like Fano varieties). At
this stage we have two criteria, one sufficient and one neces
sary, to detect the fibre-likeness of a Fano variety. These cr
iteria turn into a characterisation in the rigid case. Our mai
n applications are for surfaces, three-folds and toric varietie
s.
END:VEVENT
BEGIN:VEVENT
UID:news-685@dmi.unibas.ch
DTSTAMP:20190212T230504
DTSTART;TZID=Europe/Zurich:20150925T103000
DTEND;TZID=Europe/Zurich:20150925T120000
SUMMARY:Seminar Algebra and Geometry: Ronan Terpereau (Bonn)
LOCATION:Seminar room 00.003
DESCRIPTION:In this talk you will learn about Chevalley theorem for Lie alg
ebras, commuting scheme (associated with a Lie algebra, a thet
a-group, or a polar representation), symplectic reduction (ass
ociated with a representation of a reductive group), symplecti
c singularities and their resolutions, and about the beautiful
relations between all these things. I will mention several qu
estions remained open for decades and explained some recent ap
proaches to try to answer them. This talk will be elementary w
ith many examples and shall provide a good opportunity to lear
n about representation theory of reductive groups and the Lie
theory.
END:VEVENT
BEGIN:VEVENT
UID:news-684@dmi.unibas.ch
DTSTAMP:20190212T230517
DTSTART;TZID=Europe/Zurich:20150918T103000
DTEND;TZID=Europe/Zurich:20150918T120000
SUMMARY:Seminar Algebra and Geometry: Adrien Dubouloz (Dijon)
LOCATION:Seminar room 00.003
DESCRIPTION:An exotic affine sphere is a smooth complex affine variety whic
h is diffeomorphic to a smooth non degenerate affine quadric o
f the same dimension but not algebraically isomorphic to it. T
he smooth threefold with equation x2v + y2u = 1 is such an exo
tic sphere, and in fact is essentially the unique example know
n so far. In this talk, after a short review of the existing r
esults and open questions around these spheres, I will discuss
some strategies to construct larger families of exotic 3-sphe
res, as a well as higher dimensional exotic spheres. It time p
ermits, I will indicate some connexions between the exoticity o
f a certain 5-dimensional sphere in the sens of A1-homotopy th
eory and the stable exoticity of the Russell cubic threefold.
END:VEVENT
BEGIN:VEVENT
UID:news-703@dmi.unibas.ch
DTSTAMP:20190207T211242
DTSTART;TZID=Europe/Zurich:20150604T141500
DTEND;TZID=Europe/Zurich:20150604T154500
SUMMARY:Seminar Algebra and Geometry: Joseph Landsberg (Texas A&M University)
LOCATION:Spiegelgasse 1 (Erdgeschoss)
DESCRIPTION:In 1969, Strassen discovered the standard algorithm for multipl
ying two n×n matrices is not the optimal one. Subsequent work
has led to the astounding conjecture that as n goes to infini
ty it is nearly as easy to multiply matrices as it is to add t
hem. I will explain uses of algebraic geometry, differential g
eometry and representation theory in the study of this conject
ure.
END:VEVENT
BEGIN:VEVENT
UID:news-702@dmi.unibas.ch
DTSTAMP:20190207T210012
DTSTART;TZID=Europe/Zurich:20150529T103000
DTEND;TZID=Europe/Zurich:20150529T120000
SUMMARY:Seminar Algebra and Geometry: Christian Urech (Universität Basel)
LOCATION:Spiegelgasse 1 (Erdgeschoss)
DESCRIPTION:The Cremona group Crn of dimension n is the group of birational
transformations of projective n-space. It is interesting to s
tudy group homomorphisms from Crn to Crm. However, at the mome
nt it seems to be very difficult to find a general classificat
ion. In this talk I will recall some properties of Cr2 and def
ine what an algebraic homomorphism between Cremona groups is.
Then we will discuss in detail an example given by Gizatullin
of an embedding of Cr2 to Cr5 and some of its rich geometrical
properties.
END:VEVENT
BEGIN:VEVENT
UID:news-701@dmi.unibas.ch
DTSTAMP:20190207T205506
DTSTART;TZID=Europe/Zurich:20150522T103000
DTEND;TZID=Europe/Zurich:20150522T120000
SUMMARY:Seminar Algebra and Geometry: Charlie Petitjean (Université de Bourgogne)
LOCATION:Spiegelgasse 1 (Erdgeschoss)
DESCRIPTION:In this talk we will consider birational equivalence of non nec
essarily irreducible divisors on rational surfaces. It will b
e given an useful invariant (in this context) related to the
Kodaira dimension. All the considered constructions will be c
onnected to particular presentations of T-varieties.
END:VEVENT
BEGIN:VEVENT
UID:news-700@dmi.unibas.ch
DTSTAMP:20190207T205023
DTSTART;TZID=Europe/Zurich:20150424T103000
DTEND;TZID=Europe/Zurich:20150424T120000
SUMMARY:Seminar Algebra and Geometry: Mathieu Huruguen (EPFL, Lausanne)
LOCATION:Spiegelgasse 1 (Erdgeschoss)
DESCRIPTION:A linear algebraic group G defined over a field k is called spe
cial if every G-torsor over every field extension of k is trivi
al. In a modern language, it can be shown that the special gro
ups are those of essential dimension zero. In 1958 Grothendiec
k classified special groups in the case where the base field k
is algebraically closed. In this talk I will explain the class
ification of special reductive groups over an arbitrary field.
If time permits, I will give an application to a conjecture of
Serre.
END:VEVENT
BEGIN:VEVENT
UID:news-699@dmi.unibas.ch
DTSTAMP:20190202T205342
DTSTART;TZID=Europe/Zurich:20150410T103000
DTEND;TZID=Europe/Zurich:20150410T120000
SUMMARY:Seminar Algebra and Geometry: Hanspeter Kraft (Universität Basel)
LOCATION:Spiegelgasse 1 (Erdgeschoss)
DESCRIPTION:We show that the automorphism group of affinen-space An determi
nes An up to isomorphism: If X is a connected affine variety s
uch that Aut(X) \simeq Aut(An) as ind-groups, then X \simeq An
as varieties.We also show that every torus appears as A
ut(X) for a suitable affine variety X, but that Aut(X)
cannot be isomorphic to a semisimple group. In fact,
if Aut(X) is finitedimensional and if X \not\simeq A1, then
the connected component Aut(X)◦ is a torus.Concerning the str
ucture of Aut(An) we prove that any homomorphism Aut(An)→ G o
f ind-groups either factors through jac : Aut(An)→C∗ where
jac is the Jacobian determinant, or it is a closed immersion.
For SAut(An) := ker(jac)⊂Aut(An) we show that everynontrivial
homomorphism SAut(An)→G is a closed immersion.Finally, we p
rove that every non-trivial homomorphism φ: SAut(An)→SAut(An
) is anautomorphism, and that φ is given by conjugation with a
n element from Aut(An).
END:VEVENT
BEGIN:VEVENT
UID:news-698@dmi.unibas.ch
DTSTAMP:20190202T201415
DTSTART;TZID=Europe/Zurich:20150327T103000
DTEND;TZID=Europe/Zurich:20150327T120000
SUMMARY:Seminar Algebra and Geometry: Pierre-Marie Poloni (Universität Basel)
LOCATION:Spiegelgasse 1 (Erdgeschoss)
DESCRIPTION:Explicit counterexamples to the Laurent Cancellation Problem
END:VEVENT
BEGIN:VEVENT
UID:news-697@dmi.unibas.ch
DTSTAMP:20190202T201121
DTSTART;TZID=Europe/Zurich:20150320T103000
DTEND;TZID=Europe/Zurich:20150320T120000
SUMMARY:Seminar Algebra and Geometry: Markus Hunziker (Baylor University, Texas)
LOCATION:Spiegelgasse 1 (Erdgeschoss)
DESCRIPTION:In this talk, I will show how to solve several famous problems
(old and new) from classical invariant theory by using the stru
cture of certain categories of highest weight modules. For exam
ple, the structure of a category of highest weight modules for
the symplectic Liealgebra sp2n(C) will be used to compute minim
al free resolutions (syzygies) and Hilbertseries of the modules
of covariants for the action of the orthogonal group Ok(C) on
Ck⊕···⊕Ck( n-copies). This in turn leads to a simple cha
racterization of the Cohen–Macaulaymodules of covariants.
END:VEVENT
BEGIN:VEVENT
UID:news-696@dmi.unibas.ch
DTSTAMP:20190202T200544
DTSTART;TZID=Europe/Zurich:20150313T103000
DTEND;TZID=Europe/Zurich:20150313T120000
SUMMARY:Seminar Algebra and Geometry: Johan Björklund (Institut de Mathématiques de Jussieu)
LOCATION:Spiegelgasse 1 (Erdgeschoss)
DESCRIPTION:A real algebraic rational knot is an algebraic embedding of CP1
into CP3 by real polynomials. By applying a real projection t
o a (generic) projective plane we obtain a real rational plana
r curve of the same degree, which should be thought of as the
associated knot diagram (with appropriate decorations at doubl
e points). In classical smooth knot theory, knots and their di
agrams are closely interconnected. For each decorated diagram,
there is a corresponding knot (constructed using an appropria
te height-function). In the algebraic setting this does not ho
ld true, there exists decorated real algebraic planar curves wh
ich can not be lifted to an appropriate knot.In this talk I wi
ll explain how to associate a hyperplane arrangement to a noda
l planar rational real algebraic curve. This arrangement descr
ibes the space of nonsingular liftings and allows us to calcul
ate the homology (and thus, in particular, the number of lifti
ngs up to rigid isotopy). We will show that, up to degree 5, t
his hyperplane arrangement is a rigid isotopy invariant (of pl
anar curves) and can provide real algebraic analogues of the c
lassical Reidemeister moves. Obstructions in the case of higher
degrees will be discussed. The talk should be accessible to
nonspecialists.
END:VEVENT
BEGIN:VEVENT
UID:news-695@dmi.unibas.ch
DTSTAMP:20190202T194348
DTSTART;TZID=Europe/Zurich:20150220T103000
DTEND;TZID=Europe/Zurich:20150220T120000
SUMMARY:Seminar Algebra and Geometry: Jörg Winkelmann (Ruhr-Universität Bochum)
LOCATION:Spiegelgasse 1 (Erdgeschoss)
DESCRIPTION:Conjecturally abundance of entire curves is closely related to
abundance of rational points on projective varieties defined o
ver a number field.This is discussed in connection with algebr
aic groups and prinicipal bundles. More precisely, let X be a
projective manifold, G an algebraic group and E a G-principal
bundle on X, all defined over some number field K. Then E admi
ts a Zariskidense set of L-rational points for some finite ext
ension field iff X does. This corresponds to the homotopy lift
ing property whose complex analytic analogue allows to lift ent
ire curves.
END:VEVENT
BEGIN:VEVENT
UID:news-716@dmi.unibas.ch
DTSTAMP:20190207T232809
DTSTART;TZID=Europe/Zurich:20141219T103000
DTEND;TZID=Europe/Zurich:20141219T120000
SUMMARY:Seminar Algebra and Geometry: Amos Turchet (Chalmers Universitet, Göteborg)
LOCATION:kleiner Hörsaal
DESCRIPTION:Lang-Vojtas Conjectures are a set of deep and far reaching conj
ectures, formulated by Paul Vojta using ideas of Lang, which em
brace the distribution of solutions to Diophantine equations ov
er number fields, the behaviour of holomorphic maps into comple
x manifolds and of algebraic curves into algebraic varieties.
In the (split) function field case the conjecture predicts (wea
k) algebraic hyperbolicity for log-general type varieties.When
the completion of the variety is the projective plane the conje
cture is known both if the divisor at infinity consits of four
lines in general position (Brownawell-Masser and,independently,
Voloch) and for a conic and two lines with five singular point
s (Corvaja and Zannier). With different methods Chen and Pacie
nza-Rousseau proved that the conjecture holds in the hyperbolic
case, i.e. the complement of a very generic curve of degree a
t least 5.In the talk, after an introduction to this fascinatin
g subject, we will show how to prove the conjecture in general
for the complement of a very generic curve of degree at least f
our.The proof relies on a deformation argument applied to a con
ic and two lines and on the theory of logarithmic stable maps a
s defined by Abramovich-Chen (and independently by Gross and Si
ebert) which extends usual stable maps to the logarithmic categ
ory (in the sense of Kato and Illusie).
END:VEVENT
BEGIN:VEVENT
UID:news-715@dmi.unibas.ch
DTSTAMP:20190207T232307
DTSTART;TZID=Europe/Zurich:20141212T103000
DTEND;TZID=Europe/Zurich:20141212T120000
SUMMARY:Seminar Algebra and Geometry: Salomon Vishkautsan (ERC postdoc at Scuola Normale Superiore di Pisa)
LOCATION:kleiner Hörsaal
DESCRIPTION:In this talk we present a local-global property in arithmetic d
ynamics called strong residual periodicity, as defined by Bandm
an, Grunewald and Kunyavskii in 2010. We start with a dynamica
l system induced by an endomorphism of a quasi projective varie
ty defined over a number field. This system can be reduced mod
p for “almost all” primes in the ring of integers of t
he number field. We can then ask how the dynamics of
the global system relate to the dynamics of the system reduc
ed mod p for almost all primes p. Strong residual periodicity
occurs when points of small period exist modulo almost every pr
ime,but “cannot be explained” by the dynamics of the global
system. The aim of this talk is to present many motivating ex
amples and raise some interesting questions to encourage furthe
r research on this topic.
END:VEVENT
BEGIN:VEVENT
UID:news-714@dmi.unibas.ch
DTSTAMP:20190207T231842
DTSTART;TZID=Europe/Zurich:20141205T103000
DTEND;TZID=Europe/Zurich:20141205T120000
SUMMARY:Seminar Algebra and Geometry: Eric Edo (Université de la Nouvelle Calédonie)
LOCATION:kleiner Hörsaal
DESCRIPTION:Joint work with S. Kuroda\r\n\r\nGiven a domain R of characteri
stic p >0, there exist two subgroups of the group GAn(R) of pol
ynomial automorphisms which are natural generalisations of the
linear group GLn(R).\r\n\r\n1) The subgroup of additive automor
phisms, i.e. automorphisms with n components satisfying f(x+y)
=f(x)+f(y) where x and y are set of n variables.\r\n\r\n2) The
subgroup of automorphisms with a Jacobian matrix in GLn(R).\r\
n\r\nThe subgroup generated by the translations and automorphis
ms of the type 1 (resp. 2) is called geometrically affine (res
p. differentially affine). This group, together with the tria
ngular automorphisms, generates the subgroup of geometrically t
ame (resp. differentially tame) automorphisms. We study these
groups in dimension 2. We prove that they are different and e
ndowed with a nice structure of amalgamated product.
END:VEVENT
BEGIN:VEVENT
UID:news-713@dmi.unibas.ch
DTSTAMP:20190207T231253
DTSTART;TZID=Europe/Zurich:20141128T103000
DTEND;TZID=Europe/Zurich:20141128T120000
SUMMARY:Seminar Algebra and Geometry: Donna Testerman (EPFL, Lausanne)
LOCATION:kleiner Hörsaal
DESCRIPTION:In joint work with Ross Lawther, and building on work of Proud
and Seitz, we considered properties of CG(CG(u)) =Z(CG(u)), whe
re G is a simple algebraic group defined over an algebraically
closed field of good positive characteristic or of characterist
ic 0, and u is a unipotent element. In particular, we determin
ed the dimension of the double centralizer.In subsequent work,
Simion considered the case of bad characteristic, and determine
d the dimension for G of exceptional type.We discuss the above
work, describe the results and in particular, indicate the diff
erences between good and bad characteristic. Furthermore, in t
he case where the characteristic is good or 0 and u is an even
element (that is, the associated cocharacter has even weights o
n Lie(G)), there is a particularly nice formula for this dimens
ion. We indicate a recent case-free proof of one of the
inequalities implied by this formula, obtained by myself
in characteristic 0 and in collaboration with McNinch in posi
tive characteristic.
END:VEVENT
BEGIN:VEVENT
UID:news-712@dmi.unibas.ch
DTSTAMP:20190207T230735
DTSTART;TZID=Europe/Zurich:20141121T103000
DTEND;TZID=Europe/Zurich:20141121T120000
SUMMARY:Seminar Algebra and Geometry: Alberto Calabri (Università di Ferrara)
LOCATION:kleiner Hörsaal
DESCRIPTION:A plane curve C is said to be Cremona contractible if there exi
sts a plane Cremona transformation that contracts C to a point
. A characterization of irreducible plane curves which are Cre
mona contractible is a classical result due to Castelnuovo and
Enriques in 1900 and that has been improved by Kumar and Murt
hy in 1982. The case of two irreducible components is due to I
itaka in 1988. After reviewing these results, we will discuss
the open problem of characterizing reducible (reduced) plane c
urves which are Cremona contractible. In particular we will de
al with reduced unions of lines and we will give a classificat
ion in case the lines are at least 12.This is a joint work in p
rogress with Ciro Ciliberto (Univ. Roma "Tor Vergata").
END:VEVENT
BEGIN:VEVENT
UID:news-711@dmi.unibas.ch
DTSTAMP:20190207T230340
DTSTART;TZID=Europe/Zurich:20141114T103000
DTEND;TZID=Europe/Zurich:20141114T120000
SUMMARY:Seminar Algebra and Geometry: Immanuel Stampfli (Jacobs University, Bremen)
LOCATION:kleiner Hörsaal
DESCRIPTION:This is joint work with Peter Feller (University of Bern). Let
X be a smooth affine algebraic variety. A natural question
is, whether two algebraic embeddings f, g : X → Cm are
algebraically equivalent, i.e. whether there exists an algebra
ic automorphism φ of Cm such that φ◦f=g. Kaliman, Nori
and Srinivas gave an affirmative answer, provided that 2
dimX+ 2≤m. In this talk we discuss the following one-dimens
ional improvement under a relaxed equivalence condition.\r\n\r\
nTheorem. If f, g : X → Cm are algebraic embeddings and 2 dim
X+ 1≤m, then there exists a holomorphic automorphism φ of Cm
such that φ◦f=g.\r\n\r\nIn fact, the proof is based o
n an idea of Kaliman, with which he proved that two a
lgebraic embeddings of C into C3 are holomorphically equivalent
. In the course of this talk, we discuss this idea. Moreover
, we provide examples of algebraic embeddings into Cm that are
holomorphically non-equivalent.
END:VEVENT
BEGIN:VEVENT
UID:news-710@dmi.unibas.ch
DTSTAMP:20190207T225437
DTSTART;TZID=Europe/Zurich:20141107T103000
DTEND;TZID=Europe/Zurich:20141107T120000
SUMMARY:Seminar Algebra and Geometry: Jung Kyu Canci (Universität Basel)
LOCATION:kleiner Hörsaal
DESCRIPTION:Let K be a number field and v a non archimedean valuatio
n on K. We say that an endomorphism Φ : P1 → P1, def
ined over K, has good reduction at v if there exists a
model Ψ for Φ such that deg Ψv, the degree of the reduction
of Ψ modulo v, equals deg Ψ. We will present a criteria for t
he good reduction that is a generalization of a similar result
due to Zannier where he considered some particular Belyi coveri
ngs (i.e. unramified except above{0,1,∞}). Our result appli
es to any rational maps and is in connection with other two not
ions of good reduction, the simple and the critically good redu
ction.
END:VEVENT
BEGIN:VEVENT
UID:news-709@dmi.unibas.ch
DTSTAMP:20190207T224849
DTSTART;TZID=Europe/Zurich:20141031T103000
DTEND;TZID=Europe/Zurich:20141031T120000
SUMMARY:Seminar Algebra and Geometry: Alexandre Ramos Peon (Universität Bern)
LOCATION:kleiner Hörsaal
DESCRIPTION:As has been discussed recently in this seminar, manifolds with
the "density property" have a particularly rich group of holom
orphic automorphisms. In particular this group acts infinite-t
ransitively on the manifold. This can be generalized as follow
s: if instead of this static state, we allow the configuration
of finitely many points to vary in a holomorphic fashion, the
n there exists a "holomorphically depending" family of automor
phisms, mapping, for each parameter, the corresponding configu
ration to a pre-defined tuple of points on the manifold. This
is not trivial even in the affine case. The aim of the talk is
to introduce the notions and tools required, and of course t
o sketch a proof of a precise version of this theorem.
END:VEVENT
BEGIN:VEVENT
UID:news-708@dmi.unibas.ch
DTSTAMP:20190207T224457
DTSTART;TZID=Europe/Zurich:20141024T103000
DTEND;TZID=Europe/Zurich:20141024T120000
SUMMARY:Seminar Algebra and Geometry: Jérémy Blanc (Universität Basel)
LOCATION:kleiner Hörsaal
DESCRIPTION:Over an algebraically closed field, the conjugacy classes of t
he groups GLn and SLn are well-known. In particular, an elemen
t is diagonalisable if and only if its conjugacy class is clos
ed. The same result holds for polynomial automorphisms
of the complex plane (Furter-Maubach). In this talk, I
will present the case of the subgroup of polynomial a
utomorphisms of the affine space of Jacobian 1 (analogue of SLn
), which behaves very differently, especially in dimension 2.
END:VEVENT
BEGIN:VEVENT
UID:news-707@dmi.unibas.ch
DTSTAMP:20190207T223920
DTSTART;TZID=Europe/Zurich:20141010T103000
DTEND;TZID=Europe/Zurich:20141010T120000
SUMMARY:Seminar Algebra and Geometry: Matthias Leuenberger (Universität Bern)
LOCATION:kleiner Hörsaal
DESCRIPTION:The hypersurface X given by x2y+x+z2+t3 = 0 is called Koras-Ru
ssell cubic threefold and it played an important role in affine
algebraic geometry. It is an example of a threefold that is
diffeomorphic to R6 but not isomorphic to C3 as an algebraic v
ariety. The latter statement follows from the fact that the Ma
kar-Limanov invariant is non-trivial, which means that there is
a lack of algebraic automorphisms compared with C3. However,
in the holomorphic setting the situation is completely differe
nt: We will see that there are significantly more holomorphic
autmorphisms on the Koras-Russel cubic X than algebraic ones.
In fact the holomorphic automorphisms act transitively on the c
ubic. Actually X even has the density property, which means tha
t the group of holomorphic automorphisms is in some sense huge.
The question if X is biholomorphic to C3 is still open.
END:VEVENT
BEGIN:VEVENT
UID:news-706@dmi.unibas.ch
DTSTAMP:20190207T223523
DTSTART;TZID=Europe/Zurich:20141003T103000
DTEND;TZID=Europe/Zurich:20141003T120000
SUMMARY:Seminar Algebra and Geometry: Frank Kutzschebauch (Universität Bern)
LOCATION:kleiner Hörsaal
DESCRIPTION:Compared to the real differentiable case complex manifolds in g
eneral are more rigid, their groups of holomorphic diffeomorphi
sms are rather small (in general trivial). A long known except
ion to this behavior is affine n-space Cn for n at least 2, its
group of holomorphic diffeomorphisms is infinite dimensional.
In the late 1980’s Andersen - Lempert proved are markable th
eorem which stated in its generalized version due to Forstneric
and Rosay that any local holomorphic phase flow given on a Run
ge subset ofCncan be locally uniformly approximated by a global
holomorphic diffeomorphism. The main ingredient in the proof
was formalized by Varolin to be called the density property: T
he Lie algebra generated by complete holomorphic vector fields
is dense in the Lie algebra of all holomorphic vector fields.
In these manifolds a similar local to global approxima
tion of Andersen-Lempert type holds, It is a precise way of s
aying that the group of holomorphic diffeomorphisms is large.
In the talk we will explain how this notion is related to other
more recent flexibility notions in Complex Geometry, in partic
ular to the notion of Oka-Forstneric manifold. We will give ex
amples of manifolds with the density property and sketch applic
ations of the density property. If time permits we will explai
n criteria for the density property developed by Kaliman and th
e speaker or sketch some future plans.
END:VEVENT
BEGIN:VEVENT
UID:news-705@dmi.unibas.ch
DTSTAMP:20190207T212326
DTSTART;TZID=Europe/Zurich:20140926T103000
DTEND;TZID=Europe/Zurich:20140926T120000
SUMMARY:Seminar Algebra and Geometry: Adrien Dubouloz (Université de Bourgogne)
LOCATION:kleiner Hörsaal
DESCRIPTION:A much less studied version of Zariski Cancellation Problem ask
s whether two algebraic varieties which become isomorphic aft
er taking their products with an affine algebraic torus are i
somorphic themselves. It is easy to see that the answer is p
ositive when the two varieties are smooth algebraic curves. Fo
r higher dimensional varieties, the problem is more subtle, i
ntimately related to the geometry of families of affine ratio
nal curves on them. The only available general result so far
is that "cancellation" holds provided that one the two varieti
es is not dominantly covered by images of the punctured affin
e line, a property satisfied for instance by varieties of log
-general type. After a brief survey of various aspects of the
problem, I will present some strategies to construct smooth
affine factorial counter-examples to cancellation in any dime
nsion bigger or equal to two.
END:VEVENT
BEGIN:VEVENT
UID:news-704@dmi.unibas.ch
DTSTAMP:20190207T211858
DTSTART;TZID=Europe/Zurich:20140919T103000
DTEND;TZID=Europe/Zurich:20140919T120000
SUMMARY:Seminar Algebra and Geometry: Ronan Terpereau (Universität Mainz)
LOCATION:kleiner Hörsaal
DESCRIPTION:This talk focuses on a work initiated by Tanja Becker in her Ph
D thesis three years ago and completed recently by myself.\r\n\
r\nGiven a reductive group G acting on an affine scheme X, a H
ilbert function h, and a stability condition θ, we explain ho
w to construct the moduli space M of θ-stable (G,h)-constella
tions on X, which is a common generalization of the invariant
Hilbert scheme after Alexeev and Brion and of the moduli space
of θ-stable G-constellations for finite groups introduced by
Craw and Ishii. The main tools for this construction are the
geometric invariant theory and the invariant Quot schemes. Mor
eover, the moduli space M is naturally equipped with a morphis
m μ: M → X//G which turns to be a “nice” desingularizat
ion of the quotient X//G in many situations.
END:VEVENT
BEGIN:VEVENT
UID:news-717@dmi.unibas.ch
DTSTAMP:20190211T181031
DTSTART;TZID=Europe/Zurich:20140523T103000
DTEND;TZID=Europe/Zurich:20140523T120000
SUMMARY:Seminar Algebra and Geometry: Maria Fernanda Robayo (Universität Basel)
LOCATION:
DESCRIPTION:A birational diffeomorphism of a real projective algebraic vari
ety X is a birational map that restricts to a diffeomorphism on
the real points of X. In my talk, X will be the sphere, that i
s, the projective algebraic surface defined by x2+y2+z2=w2 in P
3R endowed with the complex conjugation, which gives it its re
al structure. The systematic investigation of real algebraic s
urfaces has already begun with Comessatti in 1912. On the othe
r hand, their automorphisms, the birational diffeomorphisms, h
ave not been classified up to now. I will give an introduction
into the subject and present the complete classification of b
irational diffeomorphisms of the sphere of prime order.
END:VEVENT
BEGIN:VEVENT
UID:news-718@dmi.unibas.ch
DTSTAMP:20190211T181323
DTSTART;TZID=Europe/Zurich:20140520T110000
DTEND;TZID=Europe/Zurich:20140520T120000
SUMMARY:Seminar Algebra and Geometry: Yuri Prokhorov (Steklov Mathematical Institute)
LOCATION:
DESCRIPTION:A group B is said to be Jordan if there is a constant J=J(B) su
ch that if for any finite subgroup G ⊂ B there exists a norma
l abelian subgroup A ⊂ G of index at most J. We discuss the f
ollowing natural problem: describe algebraic varieties for whic
h the group of birational self-maps is Jordan.
END:VEVENT
BEGIN:VEVENT
UID:news-719@dmi.unibas.ch
DTSTAMP:20190211T181650
DTSTART;TZID=Europe/Zurich:20140509T103000
DTEND;TZID=Europe/Zurich:20140509T120000
SUMMARY:Seminar Algebra and Geometry: Kevin Langlois (Institut Fourier, Grenoble)
LOCATION:
DESCRIPTION:This talk is devoted to the classification of normalized additi
ve group actions on affine spherical varieties. After recallin
g some basic notions concerning the Luna-Vust theory, we will
present a description of these actions which generalizes the c
lassical toric case. Joint work with Alexander Perepechko.
END:VEVENT
BEGIN:VEVENT
UID:news-720@dmi.unibas.ch
DTSTAMP:20190211T182549
DTSTART;TZID=Europe/Zurich:20140502T103000
DTEND;TZID=Europe/Zurich:20140502T120000
SUMMARY:Seminar Algebra and Geometry: Vincent Emery (EPFL)
LOCATION:
DESCRIPTION:I will recall the classical constructions for hyperbolic manifo
lds of finite volume, and in particular the method based on ar
ithmetic groups. I will then discuss a conjecture that states
that except in dimension n=3, the complete hyperbolic n-manifo
ld of the smallest volume is noncompact (joint work with Misha
Belolipetsky).
END:VEVENT
BEGIN:VEVENT
UID:news-721@dmi.unibas.ch
DTSTAMP:20190211T182835
DTSTART;TZID=Europe/Zurich:20140425T103000
DTEND;TZID=Europe/Zurich:20140425T120000
SUMMARY:Seminar Algebra and Geometry: Ivan Bazhov (Univesité de Genève)
LOCATION:
DESCRIPTION:Let X be an affine toric variety and Aut(X) be its automorphism
group. In the first part of the talk we give a combinatorial d
escription of Aut(X)-orbits on X in terms of the divisor class
group Cl(X). In the second part we introduce the total coordina
tes on X. The total coordinate ring provides a canonical presen
tation $\overline{X}$ → X of X as a quotient of a vector sp
ace $\overline{X}$ by a linear action of a quasitorus. We show
that the orbits of the connected component of the group Aut(X
) on X coincide with the Luna strata defined by the canonical q
uotient presentation.
END:VEVENT
BEGIN:VEVENT
UID:news-722@dmi.unibas.ch
DTSTAMP:20190211T183121
DTSTART;TZID=Europe/Zurich:20140411T103000
DTEND;TZID=Europe/Zurich:20140411T120000
SUMMARY:Seminar Algebra and Geometry: Andriy Regeta (Universität Basel)
LOCATION:
DESCRIPTION:We show that every Lie algebra automorphisms of the vector fiel
ds VF(An) of affine n-space An, of the vector fields VFc(An) wi
th constant divergence, and of the vector fields VF0(An) with d
ivergence zero is induced by an automorphism of An. As an immed
iate consequence, we get the following result due to Kulikov. I
f every injective endomorphism of the Lie algebra VF(An) is an
automorphism, then the Jacobian Conjecture holds in dimension n
.
END:VEVENT
BEGIN:VEVENT
UID:news-724@dmi.unibas.ch
DTSTAMP:20190211T185003
DTSTART;TZID=Europe/Zurich:20140321T103000
DTEND;TZID=Europe/Zurich:20140321T120000
SUMMARY:Seminar Algebra and Geometry: Jung Kyu Canci (Universität Basel)
LOCATION:
DESCRIPTION:I will present a joint work with Laura Paladino. \r\n\r\nWe c
onsider dynamical systems on the projective line given by rat
ional functions. Let $\phi$ be an endomorphism of the project
ive line defined over a global field K. We prove a bound for
the cardinality of the set of K-rational preperiodic points f
or $\phi$ in terms of the number of places of bad reduction.
The result is completely new in the function fields case and
it is an improvement of the number fields case. An important
tool is an S-unit equation theorem in 2 variables.
END:VEVENT
BEGIN:VEVENT
UID:news-725@dmi.unibas.ch
DTSTAMP:20190211T184751
DTSTART;TZID=Europe/Zurich:20140307T103000
DTEND;TZID=Europe/Zurich:20140307T120000
SUMMARY:Seminar Algebra and Geometry: Hanspeter Kraft (Universität Basel)
LOCATION:
DESCRIPTION:Closed ind-subgroups with the same Lie algebras
END:VEVENT
BEGIN:VEVENT
UID:news-726@dmi.unibas.ch
DTSTAMP:20190211T184914
DTSTART;TZID=Europe/Zurich:20140228T103000
DTEND;TZID=Europe/Zurich:20140228T120000
SUMMARY:Seminar Algebra and Geometry: Pierre-Marie Poloni (Universität Basel)
LOCATION:
DESCRIPTION:The tame automorphism group of affine 3-space is not closed
END:VEVENT
BEGIN:VEVENT
UID:news-727@dmi.unibas.ch
DTSTAMP:20190211T201430
DTSTART;TZID=Europe/Zurich:20131220T103000
DTEND;TZID=Europe/Zurich:20131220T120000
SUMMARY:Seminar Algebra and Geometry: Rafael Tiedra (Pontificia Universidad Católica de Chile)
LOCATION:
DESCRIPTION:
END:VEVENT
BEGIN:VEVENT
UID:news-728@dmi.unibas.ch
DTSTAMP:20190211T201639
DTSTART;TZID=Europe/Zurich:20131122T103000
DTEND;TZID=Europe/Zurich:20131122T120000
SUMMARY:Seminar Algebra and Geometry: Andriy Regeta (Universität Basel)
LOCATION:
DESCRIPTION:We study Lie subalgebras L of the vector fields Vecc(A2) of aff
ine 2-space A2 of constant divergence, and we classify those L
which are isomorphic to the Lie algebra aff2 of the group Aff2(
K) of affine transformations of A2. We then show that the follo
wing statements are equivalent:(a) The Jacobian Conjecture hold
s in dimension 2;(b) All Lie subalgebras L ⊂ Vecc(A2) isomorp
hic to aff2 are conjugate under Aut(A2);(c) All Lie subalgebras
L ⊂ Vecc(A2) isomorphic to aff2 are algebraic.
END:VEVENT
BEGIN:VEVENT
UID:news-729@dmi.unibas.ch
DTSTAMP:20190211T202615
DTSTART;TZID=Europe/Zurich:20131118T141500
DTEND;TZID=Europe/Zurich:20131118T154500
SUMMARY:Seminar Algebra and Geometry: Gerry Schwarz (Brandeis University)
LOCATION:
DESCRIPTION:Let $\mathfrak g$ be a simple complex Lie algebra and let G be
the corresponding adjoint group. Consider the G-module V which
is the direct sum of r copies of $\mathfrak g$. We say that
V is \emph{large\/} if r≥2 and r≥3 if G has rank 1. We s
howed that when V is large any algebraic automorphism ψ of th
e quotient Z:=V//G lifts to an algebraic mapping Ψ:V→V w
hich sends the fiber over z to the fiber over ψ(z), z ∈ Z.
(Most cases were already handled by work of Kuttler)We also sho
wed that one can choose a biholomorphic lift Ψ such that Ψ(g
v)=σ(g)Ψ(v), g ∈ G, v ∈ V, where σ is an automorphism o
f G. This leaves open the following questions: Can one lift
holomorphic automorphisms of Z? Which automorphisms lift if V
is not large? We answer the first question in the affirmati
ve and also answer the second question.
END:VEVENT
BEGIN:VEVENT
UID:news-730@dmi.unibas.ch
DTSTAMP:20190211T202907
DTSTART;TZID=Europe/Zurich:20131115T103000
DTEND;TZID=Europe/Zurich:20131115T120000
SUMMARY:Seminar Algebra and Geometry: Jan Draisma (Universiteit Eindhoven)
LOCATION:
DESCRIPTION:I will introduce the notion of Plücker variety, which is a rul
e that assigns to a pair of a natural number p and a finite-di
mensional vector space V a closed subvariety of the p-th exter
ior power of V. To be a Plücker variety, the rule should sati
sfy two axioms, one of which is functoriality in V. The Grass
mannian is the smallest nonempty Plücker variety, and the cl
ass of Plücker varieties is closed under joins, tangential va
rieties, unions, and intersections. I will sketch a proof tha
t any "bounded" Plücker variety is defined set-theoretical
ly by equations of bounded degree, and in fact by finitely ma
ny equations up to symmetry. So far, this statement was unkno
wn even for the first secant variety of the Grassmannian. The
talk is based on joint work with Rob Eggermont, and inspired
by Snowden's work on Delta-varieties.
END:VEVENT
BEGIN:VEVENT
UID:news-731@dmi.unibas.ch
DTSTAMP:20190211T203228
DTSTART;TZID=Europe/Zurich:20131108T103000
DTEND;TZID=Europe/Zurich:20131108T120000
SUMMARY:Seminar Algebra and Geometry: Matthias Leuenberger (Universität Bern)
LOCATION:
DESCRIPTION:For Cn the Lie algebra generated by locally nilpotent derivati
ons (LNDs) is equal to the Lie algebra of all divergence free
algebraic vector fields. It is not known which other smooth a
ffine varieties have this property. We will see an answer to
this question for smooth surfaces given by xy=p(z): It turns
out that here the Lie algebra generated by LNDs is a subalgeb
ra of codimension deg(p)-2 inside the divergence free vector
fields.
END:VEVENT
BEGIN:VEVENT
UID:news-732@dmi.unibas.ch
DTSTAMP:20190211T203628
DTSTART;TZID=Europe/Zurich:20131101T103000
DTEND;TZID=Europe/Zurich:20131101T120000
SUMMARY:Seminar Algebra and Geometry: Alberto Calabri (Università di Ferrara)
LOCATION:
DESCRIPTION:Fixed a positive integer d, plane Cremona transformations of de
gree d form a quasi-projective variety. Its decomposition in i
rreducible components, their dimension and the properties of t
he general element of each irreducible component will be prese
nted. Its connectedness will be discussed for small values of d
. This is joint work with Cinzia Bisi and Massimiliano Mella.
END:VEVENT
BEGIN:VEVENT
UID:news-733@dmi.unibas.ch
DTSTAMP:20190211T203934
DTSTART;TZID=Europe/Zurich:20131025T103000
DTEND;TZID=Europe/Zurich:20131025T120000
SUMMARY:Seminar Algebra and Geometry: Roland Lötscher (Universität München)
LOCATION:
DESCRIPTION:Essential dimension of fibered categories
END:VEVENT
BEGIN:VEVENT
UID:news-734@dmi.unibas.ch
DTSTAMP:20190211T204230
DTSTART;TZID=Europe/Zurich:20131018T103000
DTEND;TZID=Europe/Zurich:20131018T120000
SUMMARY:Seminar Algebra and Geometry: Pascal Rolli (ETH Zürich)
LOCATION:
DESCRIPTION:A quasimorphism (QM) is a real-valued function on a group that
almost behaves like a homomorphism. Non-trivial QMs exist when
ever the group has some features of negative curvature, for ex
ample when it is Gromov hyperbolic. I will discuss old and new
constructions of QMs, using combinatorial, geometric and alge
braic ideas. In a second part I will talk about the relation t
o cohomology. To each QM there is an associated class in the g
roup's second bounded cohomology $H^2_b$. One of our QM constr
uctions yields linear isometric embeddings so called defect spa
ces into $H^2_b$. These spaces have an interesting geometry, t
hey are l∞ spaces equipped with an exotic norm.
END:VEVENT
BEGIN:VEVENT
UID:news-735@dmi.unibas.ch
DTSTAMP:20190211T204531
DTSTART;TZID=Europe/Zurich:20131011T103000
DTEND;TZID=Europe/Zurich:20131011T120000
SUMMARY:Seminar Algebra and Geometry: Isac Hedén (Universität Basel)
LOCATION:
DESCRIPTION:We study complex affine Ga-threefolds X with the affine plane A
2 as their algebraic quotient, and which are a principal bundle
over the punctured affine plane A2*. Changing the point of vie
w, we look for affine extensions of Ga-principal bundles over A
2* that are obtained by adding an extra fiber to the bundle pro
jection over the origin. It turns out that SL2 plays a special
role, since every Ga-principal bundle can be obtained as a pull
back of SL2. We construct two series of such extensions which g
eneralize the basic extension obtained by adding a zero section
to SL2 viewed as a Gm-principal bundle over P1 x P1 \ Δ.
END:VEVENT
BEGIN:VEVENT
UID:news-736@dmi.unibas.ch
DTSTAMP:20190211T211438
DTSTART;TZID=Europe/Zurich:20130531T103000
DTEND;TZID=Europe/Zurich:20130531T120000
SUMMARY:Seminar Algebra and Geometry: Adrien Dubouloz (Université de Bourgogne)
LOCATION:
DESCRIPTION:A result of Makar-Limanov asserts that if X an affine variety w
ithout nontrivial action of the additive group, then there ar
e no additive group actions on the cylinder X x A1 besides th
e obvious ones by generic translations on the second factor.
In this talk, I will give examples which illustrate how badly
this property can fail for higher dimensional cylinders. The
recipe will involve, among other ingredients, rigid affine c
ubic surfaces, some Ga-linearized line bundles and a structur
e theorem for vector bundles over rational affine surfaces, a
ll mixed together through a variant of the overused Danielews
ki fiber product trick.
END:VEVENT
BEGIN:VEVENT
UID:news-737@dmi.unibas.ch
DTSTAMP:20190211T211745
DTSTART;TZID=Europe/Zurich:20130524T103000
DTEND;TZID=Europe/Zurich:20130524T120000
SUMMARY:Seminar Algebra and Geometry: Andrea Marinatto (Università di Udine)
LOCATION:
DESCRIPTION:Let K be a perfect field of characteristic not equal to two, $\
bar{K}$ an algebraic closure of K and let GK be the Galois grou
p of the extension $\bar{K}/K$. Let T be a n-point set in $P1
(\bar{K})$. The field of moduli of T is contained in each fiel
d of definition but it is not necessarily a field of definitio
n. In this seminar we show that point sets of odd cardinality
n≥5 in $P1(\bar{K})$ with field of moduli K are defined ov
er their field of moduli. We, also, show that, except for the
special case of the 4-point sets, this does not hold in general
for point sets of even cardinality n≥6.
END:VEVENT
BEGIN:VEVENT
UID:news-738@dmi.unibas.ch
DTSTAMP:20190211T211954
DTSTART;TZID=Europe/Zurich:20130517T103000
DTEND;TZID=Europe/Zurich:20130517T120000
SUMMARY:Seminar Algebra and Geometry: Susanna Zimmermann (Universität Basel)
LOCATION:
DESCRIPTION:Geometry and Invariants of the Affine Group
END:VEVENT
BEGIN:VEVENT
UID:news-739@dmi.unibas.ch
DTSTAMP:20190211T212734
DTSTART;TZID=Europe/Zurich:20130503T103000
DTEND;TZID=Europe/Zurich:20130503T120000
SUMMARY:Seminar Algebra and Geometry: Sergei Kovalenko (Ruhr-Universität Bochum)
LOCATION:
DESCRIPTION:Gizatullin surfaces are normal affine surfaces completable by a
zigzag, i. e. by a linear chain of smooth rational curves. An
equivalent characterization of such surfaces V, except for th
e surface C∗ × C∗, is that the automorphism group acts wit
h a big orbit O, i. e. V \ O is finite. Considering some exampl
es of Gizatullin surfaces like the affine plane A2 or the Danie
lewski surfaces V = {xy − P(z) = 0} ⊆ A3 it follows that th
e big orbit O coincides with the smooth locus Vreg . Gizatullin
formulated in his pioneer works the following conjecture: \r\n
\r\nConjecture (Gizatullin): The big orbit of a Gizatullin surf
ace V coincides with its smooth locus, i. e. O = Vreg . \r\n\r\
nWe show that the action of the automorphism group of a smooth
Gizatullin surface with a distinguished and rigid extended div
isor is not transitive in general. Thus such surfaces represen
t counterexamples to Gizatullin’s conjecture. For such surfa
ces we give an explicit orbit decomposition of the natural act
ion of the automorphism group. Moreover, the automorphism grou
p of such smooth Gizatullin surfaces can be represented as an
amalgamated product of two automorphism subgroups.
END:VEVENT
BEGIN:VEVENT
UID:news-740@dmi.unibas.ch
DTSTAMP:20190211T213008
DTSTART;TZID=Europe/Zurich:20130426T103000
DTEND;TZID=Europe/Zurich:20130426T120000
SUMMARY:Seminar Algebra and Geometry: Arnaud Beauville (Université de Nice)
LOCATION:
DESCRIPTION:The Lüroth problem asks whether every field K with C ⊂ K ⊂
C(x1,...,xn) is of the form C(y1,...,yp). In geometric terms,
if an algebraic variety can be parametrized by rational func
tions, can one find a one-to-one such parametrization? After
a brief historical survey, I will recall the counter-examples
found in the 70's; then I will describe a quite simple (and ne
w) counter-example, and its application to the study of finit
e simple groups of birational automorphisms of P3.
END:VEVENT
BEGIN:VEVENT
UID:news-741@dmi.unibas.ch
DTSTAMP:20190211T213334
DTSTART;TZID=Europe/Zurich:20130419T103000
DTEND;TZID=Europe/Zurich:20130419T120000
SUMMARY:Seminar Algebra and Geometry: Jan Draisma (Technische Universiteit Eindhoven)
LOCATION:
DESCRIPTION:The maximum-likelihood degree of a smooth, locally closed subva
riety X of a torus (C*)n is the number of critical points on X
of any monomial function (p1,...,pn) --> p1u1... pnunwith
u in Nn sufficiently general. This notion is motivated by stat
istics, where X is a family of probability distributions on {1,
...,n}, u records observed data, and the maximum-likelihood est
imate for p given u is one of the critical points of the functi
on above.In recent work by Hauenstein, Rodriguez, and Sturmfel
s the maximum-likelihood degree of determinantal varieties was
studied. Extensive computations using numerical algebraic geo
metry led to the conjecture that the maximum-likelihood degree
of the variety of rank-r matrices whose entries add up to 1 e
quals that of the variety of corank-(r-1) matrices whose entri
es add up to 1. I will present a proof of that conjecture, and
variations of it for symmetric and skew-symmetric matrices.
Joint work with Jose Rodriguez.
END:VEVENT
BEGIN:VEVENT
UID:news-742@dmi.unibas.ch
DTSTAMP:20190211T221148
DTSTART;TZID=Europe/Zurich:20130412T103000
DTEND;TZID=Europe/Zurich:20130412T120000
SUMMARY:Seminar Algebra and Geometry: Antoine Ducros (Université Paris 6)
LOCATION:
DESCRIPTION:If one tries to mimic naively in the p-adic setting what is u
sually done in complex analytic geometry, one immediately faces
big problems, because p-adic fields are totally disconnected.
Hence in order to develop a relevant p-adic geometry, a more su
btle approach is needed. In this talk, I will begin with a pr
esentation of that of Berkovich. It roughly consists of 'addi
ng plenty of points' to the usual p-adic spaces so that they g
et good topological properties (like local path-connectedness
). I will describe some Berkovich spaces associated with simp
le varieties (the projective line, the algebraic curves....).
\r\nThen I will try to illustrate the following slogan: 'to see
the good analog of a complex object in the p-adic world, one
often has to work with Berkovich spaces', through three examp
les: spectral theory; dynamical systems; and the theory of rea
l (p,q)-forms and related notions (integrals, boundary integr
als, curvature forms of metrized line bundles) that as been r
ecently developped in the framework of Berkovich spaces by Ch
ambert-Loir and myself, and that I will try to describe in som
e detail.
END:VEVENT
BEGIN:VEVENT
UID:news-743@dmi.unibas.ch
DTSTAMP:20190211T221406
DTSTART;TZID=Europe/Zurich:20130405T103000
DTEND;TZID=Europe/Zurich:20130405T120000
SUMMARY:Seminar Algebra and Geometry: Alexander Perepechko (Moscow State University and Institut Fourier, Grenoble)
LOCATION:
DESCRIPTION:Infinite transitivity on universal torsors
END:VEVENT
BEGIN:VEVENT
UID:news-744@dmi.unibas.ch
DTSTAMP:20190211T221955
DTSTART;TZID=Europe/Zurich:20130315T103000
DTEND;TZID=Europe/Zurich:20130315T120000
SUMMARY:Seminar Algebra and Geometry: Christian Urech (Universität Basel)
LOCATION:
DESCRIPTION:The affine Cremona group $\mathcal{G}_n$ is the group of polyno
mial automorphisms of An. Hanspeter Kraft and Immanuel Stampf
li showed that every automorphism of $\mathcal{G}_n$ as a gro
up is inner up to field automorphism if restricted to the su
bgroup of tame automorphisms. First I will sketch a proof of t
his theorem. \r\n\r\n In a next step we only consider those au
tomorphisms of $\mathcal{G}_n$ that also respect its addition
al algebraic structure as an ind-group. It turns out that the
se are exactly the inner automorphisms of $\mathcal{G}_n$. To
prove this I will follow an idea recently presented by Belov
-Kanel and Yu, which uses tame approximation. \r\n\r\nThese a
re results from my Master's thesis under the supervision of Han
speter Kraft.
END:VEVENT
BEGIN:VEVENT
UID:news-745@dmi.unibas.ch
DTSTAMP:20190211T223128
DTSTART;TZID=Europe/Zurich:20130308T103000
DTEND;TZID=Europe/Zurich:20130308T120000
SUMMARY:Seminar Algebra and Geometry: Vladimir Bavula (University of Sheffield)
LOCATION:
DESCRIPTION:An analogue of the Conjecture of Dixmier is true for the algebr
a of polynomial integro-differential operators \r\n\r\nIn 1968,
Dixmier posed six problems for the algebra of polynomial diff
erential operators, i.e. the Weyl algebra. In 1975, Joseph solv
ed the third and sixth problems and, in 2005, I solved the fif
th problem and gave a positive solution to the fourth problem
but only for homogeneous differential operators. The remaining
three problems are still open. The first problem/conjecture o
f Dixmier (which is equivalent to the Jacobian Conjecture as w
as shown in 2005-07 by Tsuchimito, Belov and Kontsevich) claim
s that the Weyl algebra 'behaves' as a finite field extensio
n. In more detail, the first problem/conjecture of Dixmier ask
s: is it true that an algebra endomorphism of the Weyl algebra
is an automorphism? In 2010, I proved that this question has a
n affirmative answer for the algebra of polynomial integro-dif
ferential operators. In my talk, I will explain the main ide
as, the structure of the proof and recent progress on the firs
t problem/conjecture of Dixmier. \r\n\r\nThe group of automorph
isms of the algebra of one-sided inverses of a polynomial alg
ebra \r\n\r\nThe algebra Sn of one-sided inverses of a polynomi
al algebra Pn in n variables is obtained from Pn by adding com
muting, left (but not two-sided) inverses of its canonical gen
erators. Ignoring non-Noetherian property, the algebra Sn belon
gs to a family of algebras like the n'th Weyl algebra and the
polynomial algebra in 2n variables (e.g., the Gelfand-Kirillov
dimension of Sn is 2n).The group of automorphisms Gn of Snis h
uge. Recently, I found the group Gn and its explicit generator
s. In my talk, I will explain the ideas behind the proofs an
d I will do an overview of the basic properties of these algeb
ras and their connections with polynomials, differential and i
ntegro-differential operators.
END:VEVENT
BEGIN:VEVENT
UID:news-746@dmi.unibas.ch
DTSTAMP:20190211T223007
DTSTART;TZID=Europe/Zurich:20130301T103000
DTEND;TZID=Europe/Zurich:20130301T120000
SUMMARY:Seminar Algebra and Geometry: Jérémy Blanc (Universität Basel)
LOCATION:
DESCRIPTION:A classical theorem says that del Pezzo surfaces are, with the
exception of the smooth quadric, blow-ups of the plane into a
t most 8 points, such that no three are collinear, no 6 on th
e same conic and no 8 on the same cubic, singular at one of
the 8 points. Moreover, a general set of at most 8 points of t
he plane satisfies these conditions. \r\n\r\nI will describe
the theorem, give the proof, and describe the generalisation
to dimension 3, by considering blow-ups of points and curves
in the projective space. We can get similar descriptions, the
conditions of generality are now in terms of multisecant lin
es, conic or twisted cubics.\r\n\r\nJoint work with Stéphane L
amy.
END:VEVENT
BEGIN:VEVENT
UID:news-747@dmi.unibas.ch
DTSTAMP:20190211T231411
DTSTART;TZID=Europe/Zurich:20121214T103000
DTEND;TZID=Europe/Zurich:20121214T120000
SUMMARY:Seminar Algebra and Geometry: Ronan Terpereau (Université Joseph Fourier, Grenoble)
LOCATION:
DESCRIPTION:Let G be a classical group (SL(V), GL(V), O(V),...) and X be th
e direct sum of p copies of the standard representation of G an
d q copies of its dual representation, where p and q are positi
ve integers. We consider the invariant Hilbert scheme, denoted
H, which parametrizes the G-stable closed subschemes Z of X suc
h that k[Z] is isomorphic to the regular representation of G.
\r\n\r\n In this talk, we will see that H is a smooth v
ariety when the dimension of V is small, but that H is generall
y singular. When H is smooth, the Hilbert-Chow morphism H -> X/
/G is a canonical resolution of the singularities of the catego
rical quotient X//G (=Spec(k[X]G)). Then it is natural to ask w
hat are the good geometric properties of this resolution (for i
nstance if it is crepant). \r\n\r\n To finish, we will men
tion some analogue results in the symplectic setting, that is t
o say by letting p=q and replacing X by the zero fiber of the
moment map. The quotients that we get by doing this are isomor
phic to the closures of nilpotent orbits, and the Hilbert-Chow
morphism is a resolution of their singularities (sometimes a
symplectic one).
END:VEVENT
BEGIN:VEVENT
UID:news-748@dmi.unibas.ch
DTSTAMP:20190211T231746
DTSTART;TZID=Europe/Zurich:20121207T103000
DTEND;TZID=Europe/Zurich:20121207T120000
SUMMARY:Seminar Algebra and Geometry: Matthieu Jacquemet (Université de Fribourg)
LOCATION:
DESCRIPTION:We are interested in estimating the volume of a hyperbolic n-po
lyhedron by knowing its f-vector, and its 'inradius'. In fact
, exact volume formulas are difficult to find for n>2. However,
in dimension 3, there is a volume estimate by Fejes-Toth (fo
r spaces of constant curvature) in terms of the inradius and
the f-vector, showing some deficiencies. A natural question i
n this context (but also in general) is the inradius of a hyper
bolic simplex. I shall start with a (partial) overview of the
context of hyperbolic orbifolds/manifolds and their volume.
In a second part, I shall discuss Fejes-Toth's result and giv
e some new results for compact simplices in any dimensions.
END:VEVENT
BEGIN:VEVENT
UID:news-749@dmi.unibas.ch
DTSTAMP:20190211T232040
DTSTART;TZID=Europe/Zurich:20121123T103000
DTEND;TZID=Europe/Zurich:20121123T120000
SUMMARY:Seminar Algebra and Geometry: Jung Kyu Canci (Universität Basel)
LOCATION:
DESCRIPTION:We will see some results in diophantine geometry and the
ir applications to some problems in arithmetic of dynamica
l systems. These problems are related to the Morton and S
ilverman's Uniform Boundedness Conjecture on preperiodic poi
nts for rational maps.
END:VEVENT
BEGIN:VEVENT
UID:news-750@dmi.unibas.ch
DTSTAMP:20190211T232355
DTSTART;TZID=Europe/Zurich:20121109T103000
DTEND;TZID=Europe/Zurich:20121109T120000
SUMMARY:Seminar Algebra and Geometry: Yves de Cornulier (Laboratoire de Mathématiques d'Orsay)
LOCATION:
DESCRIPTION:I will give "algebraic presentations" of algebraic groups, and
develop, as consequences: Abels' criterion for a p-adic algebr
aic group to be compactly presented, and polynomial estimates
on the Dehn function of real or p-adic algebraic groups.
END:VEVENT
BEGIN:VEVENT
UID:news-751@dmi.unibas.ch
DTSTAMP:20190211T235825
DTSTART;TZID=Europe/Zurich:20121026T103000
DTEND;TZID=Europe/Zurich:20121026T120000
SUMMARY:Seminar Algebra and Geometry: Émilie Dufresne (Universität Basel)
LOCATION:
DESCRIPTION:The study of separating invariants is a new trend in Invariant
Theory and a return to its roots: invariants as a classificati
on tool. Rather than considering the whole ring of invariants,
one considers a separating set, that is, a set of invariants
whose elements separate any two points which can be separated
by invariants. One of the appeals of this new approach is that
separating invariants can have better structural and computat
ional properties than the ring of invariants. For example, the
re always exist a finite separating set, no matter the group.
In this talk, we study well-behaved separating algebras for in
variants of representations of finite groups. In particular, we
show that if there exists a polynomial separating algebra, th
e the group action must be generated by (pseudo-)reflections.
END:VEVENT
BEGIN:VEVENT
UID:news-752@dmi.unibas.ch
DTSTAMP:20190212T000335
DTSTART;TZID=Europe/Zurich:20121012T103000
DTEND;TZID=Europe/Zurich:20121012T120000
SUMMARY:Seminar Algebra and Geometry: Hanspeter Kraft (Universität Basel)
LOCATION:
DESCRIPTION:Let X,Y be two complex varieties and assume that there is an (
abstract) isomorphism of the semigroups End(X) and End(Y). If o
ne of the varieties is affine and contains a copy of the affin
e line, then X and Y are isomorphic, up to a field automorphi
sm. I will explain the proof of this amazing result which is
base on an old theorem due to Dick Palais.
END:VEVENT
BEGIN:VEVENT
UID:news-753@dmi.unibas.ch
DTSTAMP:20190212T001023
DTSTART;TZID=Europe/Zurich:20121005T103000
DTEND;TZID=Europe/Zurich:20121005T120000
SUMMARY:Seminar Algebra and Geometry: Matteo Ruggiero (École polytechnique)
LOCATION:
DESCRIPTION:In the last years valuative theory has been used (see the work
s of Favre, Jonsson, Boucksom, and others) to study several
dynamical properties in both local and global settings. I sh
all present a joint work with William Gignac, on the growth of
attracting rates for iterates of a superattracting germ in C
2. If time allows, I will present other applications and exampl
es.
END:VEVENT
BEGIN:VEVENT
UID:news-754@dmi.unibas.ch
DTSTAMP:20190212T001338
DTSTART;TZID=Europe/Zurich:20120601T103000
DTEND;TZID=Europe/Zurich:20120601T120000
SUMMARY:Seminar Algebra and Geometry: Danny de Jesus Gomez (Universität Osnabrück)
LOCATION:
DESCRIPTION:Let R be a commutative ring with unity; f,f1,...,fn elements o
f R (data) and I=(f1,...,fn), the corresponding ideal of R. T
he Forcing Algebra A:=R[T1,...,Tn]/(f1T1+...+fnTn-f) for the d
ata f,f1,...,fn is the most "natural" R-algebra such that f be
longs to the expansion of I in A. On this talk we present crit
eria and properties over the base ring R and the data in order
to guarantee the connectedness of the corresponding forcing s
cheme Spec A. In particular, we present an arithmetical Crit
eria for Principal Ideals Domains. Besides, we rewrite the con
dition of belonging to the integral closure of I by means of
the universal connectedness of the canonical morphism between
the forcing scheme and the base space Spec R. Finally, we study
the local nature (over the base) of the connectedness in a ve
ry general setting.
END:VEVENT
BEGIN:VEVENT
UID:news-755@dmi.unibas.ch
DTSTAMP:20190212T002538
DTSTART;TZID=Europe/Zurich:20120525T103000
DTEND;TZID=Europe/Zurich:20120525T120000
SUMMARY:Seminar Algebra and Geometry: Milena Hering (University of Connecticut)
LOCATION:
DESCRIPTION:The action of the two-dimensional torus on the plane extends t
o an action of the torus on the Hilbert scheme of points in the
plane. The T-graph is the graph whose vertices are the fixed
points under the torus action (corresponding to monomial idea
ls), where two such vertices are connected by an edge if there
exists a one-dimensional torus orbit whose closure contains t
he corresponding fixed points. I will describe some conditions
for the existence of an edge between two given vertices. \r\n
\r\nThis is joint work with Diane Maclaga.
END:VEVENT
BEGIN:VEVENT
UID:news-756@dmi.unibas.ch
DTSTAMP:20190212T002832
DTSTART;TZID=Europe/Zurich:20120518T103000
DTEND;TZID=Europe/Zurich:20120518T120000
SUMMARY:Seminar Algebra and Geometry: Vladimir Tsygankov, Gutenberg Universität, Mainz)
LOCATION:
DESCRIPTION:I will survey briefly known results about the finite subgroup
s in the plane Cremona group over field of complex numbers. Th
e emphasis will be done on the so called ''geometrical method'
'. By the method the study of the finite subgroups is reduced
to study of so called the ''minimal'' finite subgroups of au
tomorphism groups of nonsingular del Pezzo surfaces and conic b
undles. \r\n\r\nThe description by equations of nonsingular De
l Pezzo surfaces in weighted projective spaces and description
of their automorphism groups is a very old completed problem.
The description of the ''minimal'' finite subgroups of automo
rphism groups of nonsingular del Pezzo surfaces was done by I.
V. Dolgachev and V.A. Iskovskikh in 2007. I constructed a meth
od that allows us to describe by equations in weighted proje
ctive spaces the nonsingular conic bundles with an action of '
'minimal'' subgroup of automorphisms . I applied the method in
case, when the ''minimal'' subgroup of automorphisms is nonsol
vable.
END:VEVENT
BEGIN:VEVENT
UID:news-757@dmi.unibas.ch
DTSTAMP:20190212T003452
DTSTART;TZID=Europe/Zurich:20120511T103000
DTEND;TZID=Europe/Zurich:20120511T120000
SUMMARY:Seminar Algebra and Geometry: Rosa M. Miró-Roig (Universitat de Barcelona)
LOCATION:
DESCRIPTION:In my talk, I will construct families of non-isomorphic Arithme
tically Cohen Macaulay (ACM) sheaves (i.e., sheaves without in
termediate cohomology) on projective varieties. Since the semi
nal result by Horrocks characterizing ACM bundles on $\mathbb{
P}^n$ as those that split into a sum of line bundles, an impor
tant amount of research has been devoted to the study of ACM s
heaves on a given variety. ACM sheaves also provide a criter
ium to determine the complexity of the underlying variety. Mor
e concretely, this complexity can be studied in terms of the d
imension and number of families of indecomposable ACM sheaves
that it supports, namely its \emph{representation type}. Along
this line, a variety that admits only a finite number of indec
omposable ACM sheaves (up to twist and isomorphism) is called
of finite representation type. These varieties are completely
classified: They are either three or less reduced points in $\
mathbb{P}^2$, a projective space $\mathbb{P}_k^n$, a smooth qu
adric hypersurface $X\subset\mathbb{P}^n$, a cubic scroll in $
\mathbb{P}_k^4$, the Veronese surface in $\mathbb{P}_k^5$ or a
rational normal curve. On the other extreme of complexity,
we would find the varieties of wild representation type, namely
, varieties for which there exist r-dimensional families of non
-isomorphic indecomposable ACM sheaves for arbitrary large r.
In the case of dimension one, it is known that curves of wild
representation type are exactly those of genus larger or equal
than two. In dimension greater or equal than two few examples
are known ans in may talk, I will give a brief account of the
known results.
END:VEVENT
BEGIN:VEVENT
UID:news-758@dmi.unibas.ch
DTSTAMP:20190212T003732
DTSTART;TZID=Europe/Zurich:20120504T103000
DTEND;TZID=Europe/Zurich:20120504T120000
SUMMARY:Seminar Algebra and Geometry: Clemens Berger (Université de Nice)
LOCATION:
DESCRIPTION:The Euler characteristic of a cell complex is often thought of
as the alternating sum of the number of cells of each dimensio
n. When the complex is infinite, the sum diverges. Nevertheles
s, this infinite sum can sometimes be evaluated using generati
ng power series. We show that this is in particular the case w
hen the complex is the simplicial nerve of a finite category.
We discuss the relationship with Rota's notion of Moebius inve
rsion and Noguchi's notion of zeta-function of a finite catego
ry.This talk is based on joint work with Tom Leinster (Glasgow)
.
END:VEVENT
BEGIN:VEVENT
UID:news-759@dmi.unibas.ch
DTSTAMP:20190212T003913
DTSTART;TZID=Europe/Zurich:20120420T103000
DTEND;TZID=Europe/Zurich:20120420T120000
SUMMARY:Seminar Algebra and Geometry: Tommy Wuxing Cai (Universität Basel)
LOCATION:
DESCRIPTION:We give a formula generalizing Newton's formula on symmetric fu
nctions. As an application of it, we give a simple proof of th
e existence of Macdonald symmetric functions and compute the M
acdonald functions of shapes of two-row partitions.
END:VEVENT
BEGIN:VEVENT
UID:news-760@dmi.unibas.ch
DTSTAMP:20190212T004214
DTSTART;TZID=Europe/Zurich:20120413T103000
DTEND;TZID=Europe/Zurich:20120413T120000
SUMMARY:Seminar Algebra and Geometry: Kevin Langlois (Institut Fourier, Grenoble)
LOCATION:
DESCRIPTION:Let A=C[f1,..., fr] be an integral algebra of finite type over
the field of complex numbers. Using the elements f1,..,fr it
is difficult in general to describe the normalization of A. I
n this talk, we provide some examples whenever A is a multigrad
ed algebra. Consider the group T=C*×...×C*=(C*)n given by t
he componentwise multiplication. We say that T is an algebraic
torus of dimension n. Let M be the character lattice of T. Then
a T-action on X=Spec A is equivalent to endow A with a M-gradu
ation. We classify the M-graded algebras A by a number called c
omplexity. Geometrically, it corresponds to the codimension o
f general T-orbits in X. Algebraically, the complexity is som
ehow "the thickness of graded pieces" of the algebra A. The
problem of normalization for complexity zero case is well known
(monomial or toric case). For the complexity one, the nor
malization of A admits a construction due to Timashev and Al
tmann-Hausen in terms of polyhedral divisors over an algebraic
smooth curve. Taking homogeneous generators, we will expl
ain how to build the polyhedral divisor corresponding to the
normalization of A. Assume that A is normal. Then A is give
n by a polyhedral divisor. A similar problem arises for the
integral closure of homogeneous ideals. We will give an answe
r for the complexity one case. We will provide also a classific
ation of homogeneous integrally closed ideals of A.
END:VEVENT
BEGIN:VEVENT
UID:news-761@dmi.unibas.ch
DTSTAMP:20190212T004459
DTSTART;TZID=Europe/Zurich:20120330T103000
DTEND;TZID=Europe/Zurich:20120330T120000
SUMMARY:Seminar Algebra and Geometry: Alexandru Constantinescu (Universität Basel)
LOCATION:
DESCRIPTION:On h-vectors of matroids
END:VEVENT
BEGIN:VEVENT
UID:news-762@dmi.unibas.ch
DTSTAMP:20190212T004749
DTSTART;TZID=Europe/Zurich:20120316T103000
DTEND;TZID=Europe/Zurich:20120316T120000
SUMMARY:Seminar Algebra and Geometry: Julie Déserti and Matey Mateev (Universität Basel)
LOCATION:
DESCRIPTION:T.B.A.
END:VEVENT
BEGIN:VEVENT
UID:news-763@dmi.unibas.ch
DTSTAMP:20190212T004933
DTSTART;TZID=Europe/Zurich:20120309T103000
DTEND;TZID=Europe/Zurich:20120309T120000
SUMMARY:Seminar Algebra and Geometry: Thomas Kahle (ETH Zürich)
LOCATION:
DESCRIPTION:In a polynomial ring, a binomial says that two monomials are sc
alar multiples of each other. Forgetting about the scalars,
a binomial ideal describes an equivalence relation on the mono
id of exponents. \r\n\r\nIdeally one would want to carry out
algebraic computations, such as primary decomposition of bino
mial ideals, entirely in this combinatorial language. We wil
l present such a calculus, enabling one to compute by looking
at pictures of monoids.
END:VEVENT
BEGIN:VEVENT
UID:news-765@dmi.unibas.ch
DTSTAMP:20190212T165841
DTSTART;TZID=Europe/Zurich:20111216T103000
DTEND;TZID=Europe/Zurich:20111216T120000
SUMMARY:Seminar Algebra and Geometry: Paolo Mantero (Purdue University)
LOCATION:
DESCRIPTION:A vast part of literature on CI-linkage has addressed questions
relative to the most relevant, and well-behaved, class of idea
ls in linkage: licci ideals. However, for a non-licci ideal I,
there are few results describing the structure of the linkage c
lass of I.In this talk we introduce a theoretical definition fo
r 'minimal' representatives in any even linkage class. We show
that these idealsexist under reasonable assumptions on the link
age class, and, in general, if they exist they are essentially
unique.We then show that these ideals minimize homological inva
riants (e.g. Betti numbers, multiplicity, etc.) and they enjoy
the best homological and local properties among all the ideals
in their even linkage class. This justifies why they are, in so
me sense, the `best' possible ideals in the even linkage class.
We provide several classes of ideals that are the minimal repre
sentatives of their even linkage classes (including determinant
al ideals) and, if time permits, show an easy application to pr
oduce more evidence towards the Buchsbaum-Eisenbud-Horrocks Con
jecture.
END:VEVENT
BEGIN:VEVENT
UID:news-766@dmi.unibas.ch
DTSTAMP:20190212T170052
DTSTART;TZID=Europe/Zurich:20111209T103000
DTEND;TZID=Europe/Zurich:20111209T120000
SUMMARY:Seminar Algebra and Geometry: Volkmar Welker (Philipps-Universität Marburg)
LOCATION:
DESCRIPTION:Generalized Inversion Statistics
END:VEVENT
BEGIN:VEVENT
UID:news-767@dmi.unibas.ch
DTSTAMP:20190212T170340
DTSTART;TZID=Europe/Zurich:20111202T103000
DTEND;TZID=Europe/Zurich:20111202T120000
SUMMARY:Seminar Algebra and Geometry: Andriy Regeta (Technische Universität Kaiserslautern)
LOCATION:
DESCRIPTION:(joint work with A. Petravchuk and O. Iena)Let us consider the
Lie algebra Wn=Wn(K) of all K-derivations of the polynomial rin
g K[x1,...,xn]. We discuss here a classof subalgebras in W2(K)
over an algebraically closed field of characteristic zero.\r\nL
et us consider natural action of the Lie algebra W2(K) on the f
ield of rational functions K(x,y). Recall that every derivation
Dof W2(K) of the ring K[x,y] can be uniquely extended to a der
ivation of the field K(x,y). It is natural to consider for afix
ed rational function u in K(x,y)\K the set A_{W2}(u) of all der
ivations D of W2 such that D(u)=0. This set is called the annih
ilatorof u in W2(K). It is a Lie subalgebra of W2(K) and at the
same time a K[x,y]-submodule of the K[x,y]-module W2(K).We sho
w that it is a free K[x,y]-module of rank 1 and describe centra
lizers of elements and the maximal abelian subalgebras of theLi
e algebra A_{W2}(u).
END:VEVENT
BEGIN:VEVENT
UID:news-768@dmi.unibas.ch
DTSTAMP:20190212T170631
DTSTART;TZID=Europe/Zurich:20111118T103000
DTEND;TZID=Europe/Zurich:20111118T120000
SUMMARY:Seminar Algebra and Geometry: Rafael Andrist (Universität Wuppertal)
LOCATION:
DESCRIPTION:A Danielewski surface is given as the hypersurface $x y = f(z)$
in $\mathbb{C}^3$, where $f$ is a polynomial with only simpl
e zeroes. Such a surface enjoys the Density Property, i.e. th
e Lie algebra generated by the complete holomorphic vector fi
elds is dense in the Lie algebra of all holomorphic vector fi
elds.In case of a Danielewski surface the so-called overshear
group is dense in the group of holomorphic automorphisms. We
describe the group structure of the overshear group with the
help of Nevanlinna theory.
END:VEVENT
BEGIN:VEVENT
UID:news-769@dmi.unibas.ch
DTSTAMP:20190212T171244
DTSTART;TZID=Europe/Zurich:20111111T103000
DTEND;TZID=Europe/Zurich:20111111T120000
SUMMARY:Seminar Algebra and Geometry: Winfried Bruns (Universität Osnabrück)
LOCATION:
DESCRIPTION:We report on joint work with Chr. Krattenthaler and J. Uliczka.
Stanley decompositions of multigraded modules M over polynomial
s rings have been discussed intensively in recent years. There
is a natural notion of depth that goes with a Stanley decomposi
tion, called the Stanley depth. Stanley conjectured that the St
anley depth of a module M is always at least the (classical) de
pth of M. We introduce a weaker type of decomposition, which we
call Hilbert decomposition, since it only depends on the Hilbe
rt function of M, and an analogous notion of depth, called Hilb
ert depth. Since Stanley decompositions are Hilbert decompositi
ons, the latter set upper bounds to the existence of Stanley de
compositions.The advantage of Hilbert decompositions is that th
ey are easier to find.We test our new notion on the syzygy modu
les of the residue class field of K[X1,...,Xn] (as usual identi
fied with K). Writing M(n,k) for the k-th syzygy module, we sho
w that the Hilbert depth of M(n,1), namely the irrelevant maxim
al ideal, is $\lfloor(n+1)/2\rfloor$. Furthermore, we show that
, for $n > k \ge \lfloor n/2\rfloor$, the (multigraded) Hilbert
depth of M(n,k) is equal to n-1. We conjecture that the same h
olds for the Stanley depth. For the range n/2 > k > 1, it seems
impossible to come up with a compact formula for the Hilbert d
epth. Instead, we provide very precise asymptotic results as n
becomes large.Related ideals are the powers of the irrelevant m
aximal ideal. For them the (standard graded) Hilbert depth can
be computedprecisely.
END:VEVENT
BEGIN:VEVENT
UID:news-770@dmi.unibas.ch
DTSTAMP:20190212T171622
DTSTART;TZID=Europe/Zurich:20111104T000000
DTEND;TZID=Europe/Zurich:20111104T000000
SUMMARY:Seminar Algebra and Geometry: Hanspeter Kraft and Maria Fernanda Robayo (Universität Basel), Two 30 minutes talks
LOCATION:
DESCRIPTION:T.B.A.
END:VEVENT
BEGIN:VEVENT
UID:news-771@dmi.unibas.ch
DTSTAMP:20190212T171920
DTSTART;TZID=Europe/Zurich:20111028T000000
DTEND;TZID=Europe/Zurich:20111028T000000
SUMMARY:Seminar Algebra and Geometry: Naihuan Jing (North Carolina State Univ. and South China Univ. of Technology)
LOCATION:
DESCRIPTION:Exactly 100 years ago Schur constructed irreducible characters
of the spin groups of the symmetric group and introduced the f
amous Schur Q-functions. Using ideas from affine Lie algebras
and McKay correspondence, Frenkel, Jing and Wang fixed the irr
educible characters for the spin group of the wreath products
for the symmetric group without knowing the complete character
table. In this work we will generalize Schur's work and compl
etely determine all character values for the spin wreath produ
cts in type A case. This is joint work with X. Hu.
END:VEVENT
BEGIN:VEVENT
UID:news-772@dmi.unibas.ch
DTSTAMP:20190212T172107
DTSTART;TZID=Europe/Zurich:20111021T000000
DTEND;TZID=Europe/Zurich:20111021T000000
SUMMARY:Seminar Algebra and Geometry: Émilie Dufresne and Jérémy Blanc (Universität Basel), Two 30 minutes talks
LOCATION:
DESCRIPTION:T.B.A.
END:VEVENT
BEGIN:VEVENT
UID:news-773@dmi.unibas.ch
DTSTAMP:20190212T172233
DTSTART;TZID=Europe/Zurich:20111014T000000
DTEND;TZID=Europe/Zurich:20111014T000000
SUMMARY:Seminar Algebra and Geometry: Alexandru Constantinescu and Pierre-Marie Poloni (Universität Basel), Two 30 minutes talks
LOCATION:
DESCRIPTION:T.B.A.
END:VEVENT
BEGIN:VEVENT
UID:news-774@dmi.unibas.ch
DTSTAMP:20190212T172423
DTSTART;TZID=Europe/Zurich:20111007T000000
DTEND;TZID=Europe/Zurich:20111007T000000
SUMMARY:Seminar Algebra and Geometry: Bruno Duchesne (The Hebrew University of Jerusalem)
LOCATION:
DESCRIPTION:Symmetric spaces of non-compact type (which are simply-connecte
d Riemannian manifolds of non-positive curvature with a geode
sic symmetry at each point) are classical and useful geometri
c tools to understand finite-dimensional linear representatio
ns of groups.\r\n\r\nWe will look at some infinite dimensiona
l symmetric spaces of non-positive curvature which have a rem
arkable property : they have finite rank. There exists a posi
tive integer p such that any isometrically embedded Euclidean
space has dimension at most p.\r\n\r\nThe talk will be focus
ed on the properties of these spaces and some group actions w
hich come from (non-unitary) infinite-dimensional representati
ons.
END:VEVENT
BEGIN:VEVENT
UID:news-775@dmi.unibas.ch
DTSTAMP:20190212T172550
DTSTART;TZID=Europe/Zurich:20110930T000000
DTEND;TZID=Europe/Zurich:20110930T000000
SUMMARY:Seminar Algebra and Geometry: Tommy Wuxing Cai (Universität Basel)
LOCATION:
DESCRIPTION:We first realize Jack function of rectangular shapes. Then we
introduce an iterative method to realize Jack functions of gen
eral shapes from those of rectangular shapes. As a byproduct,
we prove some special cases of Richard P. Stanley's conjectu
re about the positivity of the Littlewood-Richardson coeffici
ents of Jack functions.Contents: 1. partitions and symmetric fu
nctions 2. Some history and questions about symmetric functions
3. Vertex operator realization of rectangular Jack functions 4
. A special case of Stanley's conjecture and the realization of
general Jack functions 5. Open questions
END:VEVENT
BEGIN:VEVENT
UID:news-776@dmi.unibas.ch
DTSTAMP:20190212T172827
DTSTART;TZID=Europe/Zurich:20110527T000000
DTEND;TZID=Europe/Zurich:20110527T000000
SUMMARY:Seminar Algebra and Geometry: Olivier Serman (Université Lille 1)
LOCATION:
DESCRIPTION:Moduli spaces of principal bundles on curves, which have been i
ntroduced as non-abelian analogs of the Jacobian variety, carr
y a very rich geometry. In this talk I will focus on the singu
larities of these varieties. I will show how local factorialit
y of GIT quotients can be used to give a description of their
singular locus, and then compute the fundamental group of the
smooth locus of these moduli spaces.
END:VEVENT
BEGIN:VEVENT
UID:news-777@dmi.unibas.ch
DTSTAMP:20190212T180652
DTSTART;TZID=Europe/Zurich:20110520T000000
DTEND;TZID=Europe/Zurich:20110520T000000
SUMMARY:Seminar Algebra and Geometry: Émilie Dufresne (Universität Basel)
LOCATION:
DESCRIPTION:Nagata's famous counterexample to Hilbert's fourteenth problem
shows that the ring of invariants of an algebraic group actio
n on an affine algebraic variety is not always finitely genera
ted. In some sense, however, invariant rings are not far from
being affine. Indeed, invariant rings are always quasi-affine,
and finite separating sets always exist. We give a new method
for finding a quasi-affine variety on which the ring of regul
ar functions is equal to a given invariant ring, and give a cr
iterion to recognize separating algebras. We use the method and
criterion to construct new examples.
END:VEVENT
BEGIN:VEVENT
UID:news-778@dmi.unibas.ch
DTSTAMP:20190212T180831
DTSTART;TZID=Europe/Zurich:20110516T160000
DTEND;TZID=Europe/Zurich:20110516T170000
SUMMARY:Seminar Algebra and Geometry: Aldo Conca (University of Genova)
LOCATION:
DESCRIPTION:The goal of the talk is to explain recent results and conjectur
es regarding Koszul algebras and their syzygies. Koszul algebr
as are graded K-algebras R such that the residue filed K has
a linear R-free resolution. Koszul algebras are defined by q
uadrics. But not all algebras defined by quadric are Koszul.
However many classical algebras defined by quadrics (e.g.
the coordinate ring of the Grassmannian in its standard embed
ding) are Koszul.The main idea I will discuss is that the
syzygies of Koszul algebras have some properties in common w
ith the syzygies of algebras defined by monomialsof degree two.
END:VEVENT
BEGIN:VEVENT
UID:news-779@dmi.unibas.ch
DTSTAMP:20190212T181018
DTSTART;TZID=Europe/Zurich:20110513T000000
DTEND;TZID=Europe/Zurich:20110513T000000
SUMMARY:Seminar Algebra and Geometry: Ivan Arzhantsev (Moscow State University and Institut Fourier)
LOCATION:
DESCRIPTION:(joint work with Devrim Celik and Juergen Hausen)We begin with
a survey of known results concerning categorical quotients.Give
n an action of an affine algebraic group with only trivial cha
racters on a factorial variety, we characterize existence of c
ategorical quotient in the category of algebraic varieties. Mo
reover, allowing constructible sets as quotients, we obtain a
more general existence result, which, for example, settles the
case of a finitely generated algebra of invariants. As an app
lication, we provide a combinatorial GIT-type construction of
categorial quotients for actions on, e.g. complete, varieties
with finitely generated Cox ring via lifting to the universal
torsor.
END:VEVENT
BEGIN:VEVENT
UID:news-780@dmi.unibas.ch
DTSTAMP:20190212T181256
DTSTART;TZID=Europe/Zurich:20110506T000000
DTEND;TZID=Europe/Zurich:20110506T000000
SUMMARY:Seminar Algebra and Geometry: Andreas Maurischat (Universität Heidelberg)
LOCATION:
DESCRIPTION:In Hopf-Galois theory one can attach a Hopf algebra H (resp.
an affine group scheme G=Spec(H) ) to any finite field extensi
on. In general, however, this Hopf-algebra is not unique, but
in some cases - e.g. normal separable extensions - there are n
atural choices. In this talk, we will show how one can obtain
"natural choices" for finite extensions (even inseparable ones
) by imposing an extra structure on the field, namely a so-cal
led iterative derivation.
END:VEVENT
BEGIN:VEVENT
UID:news-781@dmi.unibas.ch
DTSTAMP:20190212T181459
DTSTART;TZID=Europe/Zurich:20110429T000000
DTEND;TZID=Europe/Zurich:20110429T000000
SUMMARY:Seminar Algebra and Geometry: Matteo Varbaro (University of Genova)
LOCATION:
DESCRIPTION:T.B.A.
END:VEVENT
BEGIN:VEVENT
UID:news-782@dmi.unibas.ch
DTSTAMP:20190212T181619
DTSTART;TZID=Europe/Zurich:20110415T000000
DTEND;TZID=Europe/Zurich:20110415T000000
SUMMARY:Seminar Algebra and Geometry: Abraham Broer (Université de Montréal)
LOCATION:
DESCRIPTION:Cohomology of line bundles on the cotangent bundle of a complet
e homogeneous space
END:VEVENT
BEGIN:VEVENT
UID:news-783@dmi.unibas.ch
DTSTAMP:20190212T181851
DTSTART;TZID=Europe/Zurich:20110408T000000
DTEND;TZID=Europe/Zurich:20110408T000000
SUMMARY:Seminar Algebra and Geometry: Enrico Schlesinger (Politecnico di Milano)
LOCATION:
DESCRIPTION:
END:VEVENT
BEGIN:VEVENT
UID:news-784@dmi.unibas.ch
DTSTAMP:20190212T182036
DTSTART;TZID=Europe/Zurich:20110401T000000
DTEND;TZID=Europe/Zurich:20110401T000000
SUMMARY:Seminar Algebra and Geometry: Lucas Fresse (Hebrew University of Jerusalem)
LOCATION:
DESCRIPTION:To a nilpotent element x in a reductive Lie algebra, one can at
tach several algebraic varieties which play roles in represent
ation theory: its nilpotent orbit; the intersection of its nil
potent orbit with a Borel subalgebra (the irreducible componen
ts of this intersection are called orbital varieties); the fib
er over x of the Springer resolution. There is a close relatio
n between the Springer fiber over x and the orbital varieties
attached to x. In this talk, we rely on this relation in order
to study two properties of orbital varieties: the smoothness,
and the property to contain a dense B-orbit. We concentrate on
type A. We provide several criteria which suggest that the t
wo mentioned properties are related. This is a joint work with
Anna Melnikov.
END:VEVENT
BEGIN:VEVENT
UID:news-785@dmi.unibas.ch
DTSTAMP:20190212T182557
DTSTART;TZID=Europe/Zurich:20110311T000000
DTEND;TZID=Europe/Zurich:20110311T000000
SUMMARY:Seminar Algebra and Geometry: Marc Chardin (Institut Mathématique de Jussieu, Paris)
LOCATION:
DESCRIPTION:In this lecture, I will present a way of computing the image of
a rational map using a free resolution of the symmetric alg
ebra. In this setting, the method was initiated by Jean-Pier
re Jouanolou, and further developed by him, by Laurent Busé
and by myself. The origin of this method is the work of peopl
e in Geometric Modeling, motivated by a simple question: how
to represent the intersection of two surfaces parametrized
by rational functions? \r\n\r\nTheir approach was first put on
firm mathematical bases by David Cox and several collaborator
s. The key point in this approach is to control the torsion
in the symmetric algebra. This is performed using a constructi
on of Herzog, Simis and Vasconcelos, that gives information on
the equations of Rees algebras.
END:VEVENT
BEGIN:VEVENT
UID:news-786@dmi.unibas.ch
DTSTAMP:20190212T182857
DTSTART;TZID=Europe/Zurich:20110304T000000
DTEND;TZID=Europe/Zurich:20110304T000000
SUMMARY:Seminar Algebra and Geometry: Nathan Ilten (Max Planck Institute)
LOCATION:
DESCRIPTION:A p-divisor on a normal variety Y is a divisor satisfying some
positivity properties, where the usual integral or rational co
efficients have been replaced by polyhedral coeffi- cients. K.
Altmann and J. Hausen have shown that there is a corresponden
ce between p-divisors and affine T-varieties, i.e. normal vari
eties together with some effective torus action. Given some T-
variety X, one can try to ”upgrade” the torus action by co
nsidering some larger torus acting on X, or ”downgrade” the
torus action by considering the action of some subtorus. I wi
ll discuss how these upgrading and downgrading procedures chan
ge the corresponding p-divisors. Time permitting, I will prese
nt an application of the upgrade procedure dealing with the p-
divisors of Cox rings of certain T-varieties. This is joint wo
rk with R. Vollmert.
END:VEVENT
BEGIN:VEVENT
UID:news-787@dmi.unibas.ch
DTSTAMP:20190212T183147
DTSTART;TZID=Europe/Zurich:20110225T000000
DTEND;TZID=Europe/Zurich:20110225T000000
SUMMARY:Seminar Algebra and Geometry: Jérôme Tambour (Université de Bourgogne)
LOCATION:
DESCRIPTION:It is not easy to construct examples of non k ̈ahler compact
complex manifold. For instance, every algebraic variety, and e
very Riemann surface, is ka ̈hler. The classical examples of
such varieties are Hopf manifolds (1948) and Calabi-Eckmann ma
nifolds (1953) which are complex structures on product of sphe
res Sp × Sq (with p and q odd). Santiago Lopez de Medrano, Al
berto Verjovsky and Laurent Meersseman gave a generalization o
f this construction. The interest of theirs manifolds, known a
s LVM manifolds, stand in the fact that it is practical to com
pute some of their topological invariants (homology and cohomo
logy for example). They are also endowed with a very nice actio
n of a compact torus.\r\n\r\nThe talk will mainly deal with a
generalization due to Bosio of the LVM manifolds, emphazing th
e combinatorial aspect of the LVM manifolds. These new manifol
ds are known as LVMB manifolds. In particular, our aim will be
to show the very strong connection between LVMB manifolds, to
ric varieties and triangulations of spheres.
END:VEVENT
BEGIN:VEVENT
UID:news-788@dmi.unibas.ch
DTSTAMP:20190212T183730
DTSTART;TZID=Europe/Zurich:20101217T000000
DTEND;TZID=Europe/Zurich:20101217T000000
SUMMARY:Seminar Algebra and Geometry: Bruno Benedetti (Technische Universität Berlin)
LOCATION:
DESCRIPTION:Shelling, constructing, counting
END:VEVENT
BEGIN:VEVENT
UID:news-789@dmi.unibas.ch
DTSTAMP:20190212T200532
DTSTART;TZID=Europe/Zurich:20101210T000000
DTEND;TZID=Europe/Zurich:20101210T000000
SUMMARY:Seminar Algebra and Geometry: Martin Kohls (Technische Universität München)
LOCATION:
DESCRIPTION:Separating invariants for the basic $G_a$-actions
END:VEVENT
BEGIN:VEVENT
UID:news-790@dmi.unibas.ch
DTSTAMP:20190212T200506
DTSTART;TZID=Europe/Zurich:20101203T000000
DTEND;TZID=Europe/Zurich:20101203T000000
SUMMARY:Seminar Algebra and Geometry: Alexandru Constantinescu (Universität Basel)
LOCATION:
DESCRIPTION:Parametrizations of Ideals in K[x,y]
END:VEVENT
BEGIN:VEVENT
UID:news-791@dmi.unibas.ch
DTSTAMP:20190212T200642
DTSTART;TZID=Europe/Zurich:20101119T000000
DTEND;TZID=Europe/Zurich:20101119T000000
SUMMARY:Seminar Algebra and Geometry: Gavin Brown (Loughborough University)
LOCATION:
DESCRIPTION:Gorenstein projections and diptych varieties
END:VEVENT
BEGIN:VEVENT
UID:news-792@dmi.unibas.ch
DTSTAMP:20190212T200734
DTSTART;TZID=Europe/Zurich:20101112T000000
DTEND;TZID=Europe/Zurich:20101112T000000
SUMMARY:Seminar Algebra and Geometry: Claudia R. Alcantara (Universidad de Guanajuato, Mexico)
LOCATION:
DESCRIPTION:Holomorphic foliations on CP2 and Geometric Invariant Theory
END:VEVENT
BEGIN:VEVENT
UID:news-793@dmi.unibas.ch
DTSTAMP:20190212T200859
DTSTART;TZID=Europe/Zurich:20101105T000000
DTEND;TZID=Europe/Zurich:20101105T000000
SUMMARY:Seminar Algebra and Geometry: Stéphane Lamy (University of Warwick / Université Lyon 1)
LOCATION:
DESCRIPTION:Birational constructions of automorphisms of affine 3-folds
END:VEVENT
BEGIN:VEVENT
UID:news-794@dmi.unibas.ch
DTSTAMP:20190212T201001
DTSTART;TZID=Europe/Zurich:20101029T000000
DTEND;TZID=Europe/Zurich:20101029T000000
SUMMARY:Seminar Algebra and Geometry: Markus Brodmann (Universität Zürich)
LOCATION:
DESCRIPTION:Projective Varieties of Low Degree
END:VEVENT
BEGIN:VEVENT
UID:news-795@dmi.unibas.ch
DTSTAMP:20190212T201207
DTSTART;TZID=Europe/Zurich:20101022T000000
DTEND;TZID=Europe/Zurich:20101022T000000
SUMMARY:Seminar Algebra and Geometry: Alvaro Liendo (Universität Basel)
LOCATION:
DESCRIPTION:Normal singularities with torus actions
END:VEVENT
BEGIN:VEVENT
UID:news-796@dmi.unibas.ch
DTSTAMP:20190212T201336
DTSTART;TZID=Europe/Zurich:20101015T000000
DTEND;TZID=Europe/Zurich:20101015T000000
SUMMARY:Seminar Algebra and Geometry: Émilie Dufresne (Universität Basel)
LOCATION:
DESCRIPTION:Additive group actions in positive chracteristic
END:VEVENT
BEGIN:VEVENT
UID:news-797@dmi.unibas.ch
DTSTAMP:20190212T201608
DTSTART;TZID=Europe/Zurich:20100929T140000
DTEND;TZID=Europe/Zurich:20100929T153000
SUMMARY:Seminar Algebra and Geometry: Frédéric Mangolte (Université d'Angers)
LOCATION:Alte Universität, room 118
DESCRIPTION:Algebraic approximations of diffeomorphisms of surfaces
END:VEVENT
BEGIN:VEVENT
UID:news-798@dmi.unibas.ch
DTSTAMP:20190212T201826
DTSTART;TZID=Europe/Zurich:20100924T000000
DTEND;TZID=Europe/Zurich:20100924T000000
SUMMARY:Seminar Algebra and Geometry: Hamid Ahmadinezhad (Universität Basel)
LOCATION:
DESCRIPTION:On rigidity of low degree del Pezzo fibrations
END:VEVENT
BEGIN:VEVENT
UID:news-809@dmi.unibas.ch
DTSTAMP:20190212T212605
DTSTART;TZID=Europe/Zurich:20100528T110000
DTEND;TZID=Europe/Zurich:20100528T123000
SUMMARY:Seminar Algebra and Geometry: Guillaume Batog (Université de Nancy)
LOCATION:
DESCRIPTION:Using polynomial invariants to compute geometric predicates
END:VEVENT
BEGIN:VEVENT
UID:news-808@dmi.unibas.ch
DTSTAMP:20190212T212410
DTSTART;TZID=Europe/Zurich:20100528T093000
DTEND;TZID=Europe/Zurich:20100528T103000
SUMMARY:Seminar Algebra and Geometry: Jan Draisma (Eindhoven University of Technology)
LOCATION:
DESCRIPTION:A tropical proof of the Brill-Noether theorem
END:VEVENT
BEGIN:VEVENT
UID:news-807@dmi.unibas.ch
DTSTAMP:20190212T212027
DTSTART;TZID=Europe/Zurich:20100521T000000
DTEND;TZID=Europe/Zurich:20100521T000000
SUMMARY:Seminar Algebra and Geometry: Jerzy Weyman (Northeastern University)
LOCATION:
DESCRIPTION:Quivers and saturation for tensor products multiplicities for c
lassical groups
END:VEVENT
BEGIN:VEVENT
UID:news-806@dmi.unibas.ch
DTSTAMP:20190212T211443
DTSTART;TZID=Europe/Zurich:20100430T000000
DTEND;TZID=Europe/Zurich:20100430T000000
SUMMARY:Seminar Algebra and Geometry: Maria Evelina Rossi (University of Genoa)
LOCATION:
DESCRIPTION:Isomorphism classes of Gorenstein local rings via Macaulay's in
verse system
END:VEVENT
BEGIN:VEVENT
UID:news-805@dmi.unibas.ch
DTSTAMP:20190212T202904
DTSTART;TZID=Europe/Zurich:20100423T000000
DTEND;TZID=Europe/Zurich:20100423T000000
SUMMARY:Seminar Algebra and Geometry: Tanja Becker (Universität Mainz)
LOCATION:
DESCRIPTION:Beispiel eines Sl_2-Hilbertschemas
END:VEVENT
BEGIN:VEVENT
UID:news-804@dmi.unibas.ch
DTSTAMP:20190212T202808
DTSTART;TZID=Europe/Zurich:20100416T110000
DTEND;TZID=Europe/Zurich:20100416T120000
SUMMARY:Seminar Algebra and Geometry: Zinovy Reichstein (University of British Columbia)
LOCATION:
DESCRIPTION:Versal actions with a twist
END:VEVENT
BEGIN:VEVENT
UID:news-803@dmi.unibas.ch
DTSTAMP:20190212T202654
DTSTART;TZID=Europe/Zurich:20100416T093000
DTEND;TZID=Europe/Zurich:20100416T103000
SUMMARY:Seminar Algebra and Geometry: Alvaro Liendo (Université de Grenoble)
LOCATION:
DESCRIPTION:A birational characterization of affine varieties with trivial
ML invariant
END:VEVENT
BEGIN:VEVENT
UID:news-802@dmi.unibas.ch
DTSTAMP:20190212T202456
DTSTART;TZID=Europe/Zurich:20100409T000000
DTEND;TZID=Europe/Zurich:20100409T000000
SUMMARY:Seminar Algebra and Geometry: Michel Brion (Université Joseph Fourier Grenoble)
LOCATION:
DESCRIPTION:Picard groups of homogeneous varieties
END:VEVENT
BEGIN:VEVENT
UID:news-801@dmi.unibas.ch
DTSTAMP:20190212T202321
DTSTART;TZID=Europe/Zurich:20100326T000000
DTEND;TZID=Europe/Zurich:20100326T000000
SUMMARY:Seminar Algebra and Geometry: Jérémy Blanc (Universität Basel)
LOCATION:
DESCRIPTION:Algebraic actions on the affine plane
END:VEVENT
BEGIN:VEVENT
UID:news-800@dmi.unibas.ch
DTSTAMP:20190212T202200
DTSTART;TZID=Europe/Zurich:20100312T140000
DTEND;TZID=Europe/Zurich:20100312T153000
SUMMARY:Seminar Algebra and Geometry: Luc Guyot (University of Göttingen)
LOCATION:
DESCRIPTION:Limits of metabelian groups
END:VEVENT
BEGIN:VEVENT
UID:news-799@dmi.unibas.ch
DTSTAMP:20190212T202010
DTSTART;TZID=Europe/Zurich:20100305T000000
DTEND;TZID=Europe/Zurich:20100305T000000
SUMMARY:Seminar Algebra and Geometry: Ulrich Derenthal (University of Freiburg)
LOCATION:
DESCRIPTION:Rational points on cubic surfaces
END:VEVENT
BEGIN:VEVENT
UID:news-820@dmi.unibas.ch
DTSTAMP:20190212T220426
DTSTART;TZID=Europe/Zurich:20091218T000000
DTEND;TZID=Europe/Zurich:20091218T000000
SUMMARY:Seminar Algebra and Geometry: Hanspeter Kraft (Basel University)
LOCATION:
DESCRIPTION:Was ist ein Faserbündel?
END:VEVENT
BEGIN:VEVENT
UID:news-819@dmi.unibas.ch
DTSTAMP:20190212T220304
DTSTART;TZID=Europe/Zurich:20091211T000000
DTEND;TZID=Europe/Zurich:20091211T000000
SUMMARY:Seminar Algebra and Geometry: Pierre-Marie Poloni (Basel University)
LOCATION:
DESCRIPTION:Some examples coming from the study of the Danielewski hypersur
faces
END:VEVENT
BEGIN:VEVENT
UID:news-818@dmi.unibas.ch
DTSTAMP:20190212T220207
DTSTART;TZID=Europe/Zurich:20091204T000000
DTEND;TZID=Europe/Zurich:20091204T000000
SUMMARY:Seminar Algebra and Geometry: Roland Lötscher (Basel University)
LOCATION:
DESCRIPTION:Essential dimension of algebraic tori
END:VEVENT
BEGIN:VEVENT
UID:news-817@dmi.unibas.ch
DTSTAMP:20190212T220108
DTSTART;TZID=Europe/Zurich:20091120T000000
DTEND;TZID=Europe/Zurich:20091120T000000
SUMMARY:Seminar Algebra and Geometry: Maike Massierer (Basel University)
LOCATION:
DESCRIPTION:Aspects of class field theory for global function fields
END:VEVENT
BEGIN:VEVENT
UID:news-816@dmi.unibas.ch
DTSTAMP:20190212T215928
DTSTART;TZID=Europe/Zurich:20091113T000000
DTEND;TZID=Europe/Zurich:20091113T000000
SUMMARY:Seminar Algebra and Geometry: Jean-Philippe Furter (Université de La Rochelle)
LOCATION:
DESCRIPTION:Normal subgroup generated by a plane polynomial automorphism
END:VEVENT
BEGIN:VEVENT
UID:news-815@dmi.unibas.ch
DTSTAMP:20190212T215827
DTSTART;TZID=Europe/Zurich:20091106T000000
DTEND;TZID=Europe/Zurich:20091106T000000
SUMMARY:Seminar Algebra and Geometry: Elisa Gorla (Basel University)
LOCATION:
DESCRIPTION:Gorestein Liaison and determinantal schemes
END:VEVENT
BEGIN:VEVENT
UID:news-814@dmi.unibas.ch
DTSTAMP:20190212T213610
DTSTART;TZID=Europe/Zurich:20091030T000000
DTEND;TZID=Europe/Zurich:20091030T000000
SUMMARY:Seminar Algebra and Geometry: Immanuel Stampfli (Basel University)
LOCATION:
DESCRIPTION:Some aspects of A^1-bundles
END:VEVENT
BEGIN:VEVENT
UID:news-813@dmi.unibas.ch
DTSTAMP:20190212T213354
DTSTART;TZID=Europe/Zurich:20091016T000000
DTEND;TZID=Europe/Zurich:20091016T000000
SUMMARY:Seminar Algebra and Geometry: Stéphane Vénéreau (Basel University)
LOCATION:
DESCRIPTION:On the Bhatwadekar-Dutta-Berson-Vénéreau Polynomials
END:VEVENT
BEGIN:VEVENT
UID:news-812@dmi.unibas.ch
DTSTAMP:20190212T213139
DTSTART;TZID=Europe/Zurich:20091009T000000
DTEND;TZID=Europe/Zurich:20091009T000000
SUMMARY:Seminar Algebra and Geometry: Jonas Budmiger (Basel University)
LOCATION:
DESCRIPTION:An open problem on the coordinate ring of a variety with group
action
END:VEVENT
BEGIN:VEVENT
UID:news-811@dmi.unibas.ch
DTSTAMP:20190212T213026
DTSTART;TZID=Europe/Zurich:20091002T000000
DTEND;TZID=Europe/Zurich:20091002T000000
SUMMARY:Seminar Algebra and Geometry: Jacopo Gandini (University of Rome 1)
LOCATION:
DESCRIPTION:Spherical orbit closures in simple projective spaces and their
normalization
END:VEVENT
BEGIN:VEVENT
UID:news-810@dmi.unibas.ch
DTSTAMP:20190212T212851
DTSTART;TZID=Europe/Zurich:20090925T000000
DTEND;TZID=Europe/Zurich:20090925T000000
SUMMARY:Seminar Algebra and Geometry: Matey Mateev (Humboldt-Universität zu Berlin)
LOCATION:
DESCRIPTION:A sufficient criterion for the asymptotic stability of depths o
f ideal transformed Rees-modules
END:VEVENT
END:VCALENDAR