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BEGIN:VEVENT
SUMMARY:Andrea Seppi (Université de Grenoble)
DTSTART:20200415T120000Z
DTEND:20200415T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/1
DESCRIPTION:Title: Examples of four-dimensional geometric transition\nby Andrea
Seppi (Université de Grenoble) as part of Sapienza A&G Seminar\n\n\nAbstr
act\nRoughly speaking\, a geometric transition is a deformation of geometr
ic structures on a manifold\, by “transitioning” between different geo
metries. Danciger introduced a new such transition\, which enables to defo
rm from hyperbolic structures to Anti-de Sitter structure\, going through
another type of real projective structures called “half-pipe”\, and pr
ovided conditions for a compact 3-manifold to admit a geometric transition
of this type. By extending a construction of Kerckhoff and Storm\, I will
describe examples of finite-volume geometric transition in dimension 4.\n
\nThis is joint work with Stefano Riolo.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/1/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Ralf Köhl (Universität Giessen)
DTSTART:20200422T120000Z
DTEND:20200422T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/2
DESCRIPTION:Title: The geometry of Kac-Moody groups\nby Ralf Köhl (Universität
Giessen) as part of Sapienza A&G Seminar\n\n\nAbstract\nKac-Moody groups
enjoy very many nice geometric properties \nvia their action on twin build
ings\, on generalized flag manifolds\, and \non highest-weight modules end
owed with an anisotropic form.\n\nIn this talk I want to topological Kac-M
oody groups by comparing their \nproperties to semisimple Lie groups. I wi
ll introduce symmetric spaces \nfor Kac-Moody groups\, I will compute the
fundamental group of \ntopological Kac-Moody groups\, and I will give evid
ence for Kostant \nconvexity.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/2/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Laura Pertusi (Università statale di Milano)
DTSTART:20200429T120000Z
DTEND:20200429T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/3
DESCRIPTION:Title: Stability conditions and moduli spaces for Kuznetsov components o
f Gushel-Mukai varieties\nby Laura Pertusi (Università statale di Mil
ano) as part of Sapienza A&G Seminar\n\n\nAbstract\nA generic Gushel-Mukai
variety X is a quadric section of a linear section of the Grassmannian Gr
(2\,5). Kuznetsov and Perry proved that the bounded derived category of X
has a semiorthogonal decomposition with exceptional objects and a non-triv
ial subcategory Ku(X)\, known as the Kuznetsov component. In this talk we
will discuss the construction of stability conditions on Ku(X) and\, conse
quently\, on the bounded derived category of X. As applications\, for X of
even-dimension\, we will construct locally complete families of hyperkaeh
ler manifolds from moduli spaces of stable objects in Ku(X) and we will de
termine when X has a homological associated K3 surface.\n\nThis is a joint
work with Alex Perry and Xiaolei Zhao.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/3/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Alexander Lytchak (Universität Köln)
DTSTART:20200520T120000Z
DTEND:20200520T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/4
DESCRIPTION:Title: Structure of non-positively curved spaces\nby Alexander Lytch
ak (Universität Köln) as part of Sapienza A&G Seminar\n\n\nAbstract\nIn
the talk I would like to discuss local geometric\, analytic and topologica
l structure of spaces with upper curvature bounds and extendible geodesics
.\nThe talk will be based on joint work with Koichi Nagano.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/4/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Michele D’Adderio (Université libre de Bruxelles)
DTSTART:20200527T120000Z
DTEND:20200527T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/5
DESCRIPTION:Title: Partial and global representations of finite groups\nby Miche
le D’Adderio (Université libre de Bruxelles) as part of Sapienza A&G Se
minar\n\n\nAbstract\nThe notions of partial actions and partial representa
tions have been extensively studied in several algebraic contexts in the l
ast 25 years.\nIn this talk we introduce these concepts and give a short o
verview of the results known for finite groups. We will briefly show how t
his theory extends naturally the classical global theory\, in particular i
n the important case of the symmetric group.\nThis is joint work with Will
iam Hautekiet\, Paolo Saracco and Joost Vercruysse.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/5/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Marco Boggi (UFMG Belo Horizonte)
DTSTART:20200506T120000Z
DTEND:20200506T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/6
DESCRIPTION:Title: Automorphisms of procongruence mapping class groups\nby Marco
Boggi (UFMG Belo Horizonte) as part of Sapienza A&G Seminar\n\n\nAbstract
\nIn this talk\, I will discuss the automorphism group of the procongruenc
e mapping class group and of the associated procongruence curve and pants
complexes. In analogy with a classical result of Ivanov for mapping class
groups\, this allows to determine the group of automorphisms of the $arith
metic$ procongruence mapping class group which satisfy a natural geometric
condition. It is a nontrivial fact that this condition holds in genus $0$
. Let $\\mathcal{M}_{0\,n}$ be the moduli space of $n$-labeled\, genus $0$
algebraic curves. It follows\, in particular\, that $\\mathrm{Out}(\\pi_1
^\\mathrm{et}(\\mathcal{M}_{0\,n}\\otimes\\Q))\\cong\\Sigma_n$ for $n\\geq
5.\\newline$\nThis talk is based on a joint work with Louis Funar and Pie
rre Lochak (cf. $\\texttt{arXiv:2004.04135}$).\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/6/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Cristiano Spotti (Aarhus Universitet)
DTSTART:20200603T120000Z
DTEND:20200603T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/7
DESCRIPTION:Title: On log Kähler-Einstein metrics\nby Cristiano Spotti (Aarhus
Universitet) as part of Sapienza A&G Seminar\n\n\nAbstract\nIn this talk I
will discuss examples and some geometric properties of KE metrics with co
ne angle singularities along possibly singular (in general worse than norm
al crossing) divisors.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/7/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Karen Vogtmann (University of Warwick)
DTSTART:20200617T120000Z
DTEND:20200617T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/8
DESCRIPTION:Title: Outer space for RAAGs\nby Karen Vogtmann (University of Warwi
ck) as part of Sapienza A&G Seminar\n\n\nAbstract\nIn recent years right-a
ngled Artin groups (RAAGs) have assumed a prominent place in geometric gro
up theory and related fields\,\nNow attention is also being focused on the
ir (outer) automorphism groups. The most well-understood examples of oute
r automorphism groups of RAAGs are $\\mathbf{GL}(n\,\\mathbb{Z})$ and $\\m
athbf{Out}(F_n)$. $\\mathbf{GL}(n\,\\mathbb{Z})$ can be profitably studie
d via its action on the symmetric space of marked lattices\, and $\\mathbf
{Out}(F_n)$ via its action on Outer space. We propose an analogous ``oute
r space'' generalizing both of these\, on which the outer automorphism gro
up of an arbitrary RAAG acts properly\, and we prove this space is contrac
tible. \n\nThis is joint work with Corey Bregman and Ruth Charney.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/8/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Marco Maculan (Paris Sorbonne)
DTSTART:20200513T120000Z
DTEND:20200513T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/9
DESCRIPTION:Title: Affine vs. Stein varieties in complex and rigid geometry\nby
Marco Maculan (Paris Sorbonne) as part of Sapienza A&G Seminar\n\n\nAbstra
ct\nSerre’s GAGA theorem states that\, on a projective complex variety\,
holomorphic objects (functions\, vector bundle and their sections\, etc.)
are algebraic. Without compactness hypothesis this is not true. Yet\, one
may wonder whether a variety that can be embedded holomorphically into an
affine space\, can be embedded therein algebraically. A classical example
of Serre shows that the answer is negative.\n\nIn an ongoing joint work w
ith J. Poineau\, we investigate what happens when one replaces the complex
numbers by the p-adic ones. Despite the formal similarities between the c
orresponding analytic theories\, the p-adic outcome is somewhat surprising
.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/9/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Valentino Tosatti (Northwestern University)
DTSTART:20200610T143000Z
DTEND:20200610T153000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/10
DESCRIPTION:Title: Metric limits of Calabi-Yau manifolds\nby Valentino Tosatti
(Northwestern University) as part of Sapienza A&G Seminar\n\n\nAbstract\nI
will discuss the problem of understanding the behavior of degenerating fa
milies of Ricci-flat Kähler metrics on a Calabi-Yau manifold\, and what t
heir possible metric limits are. I will explain what we know in general ab
out such metric limits\, what techniques are used to approach these questi
ons\, and what applications these results have. \n\nBased on joint works w
ith Gross-Zhang and with Hein.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/10/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Bojko Bakalov (North Carolina State University)
DTSTART:20200624T120000Z
DTEND:20200624T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/11
DESCRIPTION:Title: A vertex algebra construction of representations of toroidal Lie
algebras\nby Bojko Bakalov (North Carolina State University) as part
of Sapienza A&G Seminar\n\n\nAbstract\nGiven a simple finite-dimensional L
ie algebra and an automorphism of finite order\, one can construct a twist
ed toroidal Lie algebra.\nSimilarly to twisted affine Lie algebras\, which
are well-studied in the literature\, we can create representations of twi
sted toroidal Lie algebras with the help of vertex algebras. In this talk\
, I will discuss twisted modules of vertex algebras and will show how repr
esentations of twisted toroidal Lie algebras can be constructed from such
twisted modules.\n\nJoint work with Samantha Kirk.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/11/
END:VEVENT
BEGIN:VEVENT
SUMMARY:J.-P. Demailly (Université Grenoble Alpes)
DTSTART:20201007T143000Z
DTEND:20201007T153000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/12
DESCRIPTION:Title: Hermitian-Yang-Mills approach to the conjecture of Griffiths on
the positivity of ample vector bundles\nby J.-P. Demailly (Université
Grenoble Alpes) as part of Sapienza A&G Seminar\n\n\nAbstract\nGiven a ve
ctor bundle of arbitrary rank with ample determinant line bundle on a proj
ective manifold\, we propose a new elliptic system of differential equatio
ns of Hermitian-Yang-Mills type for the curvature tensor.\nThe system is d
esigned so that solutions provide Hermitian metrics with positive curvatur
e in the sense of Griffiths – and even in the dual Nakano sense. As a co
nsequence\, if an existence result could be obtained for every ample vecto
r bundle\, the Griffiths conjecture on the equivalence between ampleness a
nd positivity of vector bundles would be settled. Another outcome of the a
pproach is a new concept of volume for vector bundles.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/12/
END:VEVENT
BEGIN:VEVENT
SUMMARY:D. Greb (Universität Duisburg-Essen)
DTSTART:20201021T120000Z
DTEND:20201021T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/13
DESCRIPTION:Title: Projective flatness over klt spaces and uniformisation of variet
ies with nef anti-canonical divisor\nby D. Greb (Universität Duisburg
-Essen) as part of Sapienza A&G Seminar\n\n\nAbstract\nI will discuss a cr
iterion for the projectivisation of a reflexive sheaf on a klt space to be
induced by a projective representation of the fundamental group of the sm
ooth locus. This criterion is then applied to give a characterisation of f
inite quotients of projective spaces and Abelian varieties by Q-Chern clas
s (in)equalities and a suitable stability condition. This stability condit
ion is formulated in terms of a naturally defined extension of the tangent
sheaf by the structure sheaf. I will further examine cases in which this
stability condition is satisfied\, comparing it to K-semistability and rel
ated notions. \nThis is joint work with Stefan Kebekus and Thomas Peternel
l.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/13/
END:VEVENT
BEGIN:VEVENT
SUMMARY:I. Bauer (Universität Bayreuth)
DTSTART:20201028T130000Z
DTEND:20201028T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/14
DESCRIPTION:Title: Rigid compact complex manifolds: recent results\, questions and
conjectures\nby I. Bauer (Universität Bayreuth) as part of Sapienza A
&G Seminar\n\n\nAbstract\nThe aim of this talk is to give an update on rec
ent achievements and developments on rigid compact complex manifolds. I wi
ll start introducing different notions of rigidity and explaining the rela
tions among them. For curves all these notions coincide and the only rigid
curve is the projective line. For surfaces rigidity is still quite „rar
e“\, since the only rigid surfaces which are not minimal of general type
are Del Pezzo surfaces of degree at least 5 and Inoue surfaces. \nIn high
er dimension the geography of rigid manifolds gets much richer. \nThen I w
ill report on an answer to a more than 40 years open question of Morrow an
d Kodaira\, exhibiting an infinite series of rigid but not infinitesimally
rigid surfaces of general type. \nI will conclude addressing open questio
ns and conjectures.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/14/
END:VEVENT
BEGIN:VEVENT
SUMMARY:B. Hanke (Universität Augsburg)
DTSTART:20201104T130000Z
DTEND:20201104T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/15
DESCRIPTION:Title: Scalar positive immersions\nby B. Hanke (Universität Augsbu
rg) as part of Sapienza A&G Seminar\n\n\nAbstract\nAs shown by Gromov-Laws
on and Stolz the only obstruction to the existence of positive scalar curv
ature metrics on closed simply connected manifolds in dimensions at least
five appears on spin manifolds\, and is given by the non-vanishing of the
α-genus of Hitchin.\nWhen unobstructed we shall realise a positive scalar
curvature metric by an immersion into Euclidean space whose dimension is
uniformly close to the classical Whitney upper bound for smooth immersions
. \nOur main tool is an extrinsic counterpart of the well-known Gromov-Law
son surgery procedure for constructing positive scalar curvature metrics.
At this point we use the local flexibility lemma proven by Christian Bär
and the speaker. \nThis is joint work with Luis Florit\, IMPA (Rio de Jane
iro).\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/15/
END:VEVENT
BEGIN:VEVENT
SUMMARY:G. Röhrle (Ruhr-Universität Bochum)
DTSTART:20201014T120000Z
DTEND:20201014T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/16
DESCRIPTION:Title: On a relative version of Serre's notion of G-complete reducibili
ty\nby G. Röhrle (Ruhr-Universität Bochum) as part of Sapienza A&G S
eminar\n\n\nAbstract\nWe first review some basic results related to Serre'
s notion of G-complete reducibility for a reductive algebraic group G. We
then discuss a relative variant of this concept where we let K be a reduct
ive subgroup of G\, and consider subgroups of G which normalise the identi
ty component $K^\\circ$ of K. We show that such a subgroup is relatively G
-completely reducible with respect to K if and only if its image in the au
tomorphism group of $K^\\circ$ is completely reducible in the sense of Ser
re. This allows us to generalise a number of fundamental results from the
absolute to the relative setting. This is a report on recent joint work wi
th M. Gruchot and A. Litterick.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/16/
END:VEVENT
BEGIN:VEVENT
SUMMARY:I. van Santen (Universität Basel)
DTSTART:20201111T130000Z
DTEND:20201111T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/17
DESCRIPTION:Title: Embeddings of smooth affine varieties into algebraic groups\
nby I. van Santen (Universität Basel) as part of Sapienza A&G Seminar\n\n
\nAbstract\nThis is joint work with Peter Feller. In any category there ar
e the following fundamental problems concerning embeddings from an object
Z into another object X:\n\n(Existence) Does there exist an embedding of Z
into X? \n(Uniqueness) Having two embeddings f\, g of Z into X\, does
there exists an automorphism ψ of X such that g = ψ ◦ f?\n\nIn thi
s talk\, we will mainly focus on the first problem in the category of affi
ne varieties\, where Z is smooth and X is an algebraic group. Amongst othe
r things\, we will discuss the following result.\n\nTheorem. For every sim
ple algebraic group G and every smooth affine variety Z with dim G > 2dim
Z + 1\, there exists an embedding of Z into G.\n\nThe proof is based upon
parametric transversality results for flexible affine varieties due to Kal
iman. We will also discuss the following result\, which implies the optima
lity of the above existence result up to a possible improvement of the dim
ension bound by one. It’s proof is an adaptation of a Chow-group-based a
rgument due to Bloch\, Murthy\, and Szpiro for the affine space.\n\nPropos
ition. For every non-finite algebraic group G and every d ≥ dim G / 2\,
there exists an irreducible smooth affine variety of dimension d that does
not admit an embedding into G.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/17/
END:VEVENT
BEGIN:VEVENT
SUMMARY:J. Draisma (Universität Bern)
DTSTART:20201118T130000Z
DTEND:20201118T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/18
DESCRIPTION:Title: The geometry of polynomial functors\nby J. Draisma (Universi
tät Bern) as part of Sapienza A&G Seminar\n\n\nAbstract\nA polynomial fun
ctor P is a functor from the category of finite-dimensional vector spaces
to itself such that for every U\,V the map Hom(U\,V) -> Hom(P(U)\,P(V)) is
polynomial. In characteristic zero\, P is a direct sum of Schur functors.
\n\nThis talk concerns closed subsets of such P\, i.e.\, rules that assign
to a vector space V a closed subset X(V) of P(V) such that P(phi)X(U) is
contained in X(V) for every linear map phi:U -> V.\n\nQuite surprisingly\,
these behave very much like finite-dimensional affine varieties. For inst
ance\, they satisfy the descending chain condition and a version of Cheval
ley's theorem on constructible sets. I will discuss these results and more
. The talk is based on joint work with Arthur Bik\, Rob Eggermont\, and An
drew Snowden.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/18/
END:VEVENT
BEGIN:VEVENT
SUMMARY:C. Araujo (IMPA)
DTSTART:20201125T130000Z
DTEND:20201125T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/19
DESCRIPTION:Title: Birational geometry of Calabi-Yau pairs and 3-dimensional Cremon
a transformations\nby C. Araujo (IMPA) as part of Sapienza A&G Seminar
\n\n\nAbstract\nRecently\, Oguiso addressed the following question\, attri
buted to Gizatullin: "Which automorphisms of a smooth quartic K3 surface $
D\\subset \\mathbb P^3$ are induced by Cremona transformations of the ambi
ent space $\\mathbb{P}^3$?'' When $D\\subset \\mathbb{P}^3$ is a smooth q
uartic surface\, $(\\mathbb{P}^3\,D)$ is an example of a Calabi-Yau pair\,
that is\, a pair $(X\,D)$\, consisting of a normal projective variety $X$
and an effective Weil divisor $D$ on $X$ such that $K_X+D\\sim 0$. The ab
ove question is really about birational properties of the Calabi-Yau pair
$(\\mathbb{P}^3\,D)$. In this talk\, I will explain a general framework to
study the birational geometry of mildly singular Calabi-Yau pairs. Then I
will focus on the case of singular quartic surfaces $D\\subset \\mathbb{P
}^3$. Our results illustrate how the appearance of increasingly worse sing
ularities in $D$ enriches the birational geometry of the pair $(\\mathbb{P
}^3\, D)$\, and lead to interesting subgroups of the Cremona group of $\\m
athbb{P}^3$. This is joint work with Alessio Corti and Alex Massarenti.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/19/
END:VEVENT
BEGIN:VEVENT
SUMMARY:E. Sommers (University of Massachusetts Amherst)
DTSTART:20201209T130000Z
DTEND:20201209T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/20
DESCRIPTION:Title: The geometry of nilpotent varieties via subbundles of the cotang
ent bundle\nby E. Sommers (University of Massachusetts Amherst) as par
t of Sapienza A&G Seminar\n\n\nAbstract\nLet $G$ be a simple algebraic gro
up with flag variety $G/B$. \nThe Springer resolution is the moment map fr
om the cotangent bundle of $G/B$ to the (dual of the) Lie algebra $\\mathf
rak{g}$ of $G$. The cohomology of the fibers of this map play an importan
t role in the representation theory of $G$ over various fields.\n\nIdentif
y the cotangent bundle with the vector bundle $G \\times^B \\mathfrak{n}$\
, where $\\mathfrak{n}$ is the nilradical of the Lie algebra of $B$. Ther
e are subbundles $G \\times^B I$ for each subspace $I \\subset \\mathfrak{
n}$ that is $B$-stable\, and maps $p_I: G \\times^B I \\to \\mathfrak{g}$.
The fibers of $p_I$ are also interesting and their cohomology relates t
o an intersection cohomology complex on the image of $p_I$\, a nilpotent v
ariety. \n\nIn this talk we discuss two topics: (1) methods for computing
the cohomology of the fibers of $p_I$\; (2) a vanishing theorem/conjectur
e for the cohomology of the structure sheaf on these subbundles.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/20/
END:VEVENT
BEGIN:VEVENT
SUMMARY:G. Carron (Université de Nantes)
DTSTART:20201216T130000Z
DTEND:20201216T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/21
DESCRIPTION:Title: A Bonnet-Myers Theorem from a spectral assumption\nby G. Car
ron (Université de Nantes) as part of Sapienza A&G Seminar\n\n\nAbstract\
nWe obtain a finiteness result for the fundamental group of a closed Riema
nnian manifold $(M^n\,g)$ under the assumption that the Schrödinger opera
tor $\\Delta_g+(n-2)/\\rho$ is positive (where at $x\\in M$\, $\\rho(x)$ i
s the lowest eigenvalue of the Ricci tensor at $x$).\nIt is a joint work w
ith C. Rose (MPI Leipzig).\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/21/
END:VEVENT
BEGIN:VEVENT
SUMMARY:E. Floris (Université de Poitiers)
DTSTART:20210120T130000Z
DTEND:20210120T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/22
DESCRIPTION:Title: Connected algebraic groups acting on Fano fibrations over P^1\nby E. Floris (Université de Poitiers) as part of Sapienza A&G Seminar\
n\n\nAbstract\nLet $G$ be a connected algebraic group and $X$ a variety en
dowed with a regular action of $G$ and a Mori fibre space $X/\\mathbb P^1$
whose fibre is a Fano variety of Picard rank at least 2.\nIn this talk I
will explain why there is a proper horizontal subvariety of $X$ which is i
nvariant under the action of $G$\, alongside with some applications of thi
s result to the classification of connected algebraic subgroups of the Cre
mona group in dimension 4.\nThis is a joint work with Jérémy Blanc.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/22/
END:VEVENT
BEGIN:VEVENT
SUMMARY:K. Baur (University of Leeds)
DTSTART:20210113T130000Z
DTEND:20210113T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/23
DESCRIPTION:Title: Cluster structures for Grassmannians\nby K. Baur (University
of Leeds) as part of Sapienza A&G Seminar\n\n\nAbstract\nThe coordinate r
ing of the Grassmannian has the structure of a cluster algebra. On the oth
er side\, the category of maximal CM modules over a certain infinite dimen
sional algebra Is a cluster category associated to this cluster algebra st
ructure. We study this category\, in particular in the tame cases. We also
show how to associate frieze patterns to these cluster structures.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/23/
END:VEVENT
BEGIN:VEVENT
SUMMARY:J. Weyman (Jagiellonian University in Kraków)
DTSTART:20210127T130000Z
DTEND:20210127T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/24
DESCRIPTION:Title: Finite Free Resolutions and opposite Schubert varieties\nby
J. Weyman (Jagiellonian University in Kraków) as part of Sapienza A&G Sem
inar\n\n\nAbstract\nIn the first part of this talk I will give an update o
n the connection between perfect ideals of codimension 3 and Schubert vari
eties of exceptional groups (and more generally opposite Schubert varietie
s for Kac-Moody groups associated to T-shaped graphs $T_{pqr}$). I will al
so discuss a parallel theory which points to similar connection between Go
renstein ideals of codimension 4 with n generators and opposite Schubert v
arieties in homogeneous spaces related to a Kac-Moody group of type $\\mat
hsf E_n$.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/24/
END:VEVENT
BEGIN:VEVENT
SUMMARY:V. Balaji (Chennai Mathematical Institute)
DTSTART:20210203T130000Z
DTEND:20210203T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/25
DESCRIPTION:Title: On a "wonderful" Bruhat-Tits group scheme\nby V. Balaji (Che
nnai Mathematical Institute) as part of Sapienza A&G Seminar\n\n\nAbstract
\nWe make a universal construction of Bruhat-Tits group scheme on wonderfu
l embeddings of adjoint groups in the absolute and relative settings of ad
joint Kac-Moody groups. These group schemes have natural classifying prope
rties reflecting the orbit structure on the wonderful embeddings. A theme
which is reflected in the work is that the various wonderful embeddings gi
ve algebra-geometric realizations of the different kinds of buildings. Thi
s shows an intimate relationship between the wonderful embeddings and Bruh
at-Tits theory.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/25/
END:VEVENT
BEGIN:VEVENT
SUMMARY:A. Kuznetsov (Steklov Mathematical Institute)
DTSTART:20210210T130000Z
DTEND:20210210T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/26
DESCRIPTION:Title: Lefschetz collections and residual categories for Grassmannians<
/a>\nby A. Kuznetsov (Steklov Mathematical Institute) as part of Sapienza
A&G Seminar\n\n\nAbstract\nI will try to explain and motivate the notion o
f Lefschetz (exceptional) collections in derived categories of coherent sh
eaves and their residual categories and\, in particular\, its conjectural
relation to quantum cohomology. If time permits\, I will discuss recent pr
ogress in construction of Lefschetz collections for Grassmannians of class
ical groups. This is joint work with Maxim Smirnov.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/26/
END:VEVENT
BEGIN:VEVENT
SUMMARY:M. Jonsson (University of Michigan)
DTSTART:20210217T130000Z
DTEND:20210217T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/27
DESCRIPTION:Title: Filtrations on section rings and non-Archimedean geometry\nb
y M. Jonsson (University of Michigan) as part of Sapienza A&G Seminar\n\n\
nAbstract\nTo any polarized variety (X\,L) is associated a section ring R.
I will explain the relation between suitable classes of norms on R and fu
nctions on the Berkovich analytification of X. Time permitting\, I will di
scuss applications of this to K-stability. This is joint work with S. Bouc
ksom.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/27/
END:VEVENT
BEGIN:VEVENT
SUMMARY:R. Wentworth (University of Maryland)
DTSTART:20210303T130000Z
DTEND:20210303T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/28
DESCRIPTION:Title: Conformal limits of parabolic Higgs bundles\nby R. Wentworth
(University of Maryland) as part of Sapienza A&G Seminar\n\n\nAbstract\nG
aiotto introduced the notion of a conformal limit of a Higgs bundle and co
njectured that these should\nidentify the Hitchin component with the Oper
stratum in the deRham moduli space. \nIn the case of closed Riemann surfac
es this result was proven by Dumitrescu\, et al.\, and the limits were sho
wn to exist much more generally by Collier and the speaker. \nIn this talk
I will report on progress in the case of parabolic Higgs bundles\, which
were the context of Gaiotto's original conjecture. \nThis is joint work wi
th B. Collier and L. Fredrickson.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/28/
END:VEVENT
BEGIN:VEVENT
SUMMARY:F. Rochon (Université du Québec à Montréal)
DTSTART:20210324T130000Z
DTEND:20210324T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/29
DESCRIPTION:Title: L^2-cohomology of quasi-fibered boundary metrics\nby F. Roch
on (Université du Québec à Montréal) as part of Sapienza A&G Seminar\n
\n\nAbstract\nQuasi-fibered boundary metrics (QFB metrics) form a class of
complete metrics generalizing the class of quasi-asymptotically locally E
uclidean metrics introduced by Joyce. \nAfter reviewing what QFB metrics
are and presenting natural examples\, we will present a new approach to co
mpute the L^2-cohomology of such metrics. \nThis is based on a joint work
with Chris Kottke.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/29/
END:VEVENT
BEGIN:VEVENT
SUMMARY:J. Martens (University of Edinburgh)
DTSTART:20210407T120000Z
DTEND:20210407T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/30
DESCRIPTION:Title: Sporadic behaviour of quantum representations and rank-level dua
lity\nby J. Martens (University of Edinburgh) as part of Sapienza A&G
Seminar\n\n\nAbstract\nThe theory of conformal blocks provides us with pro
jective representations of the mapping class group. These can equivalently
also be constructed from the point of view of non-abelian theta functions
\, via the Hitchin connection. It has been known for some time that\, for
SL(2)\, these representations have infinite order\, with the exception of
some sporadic low levels. We will discuss how some of these sporadic cas
es can be understood via rank-level duality.\nThis is joint work with Baie
r\, Bolognesi and Pauly.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/30/
END:VEVENT
BEGIN:VEVENT
SUMMARY:C. Procesi (Sapienza Università di Roma)
DTSTART:20210224T130000Z
DTEND:20210224T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/31
DESCRIPTION:Title: A note on the Weingarten funtion\nby C. Procesi (Sapienza Un
iversità di Roma) as part of Sapienza A&G Seminar\n\n\nAbstract\nI will c
ompare work of Formanek on a certain construction of central polynomials w
ith that of Collins on integration on unitary groups. These two quite disj
oint topics share the construction of the same function on the symmetric g
roup\, which the second author calls Weingarten function. By joining these
two approaches we succeed in giving a simplified and very natural present
ation of both Formanek and Collins's Theory.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/31/
END:VEVENT
BEGIN:VEVENT
SUMMARY:D. Alessandrini (Columbia University)
DTSTART:20210310T130000Z
DTEND:20210310T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/32
DESCRIPTION:Title: Non commutative cluster coordinates for Higher Teichmüller Spac
es\nby D. Alessandrini (Columbia University) as part of Sapienza A&G S
eminar\n\n\nAbstract\nIn higher Teichmuller theory we study subsets of the
character varieties of surface groups that are higher rank analogs of Tei
chmuller spaces\, e.g. the Hitchin components and the spaces of maximal re
presentations.\nFock-Goncharov generalized Thurston's shear coordinates an
d Penner's Lambda-lengths to the Hitchin components\, showing that they ha
ve a beautiful structure of cluster variety.\nHere we apply similar ideas
to Maximal Representations and we find new coordinates on these spaces tha
t give them a structure of non-commutative cluster varieties\, in the sens
e defined by Berenstein-Rethak.\nThis is joint work with Guichard\, Rogozi
nnikov and Wienhard.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/32/
END:VEVENT
BEGIN:VEVENT
SUMMARY:S. Scherotzke (Université du Luxembourg)
DTSTART:20210414T120000Z
DTEND:20210414T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/33
DESCRIPTION:Title: The categorified Grothendieck-Riemann-Roch Theorem\nby S. Sc
herotzke (Université du Luxembourg) as part of Sapienza A&G Seminar\n\n\n
Abstract\nThe Chern character is a central construction which appears in t
opology\, representation theory and algebraic geometry. \nIn algebraic top
ology it is for instance used to probe algebraic K-theory which is notorio
usly hard to compute\, in representation theory it takes the form of class
ical character theory. Recently\, Toen and Vezzosi suggested a constructio
n\, using derived algebraic geometry\, which allows to unify the various C
hern characters. \nWe will categorify this Chern character. In the categor
ified picture algebraic K-theory is replaced by the category of non-commut
ative motives. It turns out that the categorified Chern character has many
interesting applications. For instance we show that the DeRham realisatio
n functor is of non-commutative origin.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/33/
END:VEVENT
BEGIN:VEVENT
SUMMARY:D. Testerman (École polytechnique fédérale de Lausanne)
DTSTART:20210428T120000Z
DTEND:20210428T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/34
DESCRIPTION:Title: Overgroups of regular unipotent elements in simple algebraic gro
ups\nby D. Testerman (École polytechnique fédérale de Lausanne) as
part of Sapienza A&G Seminar\n\n\nAbstract\nWe consider the question of de
termining reductive overgroups of regular unipotent elements in simple alg
ebraic groups and in particular give a condition which guarantees that the
overgroup does not lie in a proper parabolic subgroup of the ambient simp
le group. This is joint work with Gunter Malle\, and follows on work of Su
prunenko\, Saxl and Seitz\, and Testerman and Zalesski.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/34/
END:VEVENT
BEGIN:VEVENT
SUMMARY:R. Schoof (Università di Roma "Tor Vergata")
DTSTART:20210512T120000Z
DTEND:20210512T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/35
DESCRIPTION:Title: Finite flat group schemes over Z\nby R. Schoof (Università
di Roma "Tor Vergata") as part of Sapienza A&G Seminar\n\n\nAbstract\nFini
te flat group schemes are important in number theory.\nWe explain what we
do and don't know about their structure\nover rings of integers of number
fields\, in particular over $\\mathbb Z$.\nThis is joint work with Lassina
Dembele.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/35/
END:VEVENT
BEGIN:VEVENT
SUMMARY:A. Lonjou (Université Paris-Saclay)
DTSTART:20210317T130000Z
DTEND:20210317T140000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/36
DESCRIPTION:Title: Actions of Cremona groups on CAT(0) cube complexes\nby A. Lo
njou (Université Paris-Saclay) as part of Sapienza A&G Seminar\n\n\nAbstr
act\nA key tool to study the plane Cremona group is its action on a hyperb
olic space. Sadly\, in higher rank such an action is not available. \nRece
ntly\, in geometric group theory\, actions on CAT(0) cube complexes turned
out to be a powerful tool to study a large class of groups. \nIn this tal
k\, based on a common work with Christian Urech\, we will construct such c
omplexes on which Cremona groups of rank n act. Then\, we will see which k
ind of results on these groups we can obtain.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/36/
END:VEVENT
BEGIN:VEVENT
SUMMARY:M. Păun (Universität Bayreuth)
DTSTART:20210616T120000Z
DTEND:20210616T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/37
DESCRIPTION:Title: On extension of twisted canonical forms defined on the central f
iber of a Kähler family\nby M. Păun (Universität Bayreuth) as part
of Sapienza A&G Seminar\n\n\nAbstract\nWe will report on a joint work with
J. Cao. \nOur main result establishes the extension of twisted canonical
forms defined on an infinitesimal neighborhood of the central fiber of a K
ahler family under a natural $L^2$ hypothesis.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/37/
END:VEVENT
BEGIN:VEVENT
SUMMARY:A. Wienhard (Universität Heidelberg)
DTSTART:20210623T123000Z
DTEND:20210623T133000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/38
DESCRIPTION:Title: Where geometry meets dynamics: groups\, entropy and Hausdorff di
mension\nby A. Wienhard (Universität Heidelberg) as part of Sapienza
A&G Seminar\n\n\nAbstract\nI will discuss geometric and dynamical properti
es of actions of discrete groups on Riemannian symmetric spaces. \nI will
highlight some aspects of the interplay between geometry and dynamics\, an
d present some recent results which generalize theorems of Sullivan\, Brid
geman-Taylor\, McMullen for convex cocompact subgroups acting on hyperboli
c space in the framework of discrete subgroups of Lie groups of higher ran
k.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/38/
END:VEVENT
BEGIN:VEVENT
SUMMARY:J. Kollár (Princeton University)
DTSTART:20210630T120000Z
DTEND:20210630T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/39
DESCRIPTION:Title: Deformations of varieties of general type\nby J. Kollár (Pr
inceton University) as part of Sapienza A&G Seminar\n\n\nAbstract\nWe prov
e that small deformations of a projective variety of general type are also
projective varieties of general type\, with the same plurigenera.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/39/
END:VEVENT
BEGIN:VEVENT
SUMMARY:E. Rousseau (Aix-Marseille Université)
DTSTART:20210519T120000Z
DTEND:20210519T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/40
DESCRIPTION:Title: Numerically special varieties\nby E. Rousseau (Aix-Marseille
Université) as part of Sapienza A&G Seminar\n\n\nAbstract\nCampana intro
duced the class of special varieties as the varieties admitting no maps on
to an orbifold of general type. They are also characterized by the non-exi
stence of Bogomolov sheaves which are rank one coherent subsheaves of maxi
mal Kodaira dimension in some exterior power of the cotangent bundle. \nCa
mpana has conjectured that one can replace the Kodaira dimension by the nu
merical dimension in this characterization. \nWe prove partially this conj
ecture showing that a projective manifold admitting a rank one coherent su
bsheaf of the cotangent bundle with numerical dimension 1 is not special.
\nThis is a joint work with J.V. Pereira and F. Touzet.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/40/
END:VEVENT
BEGIN:VEVENT
SUMMARY:V. F. Zenobi (Sapienza Università di Roma)
DTSTART:20210421T120000Z
DTEND:20210421T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/41
DESCRIPTION:Title: Higher rho numbers and metrics of positive scalar curvature\
nby V. F. Zenobi (Sapienza Università di Roma) as part of Sapienza A&G Se
minar\n\n\nAbstract\nThe analytic surgery sequence is a long exact sequenc
e of K-theory groups which combines topological information (the K-homolog
y of manifolds)\, index theoretic information (the K-theory of group C*-al
gebras)\, and secondary index information (the analytic structure group of
Higson-Roe). We will see how to give a definition of terms based entirely
on algebras of pseudodifferential operators and their K-theory. We use th
is to systematically develop maps to an exact sequence of non-commutative
de Rham homology/cyclic homology. Via pairings with cyclic cohomology clas
ses\, this gives rise to new numeric secondary index invariants (higher rh
o numbers) with explicit formulas and calculation tools due to the compati
bility in the whole sequence. We use this for geometric applications. In p
articular\, we derive new information about the moduli space of Riemannian
metrics of positive scalar curvature\, where we give new lower bounds on
the number of its components.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/41/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Z. Patakfalvi (École polytechnique fédérale de Lausanne)
DTSTART:20210505T120000Z
DTEND:20210505T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/42
DESCRIPTION:Title: Replacing vanishing theorems in mixed characteristic and the Min
imal Model Program for 3-folds over excellent schemes\nby Z. Patakfalv
i (École polytechnique fédérale de Lausanne) as part of Sapienza A&G Se
minar\n\n\nAbstract\nKodaira and Kawamata-Viehweg vanishing is frequently
used to lift sections of adjoint bundles\, a crucial part of many argument
s in the classification theory of algebraic varieties\, notably in many pr
oofs of the Minimal Model Program. These vanishing theorems fail in mixed
characteristic situations\, for example for \n1.) proper\, flat schemes ov
er the p-adic numbers\, or \n2.) proper birational models of mixed charact
eristic local rings. \nI present a work that remedies this situation to so
me extent. In particular\, we are able to \na.) show Kodaira and Kawamata-
Viehweg vanishing in many situations\, \nb.) prove the 3-dimensional Mini
mal Model Program for excellent schemes\, and \nc.) draw geometric corolla
ries of point b.) to the existence of the moduli space of stable surfaces
in mixed characteristic. \nThis is a joint work with Bhargav Bhatt\, Linqu
an Ma\, Karl Schwede\, Kevin Tucker\, Joe Waldron and Jakub Witaszek.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/42/
END:VEVENT
BEGIN:VEVENT
SUMMARY:S. Riche (Université Clermont Auvergne)
DTSTART:20210526T120000Z
DTEND:20210526T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/43
DESCRIPTION:Title: Perverse sheaves on affine flag varieties\, geometry of the dual
group\, and representations\nby S. Riche (Université Clermont Auverg
ne) as part of Sapienza A&G Seminar\n\n\nAbstract\nThe description of regu
lar blocks of the category O of a complex semisimple Lie algebra in terms
of perverse sheaves on a flag variety has been a crucial tool for its stud
y\, and in particular for the proof of the Kazhdan-Lusztig character formu
la. This description has a conjectural analogue for representations of red
uctive groups over fields of positive characteristic\, as predicted by Fin
kelberg-Mirkovic\, which involves a category of perverse sheaves on the af
fine Grassmannian of the Langlands dual group\, with coefficients in a fie
ld of positive characteristic. In this talk I will present a work in progr
ess with Bezrukavnikov which we expect will lead to a proof of this conjec
ture. As a step towards this goal\, we obtain a description of tilting per
verse sheaves on the affine flag variety reminiscent of the corresponding
result for usual flag varieties due to Soergel.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/43/
END:VEVENT
BEGIN:VEVENT
SUMMARY:J. Ruppenthal (Bergische Universität Wuppertal)
DTSTART:20210609T120000Z
DTEND:20210609T130000Z
DTSTAMP:20260404T095653Z
UID:AGSeminar/44
DESCRIPTION:Title: Canonical sheaves at isolated canonical singularities\nby J.
Ruppenthal (Bergische Universität Wuppertal) as part of Sapienza A&G Sem
inar\n\n\nAbstract\nThe canonical line bundle and the corresponding canoni
cal sheaf\nbelong to the most important geometric/analytic objects\nassoci
ated to a complex manifold. They play a crucial role e.g.\nin classificati
on theory\, Serre duality or vanishing theorems.\nIf we consider singular
varieties instead of smooth manifolds\,\nthen there exist various possibil
ities to generalize the canonical sheaf\nto that setting. One can consider
for example the Grothendieck(-Barlet-Henkin-Passare)\ndualizing sheaf or
the Grauert-Riemenschneider L2-sheaf.\nIn this talk\, we will discuss anot
her possible generalization\, i.e.\,\nthe sheaf of L2 holomorphic n-forms
with a certain boundary\ncondition at the singular set. This sheaf is esse
ntial for L2-dbar-theory\non singular spaces\, but difficult to understand
. We will describe it\nexplicitly for isolated canonical Gorenstein singul
arities.\n
LOCATION:https://stable.researchseminars.org/talk/AGSeminar/44/
END:VEVENT
END:VCALENDAR