### Algebra-Number Theory Archives for Academic Year 2020

#### PolyMath14: Homogeneous length functions on groups

When: Tue, July 23, 2019 - 1:00pm
Where: Kirwan Hall 3206
Speaker: Apoorva Khare ( Indian Institute of Science (Bangalore), India) - http://www.math.iisc.ac.in/~khare/
Abstract: Consider the following three properties of a general group G:

(1) Algebra: G is abelian and torsion-free.

(2) Analysis: G is a metric space that admits a "norm", namely, a translation-invariant metric d(.,.) satisfying: d(1,g^n) = |n| d(1,g) for all g in G and integers n.

(3) Geometry: G admits a length function with "saturated" subadditivity for equal arguments: l(g^2) = 2 l(g) for all g in G.

While these properties may a priori seem different, in fact they turn out to be equivalent. The nontrivial implication amounts to saying that there does not exist a non-abelian group with a "norm".

We will discuss motivations from analysis, probability, and geometry; then the proof of the above equivalences; and finally, the logistics of how the problem was solved, via a PolyMath project that began on a blogpost of Terence Tao.

(Joint work - as D.H.J. PolyMath - with Tobias Fritz, Siddhartha Gadgil, Pace Nielsen, Lior Silberman, and Terence Tao.)

#### ANT/LGRT Organizational Meeting

When: Wed, August 28, 2019 - 2:00pm
Where: Kirwan Hall 3206
Speaker: NONE () -

#### Riemann-Roch for noncommutative complex tori

When: Wed, September 11, 2019 - 2:00pm
Where: Kirwan Hall 3206
Speaker: Jonathan Rosenberg (UMCP) -
Abstract: We discuss analogues of 3 important theorems about complex tori, C^n/L, L a lattice, for noncommutative complex tori (which we will define). The 3 basic theorems are the Riemann-Roch Theorem, the Hodge Theorem, and the characterization of when a complex torus is an abelian variety. This is joint work with V. Mathai.

#### Toroidal compacifications and incompressibility of exceptional congruence covers

When: Mon, October 7, 2019 - 2:00pm
Where: Kirwan Hall 3206
Speaker: Patrick Brosnan (UMD) - http://www2.math.umd.edu/~pbrosnan/
Abstract: Suppose a finite group G acts faithfully on an irreducible
variety X. We say that the G-variety X is compressible if there is a
dominant rational morphism from X to a faithful G-variety Y of
strictly smaller dimension. Otherwise we say that X is
incompressible. In a recent preprint, Farb, Kisin and Wolfson (FKW)
have proved the incompressibility of a large class of covers related
to the moduli space of principally polarized abelian varieties with
level structure. Their methods, which rely on the existence of
integral models for the moduli space Ag, apply to diverse examples
such as moduli spaces of curves and many Shimura varieties of Hodge
type. My talk will be about joint work with Fakhruddin and
Reichstein, where our goal is to recover some of the results of FKW
via the fixed point method from the theory of essential dimension.
More specifically, we prove incompressibilty for some Shimura
varieties by finding fixed points of finite abelian subgroups of G in
their toroidal compactifications. Our results are weaker than the
results of FKW for Hodge type Shimura varieties, because the methods
of FKW apply in cases where there is no boundary, while we need the
boundary to find the fixed points. However, our method has the
advantage of extending to many Shimura varieties which are not of
Hodge type, in particular, those associated to groups of type
E7. Moreover, by using Pink's extension of the Ash, Mumford, Rapoport
and Tai theory of toroidal compactifications to mixed Shimura
varieties, we are able to prove incompressibility for congruence
covers corresponding to certain universal famiiles: e.g., the universal
families of principally polarized abelian varieties.

#### Degeneracy locus formulas for amenable Weyl group elements

When: Wed, October 16, 2019 - 2:00pm
Where: Kirwan Hall 3206

Speaker: Harry Tamvakis, University of Maryland
Abstract: We define a class of amenable Weyl group elements in the Lie types B,
C, and D, which we propose as the analogues of vexillary permutations
in these Lie types. Our amenable signed permutations index flagged
theta and eta polynomials, which generalize the double theta and eta
polynomials of Wilson and the speaker. In geometry, we obtain
corresponding formulas for the cohomology classes of symplectic and
orthogonal degeneracy loci.

#### Delta-structures and canonical lifts in families

When: Mon, October 21, 2019 - 2:00pm
Where: Kirwan Hall 3206

Speaker: James Borger (ANU (Australia)) - https://maths-people.anu.edu.au/~borger/

Abstract: I'll explain recent joint work with Lance Gurney. We prove
that any family of ordinary abelian varieties parameterised by p-adic
formal scheme S lifts to a unique family over W(S) which admits a
delta-structure in the sense of Joyal, Buium, and Bousfield. In the
case where S is the spectrum of a perfect field of characteristic p,
this specialises to the classical result of Serre-Tate and Messing
that every ordinary abelian variety over a perfect field k lifts to a
unique one over the ring W(k) of Witt vectors together with a lift of
Frobenius.

#### On Gromov-Witten invariants of quintic threefolds

When: Mon, October 28, 2019 - 2:00pm
Where: Kirwan Hall 3206
Speaker: Felix Janda (University of Michigan ) - http://www-personal.umich.edu/~janda/
Abstract: Understanding the structure of Gromov-Witten invariants of Calabi-Yau threefolds is an important problem in enumerative geometry which has been studied since the early 90s. In my talk, I will concentrate on quintic Calabi-Yau threefolds, review the algebraic geometry behind the (now standard) computation of the genus zero invariants, and explain why it cannot be easily extended to higher genus. I will then proceed to discuss a construction (joint with Q. Chen and Y. Ruan) of new moduli spaces that can control the failure of the naive approach. In joint work with S. Guo and Y. Ruan, we use
them to prove conjectures from physics about higher genus Gromov-Witten invariants of quintic threefolds, such as the "holomorphic anomaly equations".

#### Hitchin systems, hyper-Kaehler geometry, and the P=W conjecture

When: Mon, November 11, 2019 - 2:00pm
Where: Kirwan Hall 3206
Speaker: Junliang Shen (MIT) - https://sites.google.com/view/junliang/
Abstract: Lagrangian fibrations play a crucial role in the study of hyper-Kaehler geometry and integrable systems. The P=W conjecture by de Cataldo, Hausel, and Migliorini suggests a surprising connection between the topology of Lagrangian fibrations and Hodge theory. In this talk, we will first discuss a compact version of this phenomenon, based on joint work with Andrew Harder, Zhiyuan Li, and Qizheng Yin. Then we will focus on interactions between compact and noncompact hyper-Kaehler geometry. Such connections lead to new progress on the P=W conjecture for Hitchin systems and character varieties. This is joint work with Mark de Cataldo and Davesh Maulik.
If time permits, I will further discuss the connection between P=W and Gopakumar-Vafa invariants for local curves.

#### Intersection in the Lubin-Tate tower

When: Mon, November 18, 2019 - 2:00pm
Where: Kirwan Hall 3206
Speaker: Andreas Mihatsch (MIT and Bonn) -
Abstract: We define and compute ``analytic'' intersection numbers of quadratic CM-cycles on Lubin-Tate (LT) space at infinite level. This is based on the formalism of tropical (p,q)-forms by Gubler-Künnemann and the description of the infinitel level LT-space by Scholze-Weinstein.
The intersection problem itself plays a role in the linear Arithmetic Fundamental Lemma conjecture of W. Zhang. Our approach is motivated by a recent result of Q. Li who gave a formula for the corresponding intersection numbers on formal models. A posteriori, we see that our analytically defined numbers coincide the ones from formal models.

#### Counting sheaves on singular curves and surfaces

When: Mon, November 25, 2019 - 2:00pm
Where: Kirwan Hall 3206
Speaker: Amin Gholampour (University of Maryland) - https://www.math.umd.edu/~aming
Abstract: Given a virtually smooth quasi-projective scheme M, and a morphism from M to a nonsingular quasi-projective variety B, we show it is possible to find an affine bundle M' over M that admits a perfect obstruction theory relative to B. We study the resulting virtual cycles on the fibers of M' over B and relate them to the image of the virtual cycle [M]^vir under refined Gysin homomorphisms. Our main application is when M is a moduli space of stable codimension 1 sheaves on a nonsingular projective surface or Fano threefold.

#### On The Hitchin Fibration II (Distinguished Lectures in Algebra/Number Theory)

When: Thu, December 5, 2019 - 2:00pm
Where: Kirwan Hall 3206

Speaker: Ngo Bau Chao (Univ of Chicago) -
Abstract: In the second lecture, I want to discuss the theory of non-archimedean integration on the Hitchin fibration due to Groechenig, Wyss and Ziegler. Surprisingly, calculating nonarchimedean integrals is not exactly the same as counting points and this approach gives another proof of the fundamental lemma, and this discrepancy sheds yet new lights on the theory of endoscopy. The proof is also more elementary in the sense that it does not use the theory of perverse sheaves.

#### On The Hitchin Fibration III (Distinguished Lectures in Algebra/Number Theory)

When: Fri, December 6, 2019 - 2:00pm
Where: Kirwan Hall 3206
Speaker: Ngo Bau Chao (Univ of Chicago) -
Abstract: In my third lecture, I want to report on a completely different development on the moduli space of Higgs bundles. In joint work with T.H. Chen we started exploring the structure of the Hitchin map for the moduli space of Higgs bundles over higher-dimensional varieties, which raises interesting questions on the geometry of commuting varieties.

#### Shimura varieties and independence of l for Frobenius conjugacy classes

When: Wed, December 11, 2019 - 11:00am
Where: Kirwan Hall 3206
Speaker: Rong Zhou (Yale University) -
Abstract: The Langlands program predicts a deep relationship between the world of automorphic representations and that of Galois representations. Understanding this relationship in special cases has led to proofs of many deep theorems in number theory. A central role in the program is played by certain algebraic varieties called Shimura varieties; in special cases, they arise as moduli spaces of abelian varieties with extra structure. In this talk, I will explain a recent joint work with Kisin, using Shimura varieties to prove a result on the independence of l for Frobenius conjugacy classes attached to abelian varieties.

#### ANT and Lie Grp/Rep Theory Org. meeting

When: Wed, February 12, 2020 - 2:00pm
Where: Kirwan Hall 3206
Speaker: None () -

#### Intersection pairing of cycles and biextensions

When: Mon, February 17, 2020 - 2:00pm
Where: Kirwan Hall 3206
Speaker: Yordanka Kovacheva (UMD) - http://math.umd.edu/~ykovache/
Abstract: We study the intersection of two cycles on a variety in a situation similar to the Bloch-Beilinson height pairing and Arakelov theory. The main question we answer is as follows. For a fixed cycle, find conditions on it, such that whenever we pair it with two rationally equivalent cycles (with possible multiple equivalences), we get the same equivalence between the images of the pairing. This question relates to the question of a biextension, associated to a paring of cycles. In particular, we show that Bloch's biextension of homologically trivial cycles cannot be extended to a biextension of numerically trivial cycles. As part of the proof we give an explicit expression of the Suslin-Voevodsky's isomorphism.

#### TBA

When: Wed, March 11, 2020 - 2:00pm
Where: Kirwan Hall 3206,
Speaker: Brian Lawrence (Chicago) -

#### Vertex algebras of CohFT-type

When: Mon, May 4, 2020 - 2:00pm
Where: Kirwan Hall 3206

Speaker: Nicola Tarasca (Virginia Commonwealth University) - http://people.vcu.edu/~tarascan/
Abstract: This talk will focus on geometric realizations of non-commutative algebras. I will discuss how representations of conformal vertex algebras encode information about the geometry of algebraic curves. The starting point is the Virasoro uniformization, which provides an incarnation of the Virasoro algebra in the tangent space of a tautological line bundle on the moduli space of coordinatized curves. After briefly reviewing vertex algebras, I will discuss how their representations yield new vector bundles of conformal blocks on moduli spaces of curves and new cohomological field theories. This is joint work with Chiara Damiolini and Angela Gibney.