Geometric Methods in Representation Theory Seminar
University of North Carolina at Chapel Hill
Mondays/Fridays 4pm, PH367 or PH385
The aim of this seminar is to bring speakers from this area and outside to speak on topics related to Representation Theory (specially geometric and topological methods employed in Representation Theory). The speakers are expected to give their talks at a level suitable for graduate students. The seminar is organized by Prakash Belkale, Jiuzu Hong, Shrawan Kumar and Richard Rimanyi.
2023 Fall
Date  Speaker  Affiliation  Title 

Nov 20  Jianqiao Xia  Harvard  TBA 
Nov 1719  –  –  Workshop on geometric representation theory and moduli spaces 
Nov 3  Thomas Lam  Michigan  TBA 
Oct 23 4:30pm  Szilárd Szabó  Budapest University of Technology and Economics  TBA 
Sep 8  Mikhail Kapranov  IPMU  Nspherical functors and categorification of Euler’s continuants 
Aug 25  Joseph Landsberg  Texas A&M  Linear spaces of matrices of bounded rank 
Mikhail Kapranov: Nspherical functors and categorification of Euler’s continuants Abstract: Euler’s continuants are universal polynomials expressing the numerator and denominator of a finite continued fraction in terms of its entries. Remarkably, they make an appearance in the very foundations of category theory: in the formalism of adjoint functors. More precisely, they upgrade to natural complexes of functors built out of a given functor and its iterated adjoints. Requiring exactness of some of these complexes leads to the concept of an Nspherical functor which specializes to that of an ordinary spherical functor for N=4. Such functors describe Nperiodic semiorthogonal decompositions of (enhanced) triangulated categories. Like ordinary spherical functors, they give rise to interesting selfequivalences. Conceptually, they can be seen as categorification of certain irregular differential equations (polynomial Schroedinger) in the complex plane. Joint work with T. Dyckerhoff, V. Schechtman. 
Joseph Landsberg: Linear spaces of matrices of bounded rank Abstract: A classical problem in linear algebra is to classify linear spaces of matrices such that no element of the space has full rank. Work of Eisenbud and Harris showed that the problem may be rephrased in terms of classifying sheaves on projective space with certain properties. 40 years ago spaces of bounded rank at most three were classified and there have been interesting, isolated examples of spaces discovered that are related to wellstudied objects in algebraic geometry such as instanton bundles, but there had been no progress on the classification problem. Motivated by questions in theoretical computer science and quantum information theory, H. Huang and myself revisited this problem. Using methods from algebraic geometry and commutative algebra, we classified spaces of bounded rank four. 
2023 Spring
Date  Speaker  Affiliation  Mode  Title 

Apr 28  George Lusztig  MIT  Inperson  Strata in reductive groups 
Apr 14  Ádám Gyenge  Rényi Institute Budapest  Inperson  Blowups and the quantum spectrum of surfaces 
Apr 11  Mikhail Kapranov  IPMU  Inperson  Perverse sheaves and Hopf algebras 
March 31  Anders Buch  Rutgers  Inperson  Pieri formulas for the quantum Ktheory of cominuscule Grassmannians 
March 24  Simon Riche  Clermont Auvergne  Inperson  Characters of modular representations of reductive algebraic groups 
March 10  –  –  –  Hitoshi Konno’s lecture in the sister (Physically Inspired Mathematics) seminar 
March 10  Lingfei Yi  Minnesota  Inperson  Physical rigidity of FrenkelGross connection 
Jan 27  –  –  –  Pavel Etingof’s lecture in the sister (Physically Inspired Mathematics) seminar 
Jan 20  Tommaso Botta  ETH Zurich  Inperson  Solution of qKZB equations from the geometry of Nakajima quiver varieties 
Jan 17  Dima Arinkin  University of Wisconsin  Inperson  Integrating symplectic stacks 
George Lusztig: Strata in reductive groups Abstract: : Let G be a connected reductive group over an algebraically closed field. We define a decomposition of G into finitely many strata each of which is a union of conjugacy classes of fixed dimension. The strata are indexed by a set independent of the characteristc. The strata can be described purely in terms of the Weyl group. 
Ádám Gyenge: Blowups and the quantum spectrum of surfaces Abstract: : The cup product of ordinary cohomology describes how submanifolds of a manifold intersect each other. GromovWitten invariants give rise to quantum product and quantum cohomology, which describe how subspaces intersect in a ”fuzzy”, ”quantum” way. Dubrovin observed that quantum cohomology can be used to define a flat connection on a certain vector bundle called the quantum connection. We verify a conjecture of Kontsevich on the behaviour of the spectrum of the quantum connection under blowups for smooth projective surfaces. Joint work with Szilard Szabo. 
Mikhail Kapranov: Perverse sheaves and Hopf algebras Abstract: Perverse sheaves were originally introduced as a conceptual framework for intersection homology, a (co)homology theory for singular spaces that satisfies Poincare duality. As such, they occupy an intermediate position between sheaves (coefficients for cohomology) and cosheaves (coefficients for homology), forming a selfdual category. On the other hand, Hopf algebras, or bialgebras provide an example of a selfdual structure in a purely algebraic context, being equipped both with a multiplication and a comultiplication. The talk, based on joint work with V. Schechtman, will explain a connection between these two type of structures so that universal identities among various composite (co)operations turn out to give the relations in the quivers describing perverse sheaves on configuration spaces of the complex line. In particular, this gives a relation between graded bialgebras and factorizing systems of perverse sheaves, whose instances are known in the theory of quantum groups. 
Anders Buch: Pieri formulas for the quantum Ktheory of cominuscule Grassmannians Abstract: The quantum Ktheory ring QK(X) of a flag variety X encodes the Ktheoretic GromovWitten invariants of X, defined as arithmetic genera of GromovWitten varieties parametrizing curves meeting fixed Schubert varieties. A Pieri formula means a formula for multiplication with a set of generators of QK(X). Such a formula makes it possible to compute efficiently in this ring. I will speak about a Pieri formula for QK(X) when X is a cominuscule Grassmannian, that is, an ordinary Grassmannian, a maximal orthogonal Grassmannian, or a Lagrangian Grassmannian. This formula has a simple statement in terms of order ideals in a partially ordered set that encodes the degree distance between opposite Schubert varieties. This set generalizes both Postnikov’s cylinder and Proctor’s description of the Bruhat order of X. This is joint work with P.E. Chaput, L. Mihalcea, and N. Perrin.

Simon Riche: Characters of modular representations of reductive algebraic groups Abstract: One of the main questions in the representation theory of reductive algebraic groups is the computation of characters of simple modules. A conjectural solution to this problem was proposed by G. Lusztig in 1980, and later shown to be correct assuming the base field has large characteristic. However in 2013 G. Williamson found (counter)examples showing that this answer is not correct without this assumption. In this talk I will explain a new solution to this problem, obtained in a combination of works involving (among others) P. Achar and G. Williamson, which is less explicit but has the advantage of being valid in all characteristics. 
Lingfei Yi: TBA Abstract: A Gconnection over a smooth complex curve is called physically rigid if it is determined by its local monodromies. We show that the FrenkelGross connection is physically rigid, thus confirming the de Rham version of a conjecture of HeinlothNg^oYun. The proof is based on the construction of the Hecke eigensheaf of a connection with only generic oper structure, using the localization of Weyl modules. We will review the notion of opers and give the sketch of the proof. Time permitting, we will describe a conjectural generalization of the result relating theta connections to Langlands parameters of Epipelagic representations. 
Tommaso Botta: Solution of qKZB equations from the geometry of Nakajima quiver varieties Abstract: The quantum Knizhnik–Zamolodchikov (qKZ) equations are an important family of difference equations, deeply related to the representation theory of affine quantum enveloping algebras (trigonometric quantum groups). Over the past years, Okounkov, Smirnov and their coauthors have succeeded in studying the qKZ equations via the geometry of Nakajima varieties and producing integral solutions through enumerative counts in Ktheory. The goal of this talk is to extend some of the above ideas to the elliptic setting. Firstly, I will exploit AganagicOkounkov’s theory of elliptic stable envelopes of Nakajima varieties to define a system of elliptic difference equations— the KnizhnikZamolodchikovBernard (qKZB) equations — for arbitrary quiver varieties. Then I will discuss how to produce integral presentations of their solutions. In this context, a Cohomological Hall algebra (CoHA) interpretation of the stable envelopes will replace Okounkov’s technology of enumerative counts. This talk is based on joint work in preparation with Felder and Wang.

Dima Arinkin: Integrating symplectic stacks Abstract: Shifted symplectic stacks, introduced by Pantev, Toën, Vaquie, and Vezzosi, are a natural generalization of symplectic manifolds in derived algebraic geometry. The word `shifted’ here refers to cohomological shift, which can naturally occur in the derived setting: after all, the tangent space is now not a vector space, but a complex. Several classes of interesting moduli stacks carry shifted simplectic structures. In my talk (based on a joint project with T.Pantev and B.Toën), I will present a way to generate shifted symplectic stacks. Informally, it involves integration along a (compact oriented) topological manifold X: starting with a family of shifted symplectic stacks over X, we produce a new stack of sections of this family, and equip it with a symplectic structure via an appropriate version of the Poincaré duality. 