Organizing Committee
 David Anderson
Ohio State University  Angela Gibney
Rutgers University, New Brunswick  June Huh
Stanford University  Thomas Lam
University of Michigan  Leonardo Mihalcea
Virginia Polytechnic Institute and State University
Abstract
The purpose of the workshop is to bring together a diverse group of researchers working on combinatorial and geometric aspects related to spaces with symmetries. The workshop will cover problems arising from various flavors of Schubert Calculus and enumerative geometry on flag manifolds, and problems from geometric representation theory and combinatorial Hodge theory. The topics covered include the study of LittlewoodRichardson coefficients, quantum cohomology and quantum K theory of flag manifolds, MaulikOkounkov stable envelopes and characteristic classes, conformal blocks, and combinatorics related to moduli spaces, Macdonald theory, and quiver polynomials, Soergel bimodules, Hodge theory of matroids. These are trends in a rapidly developing area, and our aim is to facilitate interactions among researchers who work on different problems but employ similar techniques, at the intersection of algebraic geometry, combinatorics, and representation theory.
Confirmed Speakers & Participants
Talks will be presented virtually or inperson as indicated in the schedule below.
 Speaker
 Poster Presenter
 Attendee
 Virtual Attendee

Adnan Abdulwahid
University of Iowa

Dan Abramovich
Brown University

Ashleigh Adams
UC Davis

Adam Afandi
Colorado State University

Tair Akhmejanov
University of CaliforniaDavis

Elie Alhajjar
US Military Academy

Ayah Almousa
Cornell University

David Anderson
Ohio State University

Federico Ardila
San Francisco State University

Ahmed Umer Ashraf
University of Western Ontario

Olga Azenhas
University of Coimbra

Matthew Baker
Georgia Institute of Technology

Prakash Belkale
University of North Carolina at Chapel Hill

Marc Besson
University of North Carolina at Chapel Hill

Christin Bibby
Louisiana State University

Sara Billey
University of Washington

Simone Billi
University of Bologna

Aram Bingham
Tulane University

Rekha Biswal
Universite Laval

Tom Braden
University of Massachusetts Amherst

Madeline Brandt
Brown University

Michel Brion
Université Grenoble Alpes

Juliette Bruce
University of California, Berkeley / MSRI

Anders Buch
Rutgers University

Amanda Burcroff
Durham University

Mahir Bilen Can
Tulane University

Eduardo Cattani
University of Massachusetts Amherst

Melody Chan
Brown University

Anastasia Chavez
University of California, Davis

Linda Chen
Swarthmore College

Sunita Chepuri
University of Michigan

María Angélica Cueto
Ohio State University

Mark Curiel
University of Hawaii at Manoa

Xinle Dai
Harvard University

Chiara Damiolini
Rutgers University

Quang Dao
University of Michigan

Colin Defant
Princeton University

Papri Dey
University of Missouri

Alexander DiazLopez
Villanova Universiry

Galen DorpalenBarry
University of Minnesota

Theo Douvropoulos
University of Massachusetts, Amherst

Anne Dranowski
Institute for Advanced Study

Nicholas Early
Institute for Advanced Study

Richard Ehrenborg
University of Kentucky

Balazs Elek
Cornell University

Ben Elias
University of Oregon

Laura Escobar
Washington University St. Louis

Christopher Eur
Stanford University

Matthew Faust
Texas A&M University

Sergey Fomin
University of Michigan

Chris Fraser
University of Minnesota

Netanel Friedenberg
Yale University

William Fulton
University of Michigan

Nir Gadish
MIT

Christian Gaetz
MIT

Pavel Galashin
University of California, Los Angeles

Shiliang Gao
University of Illinois at UrbanaChampaign

Yibo Gao
Massachusetts Institute of Technology

Deniz Genlik
The Ohio State University

Angela Gibney
Rutgers University, New Brunswick

Andrey Glubokov
Ave Maria University

Vassily Gorbounov
Higher School of Economics, Russia

Eugene Gorsky
UC Davis

Sean Griffin
Brown University

Samuel Grushevsky
Stony Brook University

Aziz Burak Guelen
The Ohio State University

Emily Gunawan
University of Oklahoma

Christian Haase
Freie Universität Berlin

Iva Halacheva
Northeastern University

Kangjin Han
DaeguGyeongbuk Institute of Sciences and Technology (DGIST)

Megumi Harada
MCMASTER UNIVERSITY

Pamela E. Harris
Williams College

Milena Hering
The University of Edinburgh

Serkan Hosten
San Francisco State University

Yifeng Huang
University of Michigan

Daoji Huang
Brown University

June Huh
Stanford University

Brian Hwang
Cornell University

Anthony Iarrobino
Northeastern University

Bogdan Ion
University of Pittsburgh

David Jensen
University of Kentucky

Minyoung Jeon
The Ohio State University

Sam Jeralds
University of North Carolina at Chapel Hill

Shuai Jiang
Virginia Tech

Michael Joswig
TU Berlin & MPI Leipzig

Nidhi Kaihnsa
Brown University

Siddarth Kannan
Brown University

Lars Kastner
Institute of Mathematics of the Technical University

Syu Kato
Kyoto University

Hannah Keese
Cornell University

Gary Kennedy
Ohio State University

Joshua Kiers
Ohio State University

Young Rock Kim
Hankuk University of Foreign Studies

Patricia Klein
University of Minnesota

Allen Knutson
Cornell University

Aleksandr Kolpakov
University of Neuchâtel

Jakub Koncki
Institute of Mathematics, Polish Academy of Sciences

Korkeat Korkeathikhun
University of North Carolina at Chapel Hill

Lukas Kühne
Max Planck Institute for Mathematics in the Sciences

Thomas Lam
University of Michigan

Matthew Larson
Stanford University

Cristian Lenart
University at Albany

Shiyue Li
Brown University

KueiNuan Lin
Penn State University, Greater Allegheny

David LowryDuda
ICERM & Brown University

Antonio Macchia
Freie Universität Berlin

Madhusudan Manjunath
INDIAN INSTITUTE OF TECHNOLOGY BOMBAY

Hannah Markwig
Eberhard Karls University of Tübingen

Mikhail Mazin
Kansas State University

Alex McDonough
Brown University

Mike Michailidis
The MathWorks Inc.

Leonardo Mihalcea
Virginia Polytechnic Institute and State University

Elizabeth Milićević
Haverford College

Fatemeh Mohammadi
Ghent University

Sophie Morel
CNRS/ENS Lyon

Jennifer Morse
University of Virginia

Chenqi Mou
Beihang University, China

Robert Muth
Washington & Jefferson College

Philippe Nadeau
Institut Camille Jordan

Hiroshi Naruse
University of Yamanashi

Gleb Nenashev
Brown University

David Oetjen
Virginia Tech

Daniel Orr
Virginia Tech

Jianping Pan
University of California, Davis

Brendan Pawlowski
University of Southern California

Sam Payne
University of Texas at Austin

Oliver Pechenik
University of Waterloo

Michael Perlman
Queen's University

Nicolas Perrin
Versailles SaintQuentinenYvelines University

Nathan Pflueger
Amherst College

Martha Precup
Washington University in St. Louis

Ying Anna Pun
University of Virginia

Jenna Rajchgot
McMaster University

Margaret Readdy
University of Kentucky

Konstanze Rietsch
King's College London

Richard Rimanyi
University of North Carolina at Chapel Hill

Colleen Robichaux
University of Illinois at UrbanaChampaign

Oliver Ruff
Kent State University

Francisco Santos
University of Cantabria

Mahrud Sayrafi
University of Minnesota

Hal Schenck
Auburn University

Anne Schilling
UC Davis

George Seelinger
University of Virginia

Oswaldo Sevilla
Centro de Investigación en Matemáticas, Guanajuato Mexico

Melissa ShermanBennett
UC Berkeley/Harvard

Mark Shimozono
Virginia Tech

Farbod Shokrieh
University of Washington

Jose Simental Rodriguez
MaxPlanck Institute for Mathematics

Connor Simpson
University of Wisconsin  Madison

Eric Sommers
University of Massachusetts

MirunaStefana Sorea
SISSA, Trieste

Frank Sottile
Texas A&M University

Avery St. Dizier
Univeristy of Illinois at UrbanaChampaign

Jessica Striker
North Dakota State University

Bernd Sturmfels
MPI Leipzig

Changjian Su
University of Toronto

Yuri Sulyma
Brown University

Mariel Supina
University of California, Berkeley

Joshua Swanson
University of California, San Diego

Daniel Tamayo Jiménez
Université ParisSaclay

Anna Tao
Brown University

Nicola Tarasca
Virginia Commonwealth University

Mojdeh Tarighat
University of Virginia

Vasu Tewari
University of Hawaii

Ayush Tewari
TU Berlin

Martin Ulirsch
Goethe University Frankfurt

Jeremy Usatine
Brown University

Ravi Vakil
Stanford University

Lorenzo Vecchi
Alma Mater Studiorum  Università di Bologna

Emanuele Ventura
University of Bern

Diego Villamizar
Tulane University

Duc Vo
Harvard University

Weikun Wang
Southern University of Science and Technology

Andrzej Weber
Uniwersity of Warsaw

Anna Weigandt
University of Michigan

Ian Whitehead
Swarthmore College

Harold Williams
University of Southern California

Lauren Williams
Harvard University

Corey Wolfe
Tulane University

Tian An Wong
University of MichiganDearborn

Cameron Wright
University of Washington

Weihong Xu
Rutgers

Damir Yeliussizov
KazakhBritish Technical University

Alexander Yong
University of Illinois at UrbanaChampaign

Semin Yoo
University of Rochester

Chi Ho Yuen
Brown University

Claudia Yun
Brown University

Changlong Zhong
State University of New York Albany

Paul ZinnJustin
The University of Melbourne
Workshop Schedule
Monday, March 22, 2021

10:00  10:15 am EDTWelcomeVirtual
 Brendan Hassett, ICERM/Brown University

10:15  10:45 am EDTHey Series, Tell Me About the Extended Delta ConjectureVirtual
 Speaker
 Jennifer Morse, University of Virginia
 Session Chair
 Leonardo Mihalcea, Virginia Polytechnic Institute and State University
Abstract
The space of diagonal harmonics $DH_n$ is an $S_n$module in two sets of $n$ variables, which arose from a representation theoretic program to study Macdonald polynomials initiated by Garsia and Haiman. The doubly graded character of $DH_n$ has both a symmetric function and a combinatorial description: Haiman proved it is $\Delta'_{e_{n1}}e_n$ for an eigenoperator $\Delta'$ of modified Macdonald polynomials, while Carlsson and Mellit established the combinatorial ``Shuffle Conjecture"" of Haiman, Haglund, Loehr, Remmel, and Ulyanov, expressing it as as a sum of LLT polynomials over Dyck paths. An expanded investigation led Haglund, Remmel and Wilson to the Extended Delta Conjecture, a combinatorial prediction for $\Delta_{h_l}\Delta'_{e_k}e_n$, allowing for any $0\leq l,k<n$. Bringing in new results about the action of the elliptic Hall algebra on symmetric functions, we reformulate the conjecture as the polynomial truncation of an identity of infinite series of $GL_l$ characters, expressed in terms of LLT series. The stronger infinite series identity is not difficult to prove using identities on nonsymmetric HallLittlewood polynomials. This is joint work with Blasiak, Haiman, Pun, and Seelinger.

11:00  11:15 am EDTBreakCoffee Break  Virtual

11:15  11:45 am EDTCastelnuovoMumford regularity of matrix Schubert varietiesVirtual
 Speaker
 Oliver Pechenik, University of Waterloo
 Session Chair
 Leonardo Mihalcea, Virginia Polytechnic Institute and State University
Abstract
Jenna Rajchgot observed that the CastelnuovoMumford regularity of matrix Schubert varieties is computed by the degrees of the corresponding Grothendieck polynomials. We give a formula for these degrees. Indeed, we compute the leading terms of the top degree pieces of Grothendieck polynomials and give a complete description of when two Grothendieck polynomials have the same top degree piece (up to scalars). Our formulas rely on some new facts about major index of permutations.

11:45 am  2:00 pm EDTLunch/Free TimeVirtual

2:00  2:30 pm EDTConformal blocks: an overviewVirtual
 Speaker
 Chiara Damiolini, Rutgers University
 Session Chair
 June Huh, Stanford University
Abstract
In this talk I will introduce simple examples of sheaves of conformal blocks arising from representations of Lie algebras and discuss how they can be used to study certain moduli spaces. I will further discuss generalizations of these sheaves which go beyond the representation theory of Lie algebras, and related works in progress.

2:45  3:15 pm EDTQuantum geometric Satake at a root of unity and Schubert calculus for G(m,m,n)Virtual
 Speaker
 Ben Elias, University of Oregon
 Session Chair
 June Huh, Stanford University
Abstract
There is a qdeformation of the reflection representation of the affine Weyl group in type A which leads to a qdeformation of many common constructions: Demazure operators, Soergel bimodules, geometric Satake, etcetera. When q is set to a root of unity, the action of the affine Weyl group factors through a finite quotient, the complex reflection group G(m,m,n), and a new kind of "Schubert calculus" appears. This talk will demonstrate this unusual construction in the wellunderstood case of affine A_1 and the mysterious case of affine A_2.

3:15  4:30 pm EDTGathertown ReceptionReception  Virtual
Tuesday, March 23, 2021

9:00  9:45 am EDTGathertown Morning CoffeeCoffee Break  Virtual

9:45  10:15 am EDTGeometry of semiinfinite flag manifoldsVirtual
 Speaker
 Syu Kato, Kyoto University
 Session Chair
 Thomas Lam, University of Michigan
Abstract
Semiinfinite flag manifolds are variants of affine flag manifolds whose Schubert cells are simultaneously infinitedimensional and infinitecodimensional. Such objects are introduced by Drinfeld and Lusztig around 1980 (as sets), and its finitedimensional approximation model was presented by FinkelbergMirkovic in 1999. Recently, we described them as an explicit indscheme of indinfinite type that satisfies a certain universal property. It lead us to a description of quantum $K$groups of partial flag manifolds and some functorial relations between them. In this talk, I will start from a brief review of affine flag manifolds/varieties, define semiinfinite flag manifolds and their natural subschemes, and then explain their combinatorial and algebrogeometric properties (trying to stress the difference from affine flag manifolds/varieties). If time allows, then I will explain how some of these properties are derived as a shadow of the homological properties of affine Lie algebras. This talks is mainly based on arXiv:1810.07106.

10:30  10:45 am EDTBreakCoffee Break  Virtual

10:45  11:35 am EDTLightning TalksVirtual
 Speakers
 Aram Bingham, Tulane University
 Papri Dey, University of Missouri
 Yifeng Huang, University of Michigan
 Michael Perlman, Queen's University
 Semin Yoo, University of Rochester
 Session Chair
 David Anderson, Ohio State University
Abstract
Combinatorics of quadratic spaces over finite fields
Semin Yoo, University of Rochester
Let $\mathbb{F}_q^n$ be the finite field with $q$ elements, where the char$F$ is not $2$. The size of the set of $k$dimensional subspaces of $\mathbb{F}_q^n$ is called $q$binomial coefficient, (or Gaussian binomial coefficient.) which has various interesting combinatorial descriptions and related works. Furthermore, the fact that makes $q$binomial coefficients more interesting is that $q$binomial coefficients have $q$analogues. $\mathbf{q}$\textbf{analogues} of quantities in mathematics involve perturbations of classical quantities using the parameter $q$ and revert to the original quantities when $q$ goes to $1$. In this talk, we add one more structure, called a quadratic form. We are mainly interested in the size of the set of $k$dimensional subspaces that have orthonormal bases, which can be also written as analogues of binomial coefficients. Various combinatorial properties of this new binomial coefficient will be discussed. We will also show what will happen in the set side if we take the limit when $q$ goes to $1$.
Mixed Hodge structure on local cohomology with support in determinantal varieties
Michael Perlman, Queen's University
Given a closed subvariety Z in a smooth variety, the local cohomology sheaves with support in Z are functorially endowed with structures as mixed Hodge modules. This implies that they are equipped with two increasing filtrations: the Hodge filtration and the weight filtration. We will discuss new calculations of these filtrations in the case when Z is a generic determinantal variety. This talk includes joint work with Claudiu Raicu.
Geometric and Combinatorial aspects of Nonlinear Algebra
Papri Dey, University of MissouriColumbia
Nonlinear algebra is an interdisciplinary area and I shall talk about some interaction among algebraic, geometric and combinatorial objects in this field.
Clans, sects, and symmetric space closure orders
Aram Bingham, Tulane University of Louisiana
We will give a combinatorial description of the closure order on Borel orbits in symmetric spaces of Hermitian type in terms of parameterizing objects called clans and their projections onto $G/P$ (sects). This description resolves part of a conjecture of Wyser on the restriction of Bruhat orders on these spaces.
A generating function for counting mutually annihilating matrices over a finite field
Yifeng Huang, University of Michigan
We count the number of pairs of n x n matrices (A, B) over a finite field such that AB=BA=0. We then give an explicit factorization of a generating function associated to this count, which in particular shows that the function can be meromorphically extended to the entire complex plane. This essentially says that the motivic formula about (the stack of) finitelength coherent sheaves on a nodal singular curve behaves as the geometry predicts. 
11:35 am  2:00 pm EDTGathertown with Lightning speakers / LunchLunch/Free Time  Virtual

2:00  2:30 pm EDTGröbner Geometry of Schubert Polynomials Through IceVirtual
 Speaker
 Anna Weigandt, University of Michigan
 Session Chair
 Angela Gibney, Rutgers University, New Brunswick
Abstract
The geometric naturality of Schubert polynomials and the related combinatorics of pipe dreams was established by Knutson and Miller (2005) via antidiagonal Gröbner degeneration of matrix Schubert varieties. We consider instead diagonal Gröbner degenerations. In this dual setting, Knutson, Miller, and Yong (2009) obtained alternative combinatorics for the class of vexillary matrix Schubert varieties. We will discuss general diagonal degenerations, relating them to an older formula of Lascoux (2002) in terms of the 6vertex ice model. Lascoux's formula was recently rediscovered by Lam, Lee, and Shimozono (2018), as "bumpless pipe dreams." We will explain this connection and discuss conjectures and progress towards understanding diagonal Gröbner degenerations of matrix Schubert varieties.

2:45  3:00 pm EDTBreakCoffee Break  Virtual

3:00  3:30 pm EDTRigid local systems and the multiplicative eigenvalue problemVirtual
 Speaker
 Prakash Belkale, University of North Carolina at Chapel Hill
 Session Chair
 Angela Gibney, Rutgers University, New Brunswick
Abstract
Local systems are sheaves which describe the behavior of solutions of differential equations. A local system is rigid if local monodromy determines global monodromy. We give a construction which produces irreducible complex rigid local systems on a punctured Riemann sphere via quantum Schubert calculus and strange duality. These local systems are unitary and arise from a study of vertices in the polytopes controlling the multiplicative eigenvalue problem for the special unitary groups SU(n) (i.e., determination of the possible eigenvalues of a product of unitary matrices given the eigenvalues of the matrices). Roughly speaking, we show that the strange duals of the simplest vertices (which can be inductively determined) of these polytopes give (all) possible unitary irreducible rigid local systems. We note that these polytopes are generalizations of the classical LittlewoodRichardson cones of algebraic combinatorics. Answering a question of Nick Katz, we show that there are no irreducible rigid local systems on a punctured Riemann sphere of rank greater than one, with finite global monodromy, all of whose local monodromies have orders dividing n, when n is a prime number.

3:45  4:15 pm EDTCyclotomic generating functionsVirtual
 Speaker
 Joshua Swanson, University of California, San Diego
 Session Chair
 Angela Gibney, Rutgers University, New Brunswick
Abstract
Generating functions are a major theme in mathematics which unify many disciplines. It is remarkably common to find a combinatorial generating function which factors as a ratio of products of $q$integers. Examples of such quotients arise from enumerative combinatorics (e.g. subset sums and $q$binomials), representation theory (the $q$Weyl dimension formula), geometry (iterated $\mathbb{P}^1$bundles), probability (random variable decompositions), commutative algebra (homogeneous systems of parameters), and more. We call such quotients "cyclotomic generating functions" (CGFs) and initiate their general study. This talk will review some of the many known constructions of CGFs and give asymptotic estimates of their coefficients. We will also highlight a range of conjectures and accessible open problems. Joint work in progress with Sara Billey.
Wednesday, March 24, 2021

9:00  9:45 am EDTGathertown Morning CoffeeCoffee Break  Virtual

9:45  10:15 am EDTDoing Schubert Calculus with Bumpless Pipe DreamsVirtual
 Speaker
 Daoji Huang, Brown University
 Session Chair
 Laura Escobar, Washington University St. Louis
Abstract
Bumpless pipe dreams were introduced by Lam, Lee, and Shimozono in the context of back stable Schubert calculus. Like ordinary pipe dreams, they compute Schubert and double Schubert polynomials. In this talk, I will give a bijective proof of Monk's rule for Schubert polynomials, and show that the proof extends easily to the proof of Monk's rule for double Schubert polynomials. As an application, I will explain how to biject bumpless pipe dreams and ordinary pipe dreams using the transition and cotransition formulas, which are specializations of Monk's rule. If time permits I will also briefly discuss some work on how bumpless pipe dreams can be used to compute products of certain Schubert polynomials that generalize the Grassmannian case.

10:30  10:45 am EDTBreakCoffee Break  Virtual

10:45  11:35 am EDTLightning TalksVirtual
 Speakers
 anastasia chavez, University of California, Davis
 Theo Douvropoulos, University of Massachusetts, Amherst
 Brian Hwang, Cornell University
 Maiko Serizawa, University of Ottawa
 Weihong Xu, Rutgers
 Session Chair
 Leonardo Mihalcea, Virginia Polytechnic Institute and State University
Abstract
Volumes of Root Zonotopes via the WLaplacian
Theo Douvropoulos, University of Massachusetts, Amherst
The normalized volume of the classical permutahedron is given by the formula n^{n2} which, ask any combinatorialist and they will tell you, agrees with the number of trees on n vertices. This coincidence is well understood; the classical permutahedron is a unimodular zonotope, on the set of positive roots of the Symmetric group S_n, and its bases are indexed by trees and hence enumerated by the determinant of a Laplacian matrix. This description of the permutahedron lends itself to a natural generalization: for a Weyl group W, the zonotope associated to the collection of positive roots of W will be called the root zonotope Z_W. These zonotopes Z_W are not unimodular, but their bases can be differentiated with respect to the reflection subgroup they generate (and its connection index). In joint work with Guillaume Chapuy (arxiv.2012.04519) we have introduced for any Weyl group W a (nxn) Laplacian matrix L_W whose spectrum encodes (nontrivially) many enumerative properties of W. In particular we will present new short formulas (ibid: Section 8.3) for the volumes of the root zonotopes Z_W involving only the Coxeter numbers of W and its reflection subgroups. Our approach is uniform and does not rely on the classification.
Effective pointcounting for polygons in flag varieties
Brian Hwang, Cornell University
There are a number of seemingly similar "big" (e.g. affine, open) spaces that arise in the context of flag varieties. Certain classes of these spaces are known to exhibit special features. For example, open Richardson varieties have a decomposition into a disjoint union of simple components and the coordinate rings of double Bruhat cells are known to be cluster algebras. There are often combinatorial objects that govern their geometry, such as subword complexes for brick manifolds or various kinds of polytopes, especially in the context of toric varieties and related deformations and degenerations. The bestiary is rich and multifarious. Many of these spaces, however, turn out to admit a description as a polygon in a flag variety, that is, a (cyclic) tuple of flags where each flag is a prescribed distance from its immediate neighbors. Here, the distance is taken not with respect to the Euclidean metric, but with a nonsymmetric Weylgroupvalued notion of "distance." Such polygons in flag varieties turn out to admit simple decompositions that can be phrased in terms of triangulations of the polygon into certain triangles with an "easyexit" property. As an illustration of this organizing principle, we show how we can use this to easily count the points of open Richardson varieties over finite fields–––recovering a classic result of Deodhar, as well as its later enhancement by Marsh and Rietsch–––and how this naturally extends to spaces like double Bruhat cells and their amalgamations.
A 'Quantum Equals Classical' Theorem for npointed (Ktheoretic) GromovWitten Invariants of Lines in Homogeneous Spaces
Weihong Xu, Rutgers University
'Quantum equals classical' refers to ways of identifying (Ktheoretic) GromovWitten invariants on a flag variety G/P with (Ktheoretic) classical invariants on a possibly different flag variety G/Q. It has been an active research topic in the last 20 years, but by far all the work has been for 3pointed invariants. We obtain a 'quantum equals classical' theorem for npointed, genus 0 (Tequivariant Ktheoretic) GromovWitten invariants of lines in G/P, which generalizes the 3pointed result of Leonardo Mihalcea and Changzheng Li from 2013. This talk is based on joint work with Anders Buch, Linda Chen, Angela Gibney, Lauren Heller, Elana Kalashnikov, and Hannah Larson.
Matroids, Positroids, and Combinatorial Characterizations
Anastasia Chavez, UC Davis
Positroids are a special class of representable matroids introduced by Postnikov in the study of the nonnegative part of the Grassmannian. Postnikov defined several combinatorial objects that index positroids. In this talk, I'll briefly introduce some of these objects and how they can combinatorially characterize matroid properties.
Twisted quadratic foldings of root systems and combinatorial Schubert calculus
Maiko Serizawa, University of Ottawa
This thesis builds on the connection of two widely studied objects in the literature, that is, foldings of finite root systems and the structure algebras of moment graphs associated with finite root systems. Given a finite crystallographic root system <I> whose Dynkin diagram has a nontrivial automorphism, it yields a new root system <l>7 by a socalled classical folding. On the other hand, Lusztig's folding (1983) folds the root system of type Es to type H4 starting from an automorphism of the root lattice of type Es. LaniniZainoulline (2018) developed the notion of a twisted quadratic folding of a root system, which describes both the classical foldings and Lusztig's folding on the same footing. Our second key object of study is the structure algebra Z(Q) of the moment graph g associated with a finite root system and its reflection group W. The structure algebra Z(Q) is an algebra over a certain polynomial ring S whose underlying module is free with a distinguished basis { a( w) I w E W} called combinatorial Schubert classes. Each Schubert class a(w) is an Svalued function on W, whose value is explicitly known for any finite reflection group W. LaniniZainoulline (2018) showed that a twisted quadratic folding <I> v; <l>7 induces an embedding of the respective Coxeter groups E : W7 Y W and a ring homomorphism c:* : Z(Q) ➔ Z(Q7 ) between the corresponding structure algebras. This thesis investigates how the induced map c:* relates the Schubert classes of the original structure algebra Z(Q) to those of the folded structure algebra Z(Q7 ). In particular, we will provide a combinatorial criterion for a Schubert class a?u) of Z(Q7 ) to admit a Schubert class a(w) of Z(Q) such that the relation c:*(a(w) ) = c · a?u) holds for some nonzero scalar c. We will also prove that c:* is surjective after an appropriate extension of the coefficient ring. 
11:35 am  2:00 pm EDTGathertown with Lightning speakers / LunchLunch/Free Time  Virtual

2:00  2:30 pm EDTDifferential operators for Schur and Schubert polynomialsVirtual
 Speaker
 Gleb Nenashev, Brown University
 Session Chair
 Alexander Yong, University of Illinois at UrbanaChampaign
Abstract
We study differential operators for Schur and back stable Schubert polynomials. Our operators are based on two operators of degree (1), which satisfy Leibniz rule. For the case of Schur functions, these two operators fully determine the product of Schur functions, i.e., it is possible to define LittlewoodRichardson coefficients only using these operators. This new point of view on Schur functions gives us an elementary proof of The Giambelli identity and JacobiTrudi identities. For the case of Schubert polynomials, we construct a larger class of decreasing operators, which are indexed by Young diagrams. Operators from this family are related to Stanley symmetric functions. In particular, we extend bosonic operators from Schur to Schubert polynomials.

2:45  3:00 pm EDTBreakCoffee Break  Virtual

3:00  3:30 pm EDTBackstable Ktheory Schubert calculusVirtual
 Speaker
 Mark Shimozono, Virginia Tech
 Session Chair
 Alexander Yong, University of Illinois at UrbanaChampaign
Abstract
We explain some properties of backstable limits of double Grothendieck polynomials. We discuss the dual basis of equivariant Khomology of the infinite Grassmannian.

3:45  4:15 pm EDTBow varietiesVirtual
 Speaker
 Richard Rimanyi, University of North Carolina at Chapel Hill
 Session Chair
 Alexander Yong, University of Illinois at UrbanaChampaign
Abstract
There are pairs of seemingly unrelated spaces such that Schubert calculus on the two spaces `match’ in some concrete sense. This duality, called 3d mirror symmetry, is best observed in equivariant elliptic cohomology. For example, the dual of the 12dimensional $T^*Gr(2,5)$ is a certain 4dimensional Nakajima quiver variety. However, the set of cotangent bundles of homogeneous spaces, or even the set of quiver varieties are not closed for 3d mirror symmetry. In this talk, based on a joint work with Y. Shou, we present a larger pool of spaces: Cherkis bow varieties. Superstring theory predicts that bow varieties are closed for 3d mirror symmetry. The combinatorics necessary to play Schubert calculus on bow varieties includes binary contingency tables and tie diagrams. The existence of an operation (called HananyWitten transition) gives bow varieties extra flexibility. We will illustrate the combinatorics and geometry of bow varieties with examples, and we will calculate cohomological and elliptic Schubert classes (rather, `stable envelopes’) to explain the `matching Schubert calculus’ phenomenon.
Thursday, March 25, 2021

9:00  9:45 am EDTGathertown Morning CoffeeCoffee Break  Virtual

9:45  10:15 am EDTRefined Dubrovin's conjecture for coadjoint varietiesVirtual
 Speaker
 Nicolas Perrin, Versailles SaintQuentinenYvelines University
 Session Chair
 Anders Buch, Rutgers University
Abstract
Let X be a Fano variety. Dubrovin’s conjecture predicts, among other things, an equivalence between the semisimplicity of QH(X) the big quantum cohomology of X and the existence of a full exceptional collection in D(X) the bounded derived category of X. Recently, Kuznetsov and Smirnov proposed a refinement of this conjecture if qH(X) the small quantum cohomology is not semisimple. I will present this conjecture, discuss it in the case of coadjoint varieties and make connections with roots systems and simple surface singularities. This is a joint work with Maxim Smirnov

10:30  10:45 am EDTBreakCoffee Break  Virtual

10:45  11:15 am EDTmotivic Chern classes of Schubert cells and applicationsVirtual
 Speaker
 Changjian Su, University of Toronto
 Session Chair
 Anders Buch, Rutgers University
Abstract
The motivic Chern class in Ktheory is a generalization of the ChernSchwartzMacPherson class in homology. As the affine Hecke algebra, the motivic Chern classes of Schubert cells have a Langlands dual description. In joint works with P. Aluffi, L. Mihalcea, and J. Schurmann, we use this description to solve some problems about the Casselman basis for the padic dual group, and relate the Euler characteristic of the motivic Chern classes to the IwahoriWhittaker functions.

11:30 am  1:30 pm EDTLunch/Free TimeVirtual

2:00  2:30 pm EDTSkewsymmetric matrix Schubert varietiesVirtual
 Speaker
 Brendan Pawlowski, University of Southern California
 Session Chair
 Thomas Lam, University of Michigan
Abstract
Imposing conditions on the ranks of upperleft corners of a skewsymmetric matrix defines a skewsymmetric matrix Schubert variety. We give Gröbner bases for the prime ideals of these varieties, and identify the corresponding initial ideals as the StanleyReisner ideals of certain explicit shellable simplicial complexes. These results are analogous to results of Knutson and Miller in the setting of ordinary matrix Schubert varieties, but the techniques are new, and can be used to give new proofs of some of their results. Based on joint work with Eric Marberg.

2:45  3:00 pm EDTBreakCoffee Break  Virtual

3:00  3:30 pm EDTCanonical Bases for Coulomb BranchesVirtual
 Speaker
 Harold Williams, University of Southern California
 Session Chair
 Thomas Lam, University of Michigan
Abstract
Coulomb branches of 4d N=2 gauge theories are a class of algebraic varieties which, together with their quantizations, appear in a variety of guises in geometry and representation theory. In this talk we outline a construction of canonical bases in the quantized coordinate rings of these varieties. These bases appear as byproducts of a more fundamental construction, a nonstandard tstructure on the dg category of coherent sheaves on the BravermanFinkelbergNakajima space of triples. The heart of this tstructure is a tensor category which is not braided, but admits renormalized rmatrices abstracting those appearing in the finite dimensional representation theory of quantum affine algebras and KLR algebras. It is intended to provide a mathematical model of the category of halfBPS line defects in the relevant gauge theory, and is inspired by earlier work of KapustinSaulina and GaiottoMooreNeitzke. On a combinatorial level, the resulting bases are expected to be controlled by specific cluster algebras, which we can confirm in simple examples. This is joint work with Sabin Cautis.

3:45  4:00 pm EDTBreakCoffee Break  Virtual

4:00  4:30 pm EDTSchubert puzzles from Lagrangian correspondencesVirtual
 Speaker
 Allen Knutson, Cornell University
 Session Chair
 Thomas Lam, University of Michigan
Abstract
Correspondences Z inside M x N induce maps on cohomology H^*(M) > H^*(N), modulo smoothness and compactness issues. We soup up the diagonal inclusion Gr(k,n) > Gr(k,n)^2 (whose induced cohomology map is multiplication) to a correspondence of cotangent bundles, which we then factor through T^*(a 2step flag manifold). Grassmannian Schubert puzzles turn out to be a calculation on this intermediate manifold. As an application of this viewpoint we get a puzzle rule for the Euler characteristic of the intersection of three generically translated Bruhat cells (in upto4step flag manifolds).
Friday, March 26, 2021

9:00  9:45 am EDTGathertown Morning CoffeeCoffee Break  Virtual

9:45  10:15 am EDTA combinatorial Chevalley formula for semiinfinite flag manifolds and its applicationsVirtual
 Speaker
 Cristian Lenart, University at Albany
 Session Chair
 David Anderson, Ohio State University
Abstract
I present a combinatorial Chevalley formula for an arbitrary weight, in the torusequivariant Kgroup of semiinfinite flag manifolds, which is expressed in terms of the socalled quantum alcove model. One application is the Chevalley formula for antidominant fundamental weights in the (small) torusequivariant quantum Ktheory of the flag manifold G/B; this has been a longstanding conjecture. I also discuss the Chevalley formula for partial flag manifolds G/P. Another application is that the socalled quantum Grothendieck polynomials indeed represent Schubert classes in the (nonequivariant) quantum Ktheory of the type A flag manifold. This is joint work with Satoshi Naito and Daisuke Sagaki.

10:30  10:45 am EDTBreakCoffee Break  Virtual

10:45  11:15 am EDTCoxeterlike elements, Schubert geometry, and multiplicityfreeness in algebraic combinatoricsVirtual
 Speaker
 Alexander Yong, University of Illinois at UrbanaChampaign
 Session Chair
 David Anderson, Ohio State University
Abstract
For a finite Coxeter system, we define “spherical elements” (extending Coxeter elements). Conjecturally, for Weyl groups, spherical elements index Levispherical Schubert varieties in G/B. In type A, this connects to key polynomials (Demazure characters), multiplicityfreeness, and “splitsymmetry” in algebraic combinatorics. This is joint work (some ongoing) with subsets of David Brewster (UIUC), Yibo Gao (MIT), and Reuven Hodges (UIUC). See arXiv:2007.09238, arXiv:2007.09229, and arXiv:2012.09749.

11:30 am  2:00 pm EDTLunch/Free TimeVirtual

2:00  2:30 pm EDTPositroids, knots, and q,tCatalan numbersVirtual
 Speaker
 Pavel Galashin, University of California, Los Angeles
 Session Chair
 Angela Gibney, Rutgers University, New Brunswick
Abstract
We relate the cohomology of open positroid varieties and their point counts over finite fields to knot homology. In particular, we show that the bigraded Poincaré polynomials of topdimensional open positroid varieties are given by rational q,tCatalan numbers. As a consequence of the curious Lefschetz property, we obtain q,tsymmetry and unimodality statements for rational q,tCatalan numbers. Joint work with Thomas Lam. No special background on the above objects will be assumed.

2:45  3:00 pm EDTBreakCoffee Break  Virtual

3:00  3:30 pm EDTPuzzle rules in cotangent Schubert calculusVirtual
 Speaker
 Paul ZinnJustin, The University of Melbourne
 Session Chair
 Angela Gibney, Rutgers University, New Brunswick
Abstract
We'll present several combinatorial rules (in terms of ``puzzles'') for the expansion of the product of motivic Segre classes (in equivariant Ktheory) and SegreSchwartzMacPherson classes (in equivariant cohomology) in partial flag varieties. We'll show how to compute such classes and puzzles using the software Macaulay2.
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