Vyjayanthi Chari
vyjayanc at ucr.edu 
Wee Liang Gan
wlgan at math.ucr.edu 
Jacob Greenstein
jacobg at ucr.edu 
Carl Mautner
carlm at ucr.edu 
January 23, 2018  
12 p.m.  Surge 284 
Jens Eberhardt (UCLA)
Category $\mathcal{O}$ and Mixed Geometry. Abstract.
Many questions about the representation theory of a complex semisimple Lie
group can be understood in terms of the category
$\mathcal{O}(\mathfrak{g})$ associated to its Lie algebra. Both categories are intimately related to the mixed geometry of the flag variety. In characteristic $0$, categories of certain mixed $\ell$adic sheaves, mixed Hodge modules or stratified mixed Tate motives provide geometric versions of the derived graded category $\mathcal{O}(\mathfrak{g})$ (Beilinson, Ginzburg, Soergel and Wendt).
Using the work of Soergel, we prove analogous statements in characteristic $p$. First, we construct an
appropriate formalism of "mixed modular sheaves", using motives in equal characteristic. We then apply
this formalism to construct a geometric version of the of the derived graded modular category
$\mathcal{O}(G)$. (This is joint work with Shane Kelly).

January 30, 2018  
12 p.m.  Surge 284 
Karina Batistelli (CIEM  CONICET, Universidad Nacional de Córdoba, Argentina)
Quasifinite highest weight modules of the "orthogonal" and "symplectic" types Lie subalgebras of the matrix quantum pseudodifferential operators Abstract. In this talk we will characterize the irreducible quasifinite highest weight modules of some subalgebras of the Lie algebra of $N\times N$ matrix quantum pseudodifferential operators.
In order to do this, we will first give a complete description of the antiinvolutions that
preserve the principal gradation of the algebra of $N\times N$ matrix quantum pseudodifferential
operators and we will describe the Lie subalgebras of its minus fixed points. We will obtain,
up to conjugation, two families of antiinvolutions that show quite different results when
$n = N$ and $n < N$. We will then focus on the study of the "orthogonal" and "symplectic"
type subalgebras found for case $n = N$, specifically the classification and realization of the
quasifinite highest weight modules.

February 6, 2018  
12 p.m.  Surge 284 
Matheus Brito (Universidade Federal do Paraná, Brazil)
BGG resolutions of prime representations of quantum affine $sl_{n+1}$. Abstract.
We study the family of prime representations of quantum affine
$sl_{n+1}$ introduced in the work of Hernandez and Leclerc which are defined
by using an $A_n$ quiver. We show that such representations admit a BGGtype
resolution where the role of the Verma module is played by the local Weyl
module. This leads to a closed formula (the Weyl character formula) for
the character of the irreducible representation as an alternating sum of
characters of local Weyl modules.

October 17, 2017  
12 p.m.  Surge 284 
Neal Livesay
Simple affine roots, lattice chains, and parahoric subgroups Abstract.
In the representation theory of $\mathrm{GL}_n(\mathbb{C})$, there are correspondences between
sets of simple roots, flags in $\mathbb{C}^n$, and parabolic subgroups, that are wellbehaved
with respect to actions by $\mathrm{GL}_n(\mathbb{C})$. These objects index a simplicial complex
$\mathcal{B}(\mathrm{GL}_n(\mathbb{C}))$, called the building of
$\mathrm{GL}_n(\mathbb{C})$. There is an affine version of the building, called the
BruhatTits building, whose simplices correspond to sets of simple affine roots,
lattice chains, and parahoric subgroups. The primary goal of this expository talk is to give lots of low
rank examples (i.e., for $\mathrm{SL}_2$, $\mathrm{SL}_3$, and $\mathrm{Sp}_4$) for each of these
correspondences. If time permits, I will describe the relation between the BruhatTits building
and a wellbehaved class of filtrations on the loop algebra called MoyPrasad filtrations.
These filtrations will be used in my next talk to define a geometric analogue of fundamental
strata (originally developed by C. Bushnell for $p$adic representation theory).

October 24, 2017  
12 p.m.  Surge 284 
Neal Livesay
Simple affine roots, lattice chains, and parahoric subgroups (cont.)

November 3, 2017  
12:101 p.m.  Surge 268 
Ivan Loseu (Northeastern University)
Deformations of symplectic singularities and Orbit method Abstract.
Symplectic singularities were introduced by Beauville in 2000. These are especially nice singular
Poisson algebraic varieties that include symplectic quotient singularities and the normalizations of
orbit closures in semisimple Lie algebras. Poisson deformations of conical symplectic singularities
were studied by Namikawa who proved that they are classified by the points of a vector space.
Recently I have proved that quantizations of a conical symplectic singularity are still classified
by the points of the same vector spaces. I will explain these results and then apply them to
establish a version of Kirillov's orbit method for semisimple Lie algebras.

November 7, 2017  
12 p.m.  Surge 284 
Bach Nguen (Louisiana State University)
Noncommutative discriminants via Poisson geometry and representation theory Abstract.
The notion of discriminant is an important tool in number theory, algebraic
geometry and noncommutative algebra. However, in concrete situations, it is
difficult to compute and this has been done for few noncommutative algebras
by direct methods. In this talk, we will describe a general method for
computing noncommutative discriminants which relates them to representation
theory and Poisson geometry. As an application we will provide explicit
formulas for the discriminants of the quantum Schubert cell algebras at
roots of unity. If time permits, we will also discuss this for the case of
quantized coordinate rings of simple algebraic groups and quantized
universal enveloping algebras of simple Lie algebras. This is joint work
with Kurt Trampel and Milen Yakimov.

November 14, 2017  
12 p.m.  Surge 284 
Ethan Kowalenko
KazhdanLusztig polynomials and Soergel bimodules Abstract.
Over the summer, I learned about Soergel bimodules at the MSRI
from Ben Elias and Geordie Williamson, who were able to use these bimodules
to solve a conjecture about KazhdanLusztig (KL) polynomials for an
arbitrary Coxeter system $(W,S)$. The aim of these talks will be twofold.
First, I want to define the KL polynomials. This will involve looking in
the Hecke algebra of a Coxeter system, and showing how the KL polynomials
describe one basis of the Hecke algebra in terms of another. The second
goal will then be to describe the category of Soergel bimodules, and to
show how they can be used to prove that the coefficients of the KL
polynomials are nonnegative.

November 16, 2017  
12 p.m.  Surge 284 
Ethan Kowalenko
KazhdanLusztig polynomials and Soergel bimodules (cont.) 
May 2, 2017  
12 p.m.  Surge 284 
Daniele Rosso
Exotic Springer Fibers Abstract.
The Springer resolution is a resolution of singularities of the variety of nilpotent elements in a semisimple Lie algebra. It is an important geometric construction in representation theory, but some of its features are not as nice if we are working in Type $C$ (Symplectic group). To make the symplectic case look more like the Type $A$ case, Kato introduced the exotic nilpotent cone and its resolution, whose fibers are called the exotic Springer fibers. We give a combinatorial description of the irreducible components of these fibers in terms of standard Young bitableaux. This is joint work with Vinoth Nandakumar and Neil Saunders.

May 16, 2017  
12 p.m.  Surge 284 
Daniele Rosso
A graphical calculus for the Jack inner product on symmetric functions Abstract.
Starting from a graded Frobenius superalgebra $B$, we consider a
graphical calculus of $B$decorated string diagrams. From this calculus we
produce algebras consisting of closed planar diagrams and of closed annular
diagrams. The action of annular diagrams on planar diagrams can be used to
make clockwise (or counterclockwise) annular diagrams into an inner product
space. I will explain how this gives a graphical realization of the space
of symmetric functions equipped with the Jack inner product. This is joint
work with Tony Licata and Alistair Savage.

June 8, 2017  
12 p.m.  Surge 284 
Yilan Tan
The irreducibility of the Verma Modules for Yangians and twisted Yangians Abstract.
In this talk, we first introduce the definition of Yangians for the reductive complex Lie
algebra $\mathfrak{gl}(N)$, describe their finitedimensional representations and give necessary
and
sufficient conditions for the irreducibility of the Verma modules. Next, the definition of
twisted Yangians is introduced. In the end, we give necessary and sufficient conditions
for the irreducibility of the Verma modules for twisted Yangian $Y^{tw}(\mathfrak{sp}_2)$.

January 17, 2017  
12 p.m.  Surge 284 
Ting Xue (Melbourne)
The Springer correspondence for symmetric spaces and Hessenberg varieties Abstract.
The Springer correspondence relates nilpotent orbits in the Lie group of a reductive algebraic group to
irreducible representations of the Weyl group. We develop a Springer theory in the case of symmetric spaces
using
Fourier transform, which relates nilpotent orbits in this setting to irreducible representations of Hecke al
gebras
at $q=1$. We discuss applications in computing cohomology of Hessenberg varieties. Examples of such varieties
include classical objects in algebraic geometry: Jacobians, Fano varieties of $k$planes in the intersection of two
quadrics, etc. This is based on joint work with Tsaohsien Chen and Kari Vilonen. 
January 24, 2017  
12 p.m.  Surge 284 
Xinli Xiao
Nakajima's double of representations of COHA Abstract.
Given a quiver $Q$ with/without potential, one can construct an
algebra structure on the cohomology of the moduli stacks of representations
of $Q$. The algebra is called Cohomological Hall algebra (COHA for short).
One can also add a framed structure to quiver $Q$, and discuss the moduli
space of the stable framed representations of $Q$. Through these geometric
constructions, one can construct two representations of Cohomological Hall
algebra of Q over the cohomology of moduli spaces of stable framed
representations. One would get the double of the representations of
Cohomological Hall algebras by putting these two representations together.
This double construction implies that there are some relations between
Cohomological Hall algebras and some other algebras. In the talk we will
focus on two specific examples: $A_1$ quiver and the Jordan quiver.

February 9, 2017  
12 p.m.  Surge 284 
Daniele Rosso
Symmetry, Combinatorics and Geometry Abstract.
Symmetry is a concept that most people grasp intuitively, and it
has important applications in several branches of mathematics, as well as
other sciences. We will focus on a surprising connection between the
geometry of subspaces of a vector space (like lines and planes) and an
algorithm that was originally defined for purposes of combinatorics
(arranging and counting things in various ways).

February 28, 2017  
12 p.m.  Surge 284 
Deniz Kus (Universität Bonn, Germany)
Graded tensor products for Lie (super)algebras Abstract.
In this talk I will discuss the construction of graded tensor products for
the current algebra associated to a Lie (super)algebra. For the
orthosymplectic Lie superalgebra we will show that these representations
can be filtered by the corresponding graded tensor products for the
underlying reductive Lie algebra. In the second part of my talk, I will
discuss the appearence of graded tensor products in PBW theory and
categorification. One of the future goals is to understand which
2representation of the categorified quantum group corresponds to graded
tensor products.

March 2, 2017  
45 p.m.  Surge 284 
Georgia Benkart (University of Wisconsin Madison)
Richard Block lecture: Tracing a Path  From Walks on Graphs to Invariant Theory Abstract.
Molien's 1897 formula for the Poincaré series of the polynomial invariants
of a finite group has given rise to many results in combinatorics, coding theory,
mathematical physics, algebraic geometry, and representation theory. This talk will focus on
analogues of Molien's formula for tensor invariants and will discuss various connections with
representation theory, and the McKay Correspondence. The approach is via walking on graphs.

March 7, 2017  
12 p.m.  Surge 284 
Ryo Fujita (Kyoto University, Japan)
Tilting modules in affine higest weight categories Abstract.
Affine highest weight category, introduced by Kleshchev, is a generalization
of the notion of highest weight category. For example, some module categories over
central completions of (degenerate) affine Hecke algebras (more generally KLR algebras of finite type) and
polynomial current Lie algebras are known to be affine highest weight.
In this talk, we consider tilting modules in affine highest weight categories and explain
that a complete collection of indecomposable tilting modules exists if our category has
a large center. As an application, we gives a simple criterion for an exact functor
between two affine highest weight categories to give an equivalence.
We can apply this criterion to the ArakawaSuzuki functor on the deformed category $\mathcal{O}$
for $\mathfrak{gl}_{m}$.

October 25, 2016  
12 p.m.  Surge 284 
Sarah Kitchen (UMich)
HarishChandra and Generalized HarishChandra Modules Abstract. The representation theory of real reductive Lie groups can be
studied by complex algebraic and geometric methods using infinitesimal approximations called HarishChandra
modules. HarishChandra modules are representations of the corresponding complex reductive Lie algebra which are
locally finite with respect to the complexification of the maximal compact subgroup of the original real Lie
group. Generalized HarishChandra modules are a generalization in which the maximal compact group is replaced by
an arbitrary reductive subalgebra of the complex Lie algebra. In this talk we will review the geometric
classification of simple HarishChandra modules, and compare this case to the setting of generalized
HarishChandra modules. This work is part of a program initiated by Ivan Penkov and Gregg Zuckerman.

(WEDNESDAY!) June 8, 2016  
12 p.m.  Surge 284 
Peter McNamara (Queensland)
Consequences of a categorified braid group action Abstract. It is well known that the braid group acts on a quantised
enveloping
algebra by algebra automorphisms. We discuss the categorification of
this braid group action and some of its consequences. Applications
include constructing reflection functors for KLR algebras, and a
theory of restricting a categorical representation along a face of a
Weyl polytope. 
September 29, 2015  
12 p.m.  Surge 284 
Yilan Tan
The local Weyl modules of Yangians Abstract. Dr. Chari and Dr. Pressley introduced the local
Weyl$
better understanding of the category of finitedimensional highest
weight representations of quantum affine algebras in 2001. Then this
notion of a local Weyl module has been extended to the
finitedimensional representations of current algebras, twisted loop
algebras and current Lie algebras on anffine varieties. Dr. Nicolas
and the author extended the notion to the finitedimensional
representations of Yangians. 
October 6, 2015  
12 p.m.  Surge 284 
Yilan Tan
Braid group actions and tensor products for Yangians Abstract. We introduce a braid group action for the finitedimensional
representations of Yangians $Y(\mathfrak{g})$, where $\mathfrak{g}$ is
a complex simple Lie algebra. It provides an efficient way to compute
certain polynomials which allows us to provide a finite set of numbers
at which the tensor product of KirillovReshetikhin modules of
Yangians may fail to be cyclic.

October 13, 2015  
12 p.m.  Surge 284 
Andrea Appel (University of Southern California)
Flat connections and KacMoody algebras Abstract. The aim of this talk is to describe the
analytic nature of the quantum groups.
More specifically, I will show how the quantum groups can be interpreted as a natural
receptacles for the monodromy representations of certain flat connections arising in the
representation theory of KacMoody algebras. 
October 20, 2015  
12 p.m.  Surge 284  Robin Walters (Northeastern University) The BernsteinSato polynomial of the Vandermonde determinant and the Strong Monodromy Conjecture Abstract. The BernsteinSato polynomial, or bfunction, is an important invariant in
singularity theory, which is difficult to compute in general. We describe a few different results towards computing the
bfunction of the Vandermonde determinant $\xi$. In 1989, Eric Opdam computed the bfunction of a related polynomial,
and we use his result to produce a lower bound for the bfunction of $\xi$. We use this lower bound to prove a
conjecture of Budur, Mustata, and Teitler for the case of finite Coxeter hyperplane arrangements, proving the Strong
Monodromy Conjecture in this case. In our second result, we use duality of some Dmodules to show that the roots of this
bfunction of $\xi$ are symmetric about 1. Finally, we use results about jumping coefficients together with Kashiwara's
proof that the roots of a bfunction are rational in order to prove an upper bound for the bfunction of $\xi$ and give
a conjectured formula. This is a joint work with Asilata Bapat. 
November 3, 2015  
12 p.m.  Surge 284 
Michael McBreen (Massachusetts Institute of Technology)
P=W for nodal curves Abstract. I will describe the P=W conjecture of de CataldoHauselMigliorini. It
concerns the moduli of local systems and higgs bundles on a smooth curve, and the relation between their cohomology
groups.
I will then discuss work in progress with Zsuzsanna Dancso and Vivek Shende to formulate and prove an analogous
conjecture for nodal curves.

November 10, 2015  
12 p.m.  Surge 284 
Alexis Bouthier (University of California, Berkeley)
Arc spaces of spherical varieties in representation theory Abstract. We will focus on two kinds of spherical varieties, the
Lmonoid and the affine closure of G/U for a connected reductive group G and U its unipotent radical.
We will first explain the joint work of the speaker with B.C. Ngo and Y. Sakellaridis which gives a way to
construct geometrically unramified local Lfactors. Nevertheless, the geometric situation is nicely defined
only globally as it is also the case for the affine closure of G/U. Locally, we need to consider arc
spaces of these spherical varieties which are infinite dimensional and for which there was no theory of
perverse sheaves on it. We will then explain the recent work of the speaker with D. Kazhdan which enables
to construct such objects and compare it with the global ones. 
December 1, 2015  
12 p.m.  Surge 284 
Carl Mautner
Geometric and combinatorial analogues of Schur algebras Abstract.
I will begin by recalling the definition and some basic properties of Schur algebras, which play an
important role in the representation theory of general linear groups and symmetric groups and are
particularly interesting in positive characteristic. I will then describe a geometric interpretation of
the category of their representations. Using this description as motivation, I will discuss joint work
with Tom Braden, in which we define two new classes of algebras. One is associated to geometric spaces
called hypertoric varieties and the other to combinatorial objects called matroids. 
May 5, 2015  
12 p.m.  Surge 284 
Ivan Loseu (Northeastern University)
Representation theory of quantized quiver varieties Abstract.
Nakajima quiver varieties are moduli spaces of certain representations of quivers. They play an important role in Algebraic Geometry, Mathematical Physics and Geometric Representation theory. Their quantizations are noncommutative associative algebras with interesting and rich representation theory conjecturally related to deep geometric properties of the underlying varieties. I will explain some reasons to be interested in that representation theory and also some results in the important special case of quantized Gieseker moduli spaces based on preprint arXiv:1405.4998. All necessary information about quiver varieties and their quantizations will be introduced during the talk.

May 26, 2015  
12 p.m.  Surge 284 
Mathew Lunde
Dissertation defense

June 2, 2015  
12 p.m.  Surge 284 
Jeffrey Wand
Dissertation defense

June 4, 2015  
12 p.m.  Surge 284 
John Dusel
Dissertation defense

June 9, 2015  
12 p.m.  Surge 284 
Peri Shereen
Dissertation defense

January 22, 2015  
12 p.m.  Surge 284 
Carl Mautner
A gentle introduction to constructible derived categories Abstract. I will briefly motivate and give a gentle introduction to the constructible derived category of sheaves on a (complex) algebraic variety. Next Tuesday, in part 2 of the talk, we will discuss some connections to the representation theory of reductive algebraic groups. 
January 27, 2015  
12 p.m.  Surge 284 
Carl Mautner
A gentle introduction to constructible derived categories 
February 5, 2015  
12 p.m.  Surge 284 
John Dusel
Balanced parabolic quotients and branching rules for Demazure crystals Abstract. We study a subset of a parabolic quotient in a simplylaced Weyl group $W$stable under an automorphism $\sigma$, which we call the balanced parabolic quotient. This subset describes the interaction between the branching rule for a Levi subalgebra, Demazure modules, and $\sigma$invariant weight spaces in $\sigma$stable simple modules for the corresponding Lie algebra. The Hasse diagram of the balanced parabolic quotient under the Bruhat order is a forest with a remarkable selfsimilarity property. We characterize an element of a balanced quotient on the level of the root system of $W$, and find that the subalgebras of the Borel associated with these elements decompose into the direct sum of two subalgebras: one contained in the Borel for a Levi subalgebra, and another consisting of $sigma$invariants. 
February 12, 2015  
12 p.m.  Surge 284 
Huafeng Zhang (Université Paris 7, France)
Asymptotic representations of quantum affine superalgebras of type $A$ Abstract. For the quantum affine superalgebra associated to a general linear Lie superalgebra, we construct inductive systems of KirillovReshetikhin modules. We endow their inductive limits with module structures over the full quantum affine superalgebra in the spirit of HernandezJimbo. Then we indicate some consequences from the asymptotic construction. 
February 1415, 2015  
Workshop on Lie algebras, Lie groups and their representations
 
February 25, 2015  
12 p.m.  Surge 284 
Jonas Hartwig
Graded algebras attached to quivers Abstract.
I will talk about certain graded algebras attached to (multi)quivers. Examples include the
$n$th Weyl algebra, as well as quotients of enveloping algebras of Lie algebras of types
$A$ and $C$. These algebras come with a canonical representation by differential operators and provide new solutions to the consistency equations for twisted generalized Weyl algebras. Properties of the quiver (Cartan matrix, equilibrium conditions, acyclicity) are directly related to properties of the corresponding algebra. We also present three new constructions (tensor products, invariant subalgebras, and folding). Finally, we discuss quantum analogs and list some open questions. Part of this talk is based on joint work with Vera Serganova.

March 3, 2015  
12 p.m.  Surge 284 
Daniele Rosso
Mirabolic quantum $\mathfrak{sl}_2$ Abstract.
BeilinsonLusztigMacPherson gave a construction of the quantum enveloping algebra $U_q(\mathfrak{sl}_n)$ (and of the $q$Schur algebras, which are certain finite dimensional quotients) as a convolution algebra on the space of pairs of partial $n$step flags. I will define a convolution product in the mirabolic setting (triples of two partial flags and a vector) and give some results about the algebra obtained in this way, when $n=2$.

March 10, 2015  
12 p.m.  Surge 284 
Wee Liang Gan
What is representation stability? Abstract.
In the last few years, a framework was developed (by Church, Ellenberg and
Farb) to discover and understand many stabilization phenomenon which occur
in sequences of representations for a family of groups. I will give an
introduction to this new theory and its applications.

October 9, 2014  
12 p.m.  Surge 284 
Daniele Rosso
Categorification from towers of algebras Abstract. If we have a sequence of associative algebras that satisfy certain conditions, we call it a tower of algebras. We will see how, in this setting, induction and restriction functors give us the structure of two dual Hopf algebras on the Grothendieck groups of the two categories respectively of all modules for the algebras and of all projective modules. 
October 14, 2014  
12 p.m.  Surge 284 
Daniele Rosso
Categorification from towers of algebras (cont.)

October 21, 2014  
12 p.m.  Surge 284 
Jacob Greenstein
Hall algebras, Nichols algebras and bases Abstract. The first part of the talk will be centered around Nichols algebras in braided tensor categories. They often appear in various algebraic contexts (for example, the upper triangular part of the quantum group, or Lusztig's algebra, is an example of a Nichols algebra). One of their nice properties is that they are generated, as algebras, by their primitive elements which, in principle, allows to find their relations explicitly. Since Hall algebras are known to provide realizations of quantum groups and are algebras and coalgebras in a braided tensor category, it is only natural to ask how far they are from being Nichols algebras. It turns out that Hall algebras of a large class of categories have the PoincaréBirkhoffWitt property and are primitively generated. In the second part of the talk we will discuss bosonisation of Nichols algebras and their doubles and a procedure for constructing bases in them, which is in turn motivated by Hall algebras and their properties. (joint work with A. Berenstein) 
October 28, 2014  
12 p.m.  Surge 284 
Jacob Greenstein
Hall algebras, Nichols algebras and bases (cont.)

November 4, 2014  
12 p.m.  Surge 284 
Jonas Hartwig
Quantum affine modules for nontwisted affine KacMoody algebras Abstract. I will talk about the results of a recent joint paper with V. Futorny and E. Wilson where we construct new irreducible weight modules over quantum affine algebras in which all weight spaces are infinitedimensional. They are obtained by parabolic induction from irreducible modules over the quantum Heisenberg subalgebra. 
November 6, 2014  
12 p.m.  Surge 284 
Peter Fiebig (FriedrichAlexanderUniversität ErlangenNürnberg, Germany, and MSRI)
Modular sheaves, modular representations and Lusztig's conjecture Abstract. After giving a quick introduction to Lusztig's conjecture on the irreducible characters of reductive algebraic groups we give an overview on recent approaches and results. In particular, we sketch a proof of the conjecture for almost all characteristics. If time permits, we will also talk about the counterexamples to the strong form of Lusztig's conjecture that were found by Williamson in 2012. 
November 13, 2014  
12 p.m.  Surge 284 
Daniel Juteau (Université de Caen, France, and MSRI)
Modular Generalized Springer Correspondence Abstract. For a reductive group $G$, the Springer correspondence is an injection from irreducible representations of the Weyl group $W$ to the simple $G$equivariant perverse sheaves on the nilpotent cone of the Lie algebra (or the unipotent variety of the group). However, in general not all simple perverse sheaves arise in this way. This led Lusztig to define a generalized Springer correspondence, involving the process of inducing cuspidal perverse sheaves from Levi subgroups. The classical correspondence is the part coming from a maximal torus. In the case of the general linear group, though, nothing new arises in this way. In my thesis I studied a modular Springer correspondence, where one takes modular representations of the Weyl group and perverse sheaves with positive characteristic coefficients. In this talk I will explain the modular version of the generalized Springer correspondence, focusing on the case of the general linear group. In the modular case there is something new, namely there is a cuspidal perverse sheaf supported by the regular nilpotent orbit when the rank is a power of the characteristic. I will also mention the most striking phenomena concerning groups of other types. This is joint work with Pramod Achar, Anthony Henderson and Simon Riche. 
November 18, 2014  
12 p.m.  Surge 284 
Mathew Lunde
Selfextensions and prime factorization of representations of quantum loop algebras Abstract. The category of finite dimensional representations of a quantum loop algebra $U_q(L\mathfrak g)$ is not semisimple. Moreover, the tensor product of irreducible representations remains irreducible generically. This leads naturally to the definition of prime objects: the factorization of irreducible objects into irreducible primes. We show that there is an interesting connection between the notion of primes and the homological properties of the category, namely, for $\mathfrak g=\mathfrak sl_2$, an irreducible representation $V$ is a tensor product of $r$ prime representations if and only if the dimension of the space of self extensions of $V$ is $r$. 
December 2, 2014  
12 p.m.  Surge 284 
Peri Shereen
A Steinberg type decomposition theorem for higher level Demazure modules Abstract. We study Demazure modules which occur in a level $\ell$ irreducible integrable representation of an affine Lie algebra. We also assume that they are stable under the action of the standard maximal parabolic subalgebra of the affine Lie algebra. We prove that such a module is isomorphic to the fusion product of «prime» Demazure modules, where the prime factors are indexed by dominant integral weights which are either a multiple of $\ell$ or take value less than $\ell$ on all simple coroots. Our proof depends on a technical result which we prove in all the classical cases and $G_2$. Calculations with mathematica show that this result is correct for small values of the level. Using our result, we show that there exist generalizations of $Q$systems to pairs of weights where one of the weights is not necessarily rectangular and is of a different level. Our results also allow us to compare the multiplicities of an irreducible representation occurring in the tensor product of certain pairs of irreducible representations, i.e., we establish a version of Schur positivity for such pairs of irreducible modules for a simple Lie algebra. 
March 4, 2014  
12 p.m.  Surge 284 
Liping Li
Introduction to DG categories Abstract.
Differential graded categories are widely used in homological algebra, representation
theory, and algebraic geometry. The purpose of these talks is to provide an introduction
to the theory of DG categories, focusing on definitions, basic properties, main results,
and its relationship to derived categories and triangulated categories. To minimize preliminaries, these talks will be selfcontained except some elementary category theory and homological algebra such as categories and functors, chain complexes and chain maps.

March 6, 2014  
12 p.m.  Surge 284 
Liping Li
Introduction to DG categories (cont.)

January 14, 2014  
12 p.m.  Surge 284 
Lisa Schneider
Oral exam

January 16, 2014  
12 p.m.  Surge 284 
Mathieu Mansuy (Université Paris VII, France)
Extremal loop weight representations of quantum toroidal algebras Abstract. We give different constructions of new integrable representations of quantum
toroidal algebras, called extremal loop weight representations. Its definition is given by Hernandez in 2009, following the one of
extremal weight representations of quantum affine algebras by Kashiwara. The aim,
like in the works of Kashiwara, is to construct finitedimensional representations of the quantum toroidal algebras,
but at roots of unity in this case.

January 21, 2014  
12 p.m.  Surge 284 
Mathieu Mansuy (Université Paris VII, France)
Extremal loop weight representations of quantum toroidal algebras (cont.) Abstract. We give different constructions of new integrable representations of quantum
toroidal algebras, called extremal loop weight representations. Its definition is given by Hernandez in 2009, following the one of
extremal weight representations of quantum affine algebras by Kashiwara. The aim,
like in the works of Kashiwara, is to construct finitedimensional representations of the quantum toroidal algebras,
but at roots of unity in this case.

January 23, 2014  
12 p.m.  Surge 284 
Vyjayanthi Chari
An introduction to quantum groups

January 28, 2014  
12 p.m.  Surge 284 
Peri Shereen
Clifford algebras and spin modules Abstract.
A fundamental module for any finite dimensional, simple Lie algebra
$\mathfrak{g}$ is an irreducible module of highest weight $\omega_i$ where
$\omega_i$ is a fundamental weight of $\mathfrak{g}$. In the case of $A_n$ it
happens that all the fundamental modules are isomorphic to exterior powers
of the natural module. In general, such isomorphisms do not always exist.
In particular, for $B_n$ and $D_n$, the fundamental modules associated to
the short root (in the case of $B_n$) and to the spin nodes (in the case of
$D_n$) are called spin modules. This talk will review the construction of
Clifford algebras and spin modules. From this construction we will exhibit
how the fundamental modules associated to the spin nodes of $D_n$ are spin
modules.

January 30, 2014  
12 p.m.  Surge 284 
Matthew O'Dell
Categorification of Algebras and Their Representations Abstract.
Categorification is the process of finding higher level structure
by replacing sets with categories, functions between sets with functors,
and relations between functions with natural transformations of functors.
This talk will focus on the categorification of algebras, and
representations of these algebras. We will define naive, weak, and strong
categorification, and look at some examples.

February 13, 2014  
12 p.m.  Surge 284 
Jiarui Fei
Highest weight categories and Macdonald polynomials Abstract.
We will follow Anton Khoroshkin's paper  Highest weight categories and Macdonald polynomials.
In the first talk, I will introduce the basic setting, including the graded Lie algebras with antiinvolution, category of finitely generated graded modules, and graded characters to the Grothendieck rings. If the module category is stratified, the Macdonaldtype polynomials can be realized via taking the character of the (proper) standard modules. This construction can be generalized to any stratified highest weight category.
However, there are many interesting nonstratified highest weight categories. For example, the bigraded Lie algebra of currents, which are related to the classical Macdonald polynomials. In those cases, we may not expect that Macdonald type polynomials are the characters of specific modules, but it is possible if we consider complexes of modules. This is the content of my second talk. I will give enough detail for those not familiar with derived categories.

February 18, 2014  
12 p.m.  Surge 284 
Aaron Lauda (University of Southern California)
Categorified quantum groups and the current algebra Abstract.
Geometric representation theory has a revealed a deep
connection between geometry and quantum groups suggesting that
quantum groups are shadows of richer algebraic structures called
categorified quantum groups. Crane and Frenkel conjectured that
these structures could be understood combinatorially and applied to
lowdimensional topology. In this lecture we will categorify quantum
groups using a simple diagrammatic calculus that requires no previous
knowledge of quantum groups. We will explain how the new diagrammatic
relations not only lift the quantum group relations to explicit isomorphisms,
they also give rise to a representation of the corresponding current algebra.

February 20, 2014  
12 p.m.  Surge 284 
Jiarui Fei
Highest weight categories and Macdonald polynomials (cont.)

October 1, 2013  
12 p.m.  Surge 284 
Dennis Hasselstrøm Pedersen (Århus universitet, Denmark)
Twisting functors Abstract.
Let $\mathfrak g$ be a semisimple finitedimensional Lie algebra and $W$ its Weyl group. I will introduce
Arkhipov's twisting functors $T_w$, $w\in W$. The twisting functors
are certain functors on the BGG
category $\mathscr O$ that can be used, for example, to construct what
is called twisted Verma modules. The functor
consists of tensoring with a "semiregular bimodule" and twisting the
action by an automorphism
corresponding to $w$. I will talk about the construction of $T_w$
and some of the properties of the
twisting functors.

October 8, 2013  
12 p.m.  Surge 284 
Jeffrey Wand
Modules wth Demazure flags and character formulae Abstract.
In these talks we study a family of finitedimensional
graded representations of the current algebra of $\mathfrak{sl}_2$ which are
indexed by partitions.
We show that for $\ell$ sufficiently large, these representations admit a
filtration by submodule where the successive quotients are Demazure modules
which occur in a level $\ell$ integrable module for $A_1^{(1)}$. We associate
to each partition and to each $\ell$ an edgelabeled directed graph which
allows us to describe in a combinatorial way the graded multiplicity of a
given level $\ell$Demazure module in the filtration. In the special case
of the partition $1^s$ and $\ell=2$, we give a closed formula for the
graded multiplicity of level two Demazure modules in a level one Demazure
module. As an application, we use our result along with the results of K.
Naoi and Lenart et al, to give the character of a $\mathfrak g$stable level
one Demazure module associated to $B_n^{(1)}$ as an explicit combination of
suitably specialized Macdonald polynomials. In the case of $\mathfrak{sl}_2$, we
also study the filtration of the level two Demazure module by level three
Demazure Modules and compute the numerical filtration multiplicities and
show that the graded multiplicites are related to (variants) of partial
theta series.

October 10, 2013  
12 p.m.  Surge 284 
Lisa Schneider
Modules wth Demazure flags and character formulae (cont.)

October 17, 2013  
12 p.m.  Surge 284 
Liping Li
Representations of modular skew group algebras Abstract.
Skew group algebras naturally generalize ordinary group algebras. Let $A$ be
a finite dimensional algebra and let $G$ be a finite subgroup of the automorphism
group of $A$. It has been shown that the skew group algebra $AG$ and $A$ share many important properties (such as finite representation type, finite global dimension, etc) if the order of $G$ is invertible. However, when the order of $G$ is not invertible, many results fail. Therefore, it is natural to ask under what conditions $AG$ and $A$ still share these properties for arbitrary $G$. The answer of this question will be described in this talk.

October 22, 2013  
12 p.m.  Surge 284 
Jiarui Fei
Vanishing cycles and cluster transformations Abstract.
For a quiver with potential, we can associate a vanishing cycle to each representation space. If there is a nice torus
action on the potential, the vanishing cycles can be expressed in terms of truncated Jacobian algebras. We study how these vanishing cycles change under the mutation of DerksenWeymanZelevinsky. The wallcrossing formula leads to a categorification of quantum cluster algebras under the assumption of existence of certain potential. This is a special case of A. Efimov's result, but our approach is more concrete and downtoearth. We also obtain a formula relating the representation Grassmannians under sinksource reflections.
In this talk, I will start with basic definitions and examples of vanishing cycles, Hall algebras, and quivers with potentials.

October 24 2013  
12 p.m.  Surge 284 
Jiarui Fei
Vanishing cycles and cluster transformations (cont.)

October 29, 2013  
12 p.m.  Surge 284 
Vyjayanthi Chari
Introduction to affine Lie algebras

October 31, 2013  
12 p.m.  Surge 284 
Vyjayanthi Chari
Introduction to affine Lie algebras

November 23, 2012  
Special session "Geometric and combinatorial aspects of representation theory"
, Western Fall Sectional meeting of the AMS
 
November 5, 2013  
12 p.m.  Surge 284 
Vyjayanthi Chari
Introduction to affine Lie algebras

November 7, 2013  
12 p.m.  Surge 284 
Vyjayanthi Chari
Introduction to affine Lie algebras

November 12, 2013  
12 p.m.  Surge 284 
Vyjayanthi Chari
Introduction to affine Lie algebras

November 14, 2013  
12 p.m.  Surge 284 
Vyjayanthi Chari
Introduction to affine Lie algebras

November 19, 2013  
12 p.m.  Surge 284 
Johanna Hennig (UCSD)
Locally finite dimensional Lie algebras Abstract.
An infinite dimensional Lie algebra is locally finite if every finitely generated subalgebra is finite dimensional. On one extreme are the simple, locally finite Lie algebras. We provide structure theorems which describe such algebras over fields of positive characteristic. On the other extreme are the maximal, locally solvable Lie algebras, which are Borel subalgebras. We provide a theorem which shows that such Lie algebras are stabilizers of maximal, generalized flags, which is a generalization of Lie's theorem. We will finish by describing some new directions in the study of these Lie algebras.

November 21, 2013  
12 p.m.  Surge 284 
Vyjayanthi Chari
Introduction to affine Lie algebras

November 26, 2013  
12 p.m.  Surge 284 
Vyjayanthi Chari
Introduction to affine Lie algebras

November 28, 2013  
12 p.m.  Surge 284 
Vyjayanthi Chari
Introduction to affine Lie algebras
