Representation Theory / Number Theory Seminar, Spring 2021

All talks online (Zoom)*. Seminar times will vary; times listed below are Mountain Time.

*For security reasons, Zoom links will not be posted here. If you would like to attend a talk, but do not have the link, please contact Gil Moss or Petar Bakic (last name at

Wednesday, January 20, 3:00-4:00pm

Speaker: Joseph Hundley, SUNY Buffalo
Title: Functorial Descent in the Exceptional Groups
Abstract: In this talk, I will discuss the method of functorial descent. This method, which was discovered by Ginzbug, Rallis and Soudry, uses Fourier coefficients of residues of Eisenstein series to attack the problem of characterizing the image of Langlands functorial liftings. We'll discuss the general structure, the results of Ginzburg, Rallis and Soudry in the classical groups, some recent attempts to extend the same ideas to the exceptional group, and challenges and new phenomena which emerge in these attempts. The new work discussed will mainly be joint with Baiying Liu. Time permitting I may also comment on unpublished work of Ginzburg and joint work with Ginzburg.

Wednesday, January 27, 3:00-4:00pm

Speaker: Lucas Mason-Brown, Oxford
Title: What is a unipotent representation?
Abstract: The concept of a unipotent representation has its origins in the representation theory of finite Chevalley groups. Let G(Fq) be the group of Fq-rational points of a connected reductive algebraic group G. In 1984, Lusztig completed the classification of irreducible representations of G(Fq). He showed:
1) All irreducible representations of G(Fq) can be constructed from a finite set of building blocks -- called `unipotent representations.'
2) Unipotent representations can be classified by certain geometric parameters related to nilpotent orbits for a complex group associated to G(Fq).
Now, replace Fq with C, the field of complex numbers, and replace G(Fq) with G(C). There is a striking analogy between the finite-dimensional representation theory of G(Fq) and the unitary representation theory of G(C). This analogy suggests that all unitary representations of G(C) can be constructed from a finite set of building blocks -- called `unipotent representations' -- and that these building blocks are classified by geometric parameters related to nilpotent orbits. In this talk I will propose a definition of unipotent representations, generalizing the Barbasch-Vogan notion of `special unipotent'. The definition I propose is geometric and case-free. After giving some examples, I will state a geometric classification of unipotent representations, generalizing the well-known result of Barbasch-Vogan for special unipotents.
This talk is based on forthcoming joint work with Ivan Loseu and Dmitryo Matvieievskyi.

Wednesday, February 17, 4:00-5:00pm

Speaker: Hiraku Atobe, Hokkaido University
Title: The Zelevinsky-Aubert duality for classical groups
Abstract: In 1980, Zelevinsky studied representation theory for p-adic general linear groups. He gave an involution on the set of irreducible representations, which exchanges the trivial representation with the Steinberg representation. Aubert extended this involution to p-adic reductive groups, which is now called the Zelevinsky-Aubert duality. It is expected that this duality preserves unitarity. In this talk, we explain an algorithm to compute the Zelevinsky-Aubert duality for odd special orthogonal groups or symplectic groups. This is a joint work with Alberto Minguez in University of Vienna.

Wednesday, March 3, 3:00-4:00pm

Speaker: Allechar Serrano López, University of Utah
Title: Counting elliptic curves with prescribed torsion over imaginary quadratic fields
Abstract: A generalization of Mazur's theorem states that there are 26 possibilities for the torsion subgroup of an elliptic curve over a quadratic extension of $\mathbb{Q}$. If $G$ is one of these groups, we count the number of elliptic curves of bounded naive height whose torsion subgroup is isomorphic to $G$ in the case of imaginary quadratic fields.

Wednesday, March 10, 3:00-4:00pm

Speaker: Justin Trias, University of East Anglia
Title: Towards an integral local theta correspondence: universal Weil module and first conjectures
Abstract: The theta correspondence is an important and somewhat mysterious tool in number theory, with arithmetic applications ranging from special values of L-functions, epsilon factors, to the local Langlands correspondence. The local variant of the theta correspondence is described as a bijection between prescribed sets of irreducible smooth complex representations of groups G_1 and G_2, where (G_1,G_2) is a reductive dual pair in a symplectic p-adic group. The basic setup in the theory (Stone-von Neumann theorem, the metaplectic group and the Weil representation) can be extended beyond complex representations to representations with coefficients in any algebraically closed field R as long as the characteristic of R does not divide p. However, the correspondence defined in this way may no longer be a bijection depending on the characteristic of R compared to the pro-orders of the pair (G_1,G_2). In the recent years, there has been a growing interest in studying representations with coefficients in as general a ring as possible. In this talk, I will explain how the basic setup makes sense over an A-algebra B, where A is the ring obtained from the integers by inverting p and adding enough p-power roots of unity. Eventually, I will discuss some conjectures towards an integral local theta correspondence. In particular, one expects that the failure of this correspondence for fields having bad characteristic does appear in terms of some torsion submodule in integral isotypic families of the Weil representation with coefficients in B.

Wednesday, March 17, 3:00-4:00pm

Speaker: Ila Varma, University of Toronto
Title: Malle's Conjecture for octic $D_4$-fields.
Abstract: We consider the family of normal octic fields with Galois group $D_4$, ordered by their discriminant. In forthcoming joint work with Arul Shankar, we verify the strong form of Malle's conjecture for this family of number fields, obtaining the order of growth as well as the constant of proportionality. In this talk, we will discuss and review the combination of techniques from analytic number theory and geometry-of-numbers methods used to prove this and related results.

Wednesday, March 24, 3:00-4:00pm

Speaker: Baiying Liu, Purdue University
Title: On recent progress on Jiang's conjecture on wave front sets of representations in Arthur packets.
Abstract: In this talk, I will introduce some recent progress on Jiang's conjecture on wave front sets of representations in Arthur packets. Jiang's conjecture is a natural generalization of Shahidi's conjecture on tempered L-packets. It shows that there is a strong connection between the structure of Arthur parameters and the wave front sets of representations in the corresponding Arthur packets. This includes some work joint with Dihua Jiang, and joint with Freydoon Shahidi.

Wednesday, March 31, 3:00-4:00pm

Speaker: Michael Griffin, BYU
Title: Moonshine
Abstract: In the 1970's, during efforts to completely classify the finite simple groups, several striking apparent coincidences emerged connecting the then-conjectural “Monster group” to the theory of modular functions. Conway and Norton turned these observed 'coincidences' into a precise conjecture known as “Monstrous Moonshine.” Borcherds proved the conjecture in 1992, embedding Monstrous Moonshine in a deeper theory of vertex operator algebras which have important physical interpretations. Fifteen years after Borcherds' proof, Witten conjectured an important role of Monstrous Moonshine in his search for a theory of pure quantum gravity in three dimensions. Under Witten's theory, the irreducible components of the Monster module represent energy states of black holes. The distribution of these energy states can be found using tools from number theory. Moonshine-phenomena have also been observed for other groups besides the Monster. These include the Umbral Moonshine conjectures of Cheng, Duncan, and Harvey which arise from the symmetry groups of each of the 24-dimensional Niemeier lattices. Recently, Moonshine for other sporadic simple groups have been shown to connect arithmetic properties of certain elliptic curves to the class numbers of certain imaginary quadratic fields.

Thursday, April 8, time TBD (Colloquium talk)

Speaker: Aaron Pollack, UCSD
Title: TBA
Abstract: TBA

Wednesday, April 14, 3:00-4:00pm

Speaker: Martin Weissman, UC Santa Cruz
Title: The compact induction theorem for rank-one p-adic groups
Abstract: A folklore conjecture predicts that when G is a p-adic group, every irreducible supercuspidal representation of G is induced from a compact-mod-center open subgroup. This was proven for GL(n) by Bushnell and Kutzko. For other groups, e.g., classical groups, tame groups, etc., the conjecture is proven for sufficiently large p thanks to hard work by many people. In this talk, I will describe a recent proof of the conjecture which applies to all groups G of relative rank one, with no assumptions about p. The method is to use the work of Schneider and Stuhler to connect supercuspidal representations to sheaves on the Bruhat-Tits tree of G, and "refine" these sheaves until the induction theorem becomes obvious.