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Members

Dougal Davis Yau Wing Li Peter McNamara Arun Ram

Kari Vilonen Ting Xue Yaping Yang Gufang Zhao

Affiliated Members

Jan de Gier Nora Ganter Alex Ghitza Christian Haesemeyer Jack Hall

Thomas Quella David Ridout Chenyan Wu Paul Zinn-Justin

We are part of the Pure mathematics group.

Peter McNamara‘s research is in categorical and geometric representation theory. He has worked on a range of topics including quantum groups and their categorifications, perverse sheaves and Schubert varieties, algebraic combinatorics and p-adic groups.

Arun Ram’s research is in the area of Combinatorial Representation Theory. Topics he has worked on include tableaux combinatorics, crystals, diagram algebras, Lie theory, quantum groups, Hecke algebras, Schubert calculus, K-theory and cohomology of flag varieties and affine flag varieties. Current projects include constructions of representations of quantum affine algebras and a study of the combinatorics of double affine Hecke algebras and Macdonald polynomials.

Kari Vilonen‘s research is in the areas of real groups, the Langlands program, and related algebraic geometry. He has worked on several aspects of the geometric Langlands program and on more foundational questions on perverse sheaves and D-modules from the microlocal point of view. His research on real groups, joint with Schmid, includes the proof of the Barbasch-Vogan conjecture and a conjectural theory of Hodge structures on representations of real groups.

Ting Xue‘s research is in the areas of representation theory and algebraic groups. She has worked on questions related to geometry of nilpotent orbits and Springer theory, including small or bad characteristics. She is also interested in combinatorics arising from representation theory.

Yaping Yang‘s research lies in the area of Lie algebras, geometric representation theory, quantum groups, and the related geometry and topology. Her current work includes Knizhnik-Zamolodchikov equations, Cohomological Hall algebras, and vertex algebras associated to toric Calabi-Yau 3-folds.

Gufang Zhao‘s research lies at the interface between algebraic geometry and representation theory. More specifically, he has been working on projects concerning derived category of coherent sheaves, oriented cohomology theories of algebraic varieties, and their applications in representation theory. He is also fond of varieties of local systems and instantons, quantum integrable systems, and related aspects in mathematical physics.

Selected papers

- - M. Lanini and P. J. McNamara, Singularities of Schubert varieties within a right cell. arXiv2003.08616.
  - P. J. McNamara, Representation Theory of Geometric Extension Algebras, arXiv:1701.07949.
  - P. J. McNamara. Representations of Khovanov-Lauda-Rouquier Algebras III: Symmetric Affine Type. arXiv. Math. Z. 287 (2017), no. 1-2, 243–286.
  - Peter J. McNamara, Finite Dimensional Representations of Khovanov-Lauda-Rouquier Algebras I: Finite Type, arXiv. J. Reine Angew. Math. 707 (2015), 103–124.
  - D. George, Arun Ram, J. Thompson and R. Volkas, Symmetry breaking, subgroup embeddings and the Weyl group, arXiv1203.1048, Physical Review D 87 105009 (2013) [14 pages]
  - Z. Daugherty, Arun Ram and R. Virk, Affine and degenerate affine BMW algebras: The center, arXiv1105.4207, Osaka J. Math 51 (2014), 257-283.
  - A. Kleshchev, A. Mathas, and Arun Ram, Universal Specht modules for cyclotomic Hecke algebras , arXiv1102.3519, Proc. London Math. Soc. (3) 105 (2012) 1245-1289.
  - P. Diaconis and Arun Ram, A probabilistic interpretation of the Macdonald polynomials, arXiv1007.4779, The Annals of Probability 40 (2012) Vol. 40 No. 5, 1861-1896.
  - Roman Bezrukavnikov and Kari Vilonen, Koszul Duality for Quasi-split Real Groups, arXiv:1510.08343, Inventiones Mathematicae, 226 (2021),139-193.
  - Masaki Kashiwara and Kari Vilonen, Microdifferential systems and the codimension-three conjecture. Ann. of Math. (2) 180 (2014) no. 2, 573-620.
  - Wilfried Schmid and Kari Vilonen, Hodge theory and unitary representations of reductive Lie groups. Frontiers of mathematical sciences, 397-420, Int. Press, Somerville, MA, 2011, arXiv
  - Kari Vilonen and Ting Xue, Character sheaves for classical symmetric spaces. With an appendix by Dennis Stanton. Represent. Theory 26 (2022), 1097-1144.
  - Tsao-Hsien Chen, Kari Vilonen and Ting Xue, Springer correspondence for the split symmetric pair in type A , Compos. Math. 154 (2018), no. 11, 2403-2425. arXiv
  - Tsao-Hsien Chen, Kari Vilonen and Ting Xue, On the cohomology of Fano varieties and the Springer correspondence, With an appendix by Dennis Stanton. Adv. Math. 318 (2017), 515-533. arXiv.
  - Ting Xue, Combinatorics of the Springer correspondence for classical Lie algebras and their duals in characteristic 2. Adv. Math. 230 (2012) no. 1, 229–262.
  - Marc Levine, Yaping Yang, and Gufang Zhao, Algebraic Elliptic cohomology theory and flops 1, appendix by Joël Riou. Mathematische Annalen volume 375, pages 1823–1855 (2019).
  - Miroslav Rapcak, Yan Soibelman, Yaping Yang, and Gufang Zhao, Cohomological Hall algebras, vertex algebras and instantons. Communications in Mathematical Physics volume 376, pages 1803–1873 (2020)
  - Yaping Yang, and Gufang Zhao, The cohomological Hall algebras for a preprojective algebra. Proc. Lond. Math. Soc. 116, 1029-1074.

Representation theory seminar 2023 Semester 1

Organisers: Dougal Davis, Kari Vilonen, Ting Xue

Place and time: Peter Hall 162, Thursdays 2:15 – 4:15pm

Please contact one of the organisers to be added to the mailing list.

Upcoming Seminars

May 18: Arun Ram (University of Melbourne) Lusztig varieties and Macdonald polynomials

Place and time: Peter Hall 162, 2:15pm – 3:15pm

In recent works Abreu-Nigro and Xuhua He have introduced the term Lusztig variety. I like this term, as Lusztig has many papers about these varieties. In 1997 Halverson and I computed the number of points of Type A nilpotent Lusztig varieties over finite fields in connection to characters of Hecke algebras. Recently, my study of Macdonald polynomials and central elements in Hecke algebras have led me to look at these computations again.

Past Seminars

May 11: Naoki Genra (University of Tokyo) Reduction by stages on W-algebras

Place and time: Peter Hall 162, 2:15pm – 3:15pm

Let $X$ be a Poisson variety with a Hamiltonian $G$ -action and $H$ be a normal subgroup of $G$ . Then $X//G$ is obtained by a (Hamiltonian) reduction of $X//H$ by the induced $G/H$ -action under suitable assumptions, called reduction by stages. We apply for the Slodowy slices and show that the Slodowy slice associated to $(g, O)$ is obtained by a reduction of the Slodowy slice associated to $(g, O')$ for a simple Lie algebra $g$ and nilpotent orbits $O$ , $O'$ such that $O > O'$ with some conditions. The quantum cases imply that the finite/affine W-algebras associated to $(g, O)$ are obtained by W-algebras associated to $(g, O')$ , which proves a conjecture of Morgan in finite cases and gives a conjectural generalization of results of Madsen and Ragoucy in affine cases. This is a joint work with Thibault Juillard.

May 4: Peter Fiebig (Erlangen) Representations and Binomial Coefficients

Place and time: Peter Hall 162, 2:15pm –

The talk is motivated by the „generational phenomena” that occur in the representation theory of algebraic groups in positive characteristics. The representation theory of quantum groups is known to provide a first step approximation to modular representations. Lusztig was the first to suggest that there should be „algebraic structures” that provide further steps towards modular representations beyond quantum groups. None of these structures are known today, even though some candidates have been suggested by several authors. In the talk I want to motivate the generational idea and then introduce a model category that makes the proximity of modular and quantum representations quite transparent. Using this category I want to show that the generational problem seems to be closely connected to finding generalizations of binomial coefficients.

April 27: Alexandre Minets (University of Edinburgh) A proof of P=W conjecture

Place and time: Peter Hall 162, 2:15pm – 4:15pm

Let C be a smooth projective curve. The non-abelian Hodge theory of Simpson is a homeomorphism between the character variety M_B of C and the moduli of (semi)stable Higgs bundles M_D on C. Since this homeomorphism is not algebraic, it induces an isomorphism of cohomology rings, but does not preserve finer information, such as the weight filtration. Based on computations in small rank, de Cataldo-Hausel-Migliorini conjectured that the weight filtration on H^*(M_B) gets sent to the perverse filtration on H^*(M_D), associated to the Hitchin map. In this talk, I will explain a recent proof of this conjecture, which crucially uses the action of Hecke correspondences on H^*(M_D). Based on joint work with T. Hausel, A. Mellit, O. Schiffmann.

Monday April 17: Xuhua He (Chinese University of Hong Kong) Affine Deligne-Lusztig varieties and affine Lusztig varieties

Place and time: Peter Hall 162, 4pm – 5pm

Abstract: Roughly speaking, an affine Deligne-Lusztig variety describes the intersection of a given Iwahori double coset with a Frobenius-twisted conjugacy class in the loop group; while an affine Lusztig variety describes the intersection of a given Iwahori double coset with an ordinary conjugacy class in the loop group. The affine Deligne-Lusztig varieties provide a group-theoretic model for the reduction of Shimura varieties and play an important role in the arithmetic geometry and Langlands program. The affine Lusztig varieties encode the information of the orbital integrals of Iwahori-Hecke functions and serve as building blocks for the (conjectural) theory of affine character sheaves. In this talk, I will explain a close relationship between affine Lusztig varieties and affine Deligne-Lusztig varieties, and consequently, provide an explicit nonemptiness pattern and dimension formula for affine Lusztig varieties in most cases. This talk is based on my preprint arXiv:2302.03203.

April 13: Dennis Gaitsgory (Max Planck Institute for Mathematics) Categorical geometric Langlands for D-modules

Place and time: Peter Hall 162, 2:15pm – 4:15pm

Abstract: In the talk we will describe the recently obtained proof of GLC in the context of D-modules. This is a joint project with D.Arikinkn, D. Beraldo, L.Chen, J.Faegerman, K. Lin, and S.Raskin, who made the most crucial contributions.

April 12: Dennis Gaitsgory (Max Planck Institute for Mathematics) Geometric Langlands with nilpotent singular support

Place and time: Peter Hall 162, 2:15pm – 4:15pm

Abstract: This talk will summarize the recent series of papers by Arinkin-Gaitsgory-Kazhdan-Raskin-Rozenblyum-Varshavsky.

We’ll introduce a version of the categorical geometric Langlands conjecture that makes sense for l-adic sheaves over a base of positive characteristic. We will explain the mechanism of the categorical trace of Frobenius, which allows to pass from the geometric assertion to a classical one.

March 23: Changlong Zhong (State University of New York at Albany) Oriented cohomology of the affine Grassmannian and the Peterson subalgebra

Place and time: Peter Hall 162, 2:15pm – 3:15pm

Abstract: Peterson subalgebra is a subalgebra of the (small) torus-equivariant homology of the affine Grassmannian. It was proved by Peterson and Lam-Shimozono that certain localization of this algebra is isomorphic to the quantum cohomology of the flag variety. K-theoretic analogue of this result was recently proved by Kato, as part of his study of semi-infinite flag varieties. In this talk I will talk about generalization of the Peterson subalgebra to the oriented cohomology in the sense of Levine-Morel. I will then talk about a certain localization of this subalgebra in the case of connective K-theory.

March 16: Aritra Bhattacharya (Institute of Mathematical Sciences, Chennai) Haglund’s positivity conjecture for Macdonald polynomials

Place and time: Peter Hall 162, 2:15pm – 3:15pm

Abstract: The Macdonald symmetric functions are an incredible family of symmetric functions that simultaneously generalize many known bases of symmetric functions, such as the Schur functions and the Hall-Littlewood functions. The transition matrix between the Hall-Littlewood and the Schur functions are very well studied – they are given by the Kostka-Foulkes polynomials which are polynomials in $t$ with non-negative integer coefficients. However very little is known about the transition matrix between the Macdonald functions and the Schur functions.

Haglund conjectured that the Schur coefficients of the integral form Macdonald Polynomials $J_{\lambda}(q,t)$ have the positivity property that $\dfrac{\langle J_{\lambda}(q,q^k), s_{\mu} \rangle}{(1-q)^{|\lambda|}}\in \mathbb{Z}_{\geq 0}[q]$ . We present some new partial results about this conjecture.

March 9: Alistair Savage (University of Ottawa) Diagratification

Place and time: Peter Hall 162, 2:15pm – 3:15pm

Abstract: We will explain how one can construct diagrammatic presentations of categories of representations of Lie groups and their associated quantum groups using only a small amount of information about these categories. To illustrate the technique in concrete terms, we will focus on the exceptional Lie group of type F4.

March 2: Kari Vilonen (University of Melbourne) Introductory remarks on geometric Langlands

Place and time: Peter Hall 162, 2:15pm – 4:15pm

Abstract: In preparation for the talks of Gaitsgory in April I will explain some aspects of the geometric Langlands program.

Some related notes by Ed Frenkel (sent in by Lance Gurney)

January 19: Ben Webster (University of Waterloo) Noncommutative resolutions and Coulomb branches

Place and time: Peter Hall 107, 2pm – 4pm

Abstract: Work of Braverman, Finkelberg and Nakajima provides a new window onto the world of noncommutative algebras, by constructing both new and previously known algebras as Coulomb branches. While beautiful, their construction involves some very infinite dimensional geometry. I’ll explain how we can replace this with a combinatorial construction, and how this leads us to a new perspective on noncommutative resolutions and categories of coherent sheaves.

Slides

January 12: Emanuel Scheidegger (Peking University) Aspects of modularity for Calabi-Yau threefolds

Place and time: Peter Hall 107, 2pm – 3pm

Abstract: We give an overview of some mostly conjectural aspects of modularity for Calabi-Yau threefolds. We focus on one parameter families of hypergeometric type and give computational results in terms of classical modular forms. If time permits, we show an explicit correspondence in one case. This is based on work with K. Boenisch, A. Klemm, and D. Zagier, 2203.09426.

January 5: Lisa Carbone (Rutgers University) A Lie group analog for the monster Lie algebra

Place and time: Peter Hall 107, 2pm – 3pm

Abstract: Let $\mathfrak{m}$ be the monster Lie algebra, a generalized Kac–Moody algebra. We describe various approaches to constructing an analog of a Lie group associated to $\mathfrak{m}$ . We construct a group $G(\mathfrak{m})$ , associated to $\mathfrak{m}$ , given by generators and relations. We also construct a group $U^+$ of automorphisms of a completion $\hat{\mathfrak{m}}$ of $\mathfrak{m}$ . The subgroup of $G(\mathfrak{m})$ generated by all positive imaginary roots embeds in $U^+$ .

Past Seminars:

Representation Theory Student Seminar 2021 Semester 2 and 2022 Semester 1&2

Organisers: Ting Xue, Yaping Yang, Gufang Zhao

Student organiser (2022S2): Davood Nejaty

Overview

This is a learning seminar on representation theory and related topics, aimed at 3rd year undergraduate students, MSc students, as well as PhD students in mathematics or related fields.

In the learning seminar, participants are expected to learn a topic based on the reference given, and present their work to the other participants. The audience are expected to ask questions and make comments during the presentation. Discussions tangential to the topics are welcome. The idea is for all participants to actively learn and discuss together.

This semester the topic will be Lie groups and their representations. We will follow the chapter on Lie groups by Graeme Segal in the reference below.

Reference: Carter, Roger; Segal, Graeme; Macdonald, Ian, Lectures on Lie groups and Lie algebras. With a foreword by Martin Taylor. London Mathematical Society Student Texts, 32. Cambridge University Press, Cambridge, 1995.

Time: Wednesdays 11-12

Location: Peter Hall 107 and Zoom (please contact the organisers for zoom link)

Exercises

Schedule (2022S2)

Time: Wednesdays 11-12

Oct 19/26 Kevin Fergusson The unitary and symmetric groups

Oct 12 Ali Khalili The complexification of a compact group

Oct 4 Cancelled

Sep 21 Amit Ben Harim Induced representations, continued

Sep 14 Amit Ben Harim Induced representations

Sep 7 Haris Rao The Peter-Weyl theorem, continued

Aug 31 Yuhan Gai Functions on $\mathbb{R}^n$ and $S^{n-1}$

Aug 24 Cancelled

Aug 17 Haris Rao The Peter-Weyl theorem

Schedule (2022S1)

Time: Mondays 11-12

Location: Peter Hall 107 and Zoom (please contact the organisers for zoom link)

Student organisers (2022S1): Weiying Guo and Davood Nejaty

May 30 Beaudon Anasson Maximal compact subgroups

May 23 Zhongtian Chen Compact groups and integration, continued

May 16 Zhongtian Chen Compact groups and integration

May 9 Kevin Fergusson Fourier series and Representation theory

May 2 Davood Nejaty Review, continued

Grace Yuan Bruhat decomposition Notes

Apr 11 Organisational meeting and review of previous topics

Schedule (2021S2)

Time: Wednesdays 3:15-4:15pm

Location: Zoom (please contact the organisers for link)

Dec 1 Linfeng Wei Lie’s theorems, continued. Notes

Nov 24 Linfeng Wei Lie’s theorems

Oct 27 Adam Monteleone Smooth manifolds, tangent space, one parameter subgroups and the exponential map, continued. Notes

Oct 20 Adam Monteleone Smooth manifolds, tangent space, one parameter subgroups and the exponential map, continued.

Oct 6 Adam Monteleone Smooth manifolds, tangent space, one parameter subgroups and the exponential map

Sep 29 Abraham Zhang Diagonalisation and maximal tori, continued. Notes

Sep 22 Abraham Zhang Diagonalisation and maximal tori

Sep 8 Grace Yuan Polar decomposition, Graham-Schmidt

Sep 1 Yifan Guo Homogeneous spaces, continued. Notes

Aug 25 Yifan Guo Homogeneous spaces Notes

Aug 18 Eskander Salloum $SU_2,SO_3,SL_2\mathbb{R}$ , continued. Notes

Aug 11 Eskander Salloum $SU_2,SO_3,SL_2\mathbb{R}$

Aug 4 Benjamin Gerraty Examples


Date	Topics	Reference	Speaker
Aug 4, 2021	Examples	[S] 1	Benjamin Gerraty
Aug 11/18, 2021	$SU_2,SO_3,SL_2\mathbb{R}$	[S] 2	Eskander Salloum
Aug 25/Sep 1, 2021	Homogeneous spaces	[S] 3	Yifan Guo
Sep 8, 2021	Polar decomposition, Graham-Schmidt, Bruhat decomposition	[S] 4	Grace Yuan
Sep 22/29, 2021	Diagonalisation and maximal tori	[S] 4	Abraham Zhang
Oct 6/20/27, 2021	Smooth manifolds, tangent space, one parameter subgroups and the exponential map	[S] 5	Adam Monteleone
Nov 24/Dec1, 2021	Lie’s theorems	[S] 5	Linfeng Wei
May 9/2022	Fourier series and Representation theory	[S] 6	Kevin Fergusson
May 16, 23/2022	Compact groups and integration	[S] 7	Zhongtian Chen
May 30/2022	Maximal compact subgroups	[S] 8	Beaudon Anasson
Aug 17, Sep 7, 2022	The Peter-Weyl theorem	[S] 9	Haris Rao
Aug 31, 2022	Functions on $\mathbb{R}^n$ and $S^{n-1}$	[S] 10	Yuhan Gai
Sep 14/21, 2022	Induced representations	[S] 11	Amit Ben Harim
Oct 12, 2022	The complexification of a compact group	[S] 12	Ali Khalili
Oct 19/26, 2022	The unitary groups and the symmetric groups	[S] 13	Kevin Fergusson
	The Borel-Weil theorem	[S] 14	Ali Khalili
References	[S] Graeme Segal, Lie groups, in Carter, Roger; Segal, Graeme; Macdonald, Ian, *Lectures on Lie groups and Lie algebras.* With a foreword by Martin Taylor. London Mathematical Society Student Texts, 32. Cambridge University Press, Cambridge, 1995.

Past seminar

Special lectures by Arun Ram summer 2021/22

Student seminar 2020S2 and 2021S1

Wille Liu (Academia Sinica), July 26-August 18, 2023

Peter Fiebig (Friedrich-Alexander-Universität) Apr 30-May 6, 2023

Alexandre Minets (University of Edinburgh), April 23-30, 2023

Xuhua He (Chinese University of Hong Kong), April 12-18, 2023

Dennis Gaitsgory (MPIM), April 5-17, 2023

Alistair Savage (University of Ottawa), March 7-10 (TBC), 2023

Changlong Zhong (State University of New York at Albany), Feb 28-Mar 28, 2023

Ben Webster (University of Waterloo), Jan 16-20, 2023

Andrea D’Agnolo (Padova), May 20-Jun 17, 2022

Wille Liu (MPIM), May 30-Jun 10, 2022

Cheng-Chiang Tsai (Academia Sinica), May 29-Jun 11, 2022

Dougal Davis (University of Edinburgh), Mar 30-Apr 3, 2020

Xinwen Zhu (Caltech), Nov 24-Dec 1, 2019

Cheng-Chiang Tsai (Stanford University), Nov 19-Dec 8, 2019

Geordie Williamson (Sydney), Oct 26-28, 2019

Bill Casselman (University of British Columbia), Oct 24-27, 2019

Emily Norton (MPIM), Feb 19-Mar 2, 2019

Luca Migliorini (University of Bologna), Dec 3-9, 2018

Cheng-Chiang Tsai (Stanford University), Nov 3-Dec 1, 2018

Xinwen Zhu (Caltech), Aug 23-30, 2018

Anthony Henderson (Sydney), Aug 21-24, 2018

Jessica Fintzen (University of Michigan), Nov 27-Dec 3, 2017

Sam Raskin (University of Texas at Austin) , Sep 12-22, 2017

Takuro Mochizuki (RIMS, Kyoto University), Sep 4-15, 2017

Carl Mautner (UC Riverside), Aug 16-26, 2017

Kari Vilonen was awarded a highly prestigious ARC Laureate Fellowship on Real groups and the Langlands program.
Peter McNamara gave a talk on June 25, 2020 at the informal Friday seminar at U Sydney.
Nora Ganter, Peter McNamara, Yaping Yang and Gufang Zhao are co-organising (with Masoud Kamgarpour and Peng Shan) the MATRIX workshop Frontiers in Representation Theory, 14-25 February 2022.
Ting Xue will be a speaker at AMSI Winter School 2020, New directions in representation theory, University of Queensland. (Postponed.)
Yaping Yang gave a talk titled “Cohomological Hall algebras and perverse coherent sheaves on toric Calabi-Yau 3-folds” at the GRT at Home seminar on 23 June, 2020.
Nora Ganter, Yaping Yang, and Gufang Zhao co-organized with Daniel Berwick Evans and Theo Johnson-Freyd the workshop on elliptic cohomology and physics 25-28 May 2020.
Arun Ram gave a performance of “Mendelssohn Salon 1828” with pianist Michael Leslie on
12 March 2020 at Tempo Rubato.
From Dec 2019 to Feb.29 2020, Yaping Yang and Gufang Zhao visited the Kavli Institute for the Physics and Mathematics of the Universe (IPMU), Japan. During the visit, Gufang Zhao gave a seminar talk titled “Cohomological Hall algebras and their representation theories” at the Mathematics and String Theory Seminar at IPMU.
The week of December 16-20 2019, Yaping Yang and Gufang Zhao co-organised the workshop on “Geometric Representation Theory and Quantum Field Theory” together with Hiraku Nakajima (IPMU), Peng Shan (Tsinghua), Wenbin Yan (Tsinghua) at TSIMF, Sanya, China.
Yaping Yang and Gufang Zhao visited the Perimeter Institute for Theoretical Physics, Waterloo, Canada during Feb-Mar 2019. During the week of February 25-March 1, 2019, Yaping Yang co-organised the workshop “Cohomological Hall algebras in Mathematics and Physics” at Perimeter Institute (with Kevin Costello (PI) and Yan Soibelman (KSU)).
Yaping Yang received an ARC Discovery Early Career Award (DE 190101231) in Dec 2018.
Gufang Zhao received an ARC Discovery Early Career Award (DE 190101222) in Dec 2018.