Seminars in Hecke Algebras

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Karin Erdmann: The Hemmer-Nakano Theorem and relative dominant dimension

30th May, 2024

Let ℋq(d) be the Iwahori-Hecke algebra of the symmetric group where q is a primitive ℓ-th root of unity, and let A = Sq(n,d) be the q-Schur algebra. Hemmer and Nakano proved amongst others that for ℓ ≥ 4, the Schur functor gives an equivalence between the category of A-modules with Weyl filtration, and the category of ℋq(d)-modules with dual Specht filtration, and that certain extension groups get identified. This has been a surprise and has inspired further research. In this talk we discuss some extensions of this result.

Chun-Ju Lai: Quantum Wreath Products

29th May, 2024

A quantum wreath product is the algebra produced from a given (not necessarily commutative) algebra B, a positive integer d, and a choice of certain coefficients in B ⊗ B. Important examples include variants of the Hecke algebras, such as (1) affine Hecke algebras and their degenerate version, (2) Wan-Wang’s wreath Hecke algebras, (3) Kleshchev-Muth’s affinization algebras, (4) Rosso-Savage’s (affine) Frobenius Hecke algebras, (5) endomorphism algebras arising from Elias’s Hecke-type categories, (6) Mathas-Stroppel’s Rees affine Frobenius Hecke algebras, and (7) Hu algebra, which quantizes the wreath product Sm ≀ S2 between the symmetric groups. Our goal is to develop a uniform approach to the structure and representation theory in order to encompass known results which were proved in a case by case manner. In this talk, I’ll focus on the Schur-Weyl duality and the Clifford theory. Our theory is motivated by (and has application to) the Ginzburg-Guay-Opdam-Rouquier problem on quasi-hereditary covers of Hecke algebras for complex reflection groups.

Arun Ram: Lusztig varieties and Macdonald polynomials

27th May, 2024

In type A, the Macdonald polynomials and the integral from Macdonald polynomials are related by a plethystic transformation. We interpret this plethystic transformation geometrically as a relationship between nilpotent parabolic Springer fibres and nilpotent Lusztig varieties. This points the way to a generalization of modified Macdonald polynomials and integral form Macdonald polynomials to all Lie types. But these generalizations are not polynomials, they are elements of the Iwahori-Hecke algebra of the finite Weyl group. This work concerns the generalization of, and connection between, a 1997 paper of Halverson-Ram (which counts points of nilpotent Lusztig varieties over a finite field) and a 2017 paper of Mellit (which counts points of nilpotent parabolic affine Springer fibres over a finite field).

Thomas Haines: Pavings of convolution fibres and applications

1st March, 2024

A convolution morphism is the geometric analogue of the convolution of functions in a Hecke algebra. The properties of fibres of convolution morphisms are used in a variety of ways in the geometric Langlands programme and in the study of Schubert varieties. I will explain a very general result about cellular pavings of fibres of convolution morphisms in the setting of partial affine flag varieties, as well as applications related to the very purity and parity vanishing of cohomology of Schubert varieties over finite fields, structure constants for parahoric Hecke algebras, and the (motivic) geometric Satake equivalence.

Ziqing Xiang: Quantum Wreath Products and Their Representations

19th February, 2024

We introduce a new notion called the quantum wreath product, which produces an algebra B ≀Q H(d) from a given associative algebra B, a positive integer d, and a choice Q = (R, S, ρ, σ) of parameters. Important examples include many variants of the Hecke algebras, such as the Ariki-Koike algebras, the affine Hecke algebras and their degenerate version, Wan-Wang’s wreath Hecke algebras, Rosso-Savage’s (affine) Frobenius Hecke algebras, Kleshchev-Muth’s affine zigzag algebras, and the Hu algebra that quantizes the wreath product Σm ≀ Σ2 between symmetric groups. We will discuss the bases of quantum wreath product algebras, and some of their representations.

Eric Marberg: From Klyachko models to perfect models

5th December, 2023

In this talk a "model" of a finite group or semisimple algebra means a representation containing a unique irreducible subrepresentation from each isomorphism class. In the 1980s Klyachko identified an elegant model for the general linear group over a finite field with q elements. There is an informal sense in which taking the q→1 limit of Klyachko's construction gives a model for the symmetric group, which can be extended to its Iwahori-Hecke algebra. The resulting Hecke algebra representation is a special case of a "perfect model", which is a more flexible construction that can be considered for any finite Coxeter group. In this talk, I will classify exactly which Coxeter groups have perfect models, and discuss some notable features of this classification. For example, each perfect model gives rise to a pair of related W-graphs, which are dual in types B and D but not in type A. Various interesting questions about these W-graphs remain open.

Peng Shan: Modularity for W-algebras and affine Springer fibres

31st August, 2023

We will explain a bijection between admissible representations of affine Kac-Moody algebras and fixed points in affine Springer fibres. We will also explain how to match the modular group action on the characters with the one defined by Cherednik in terms of double affine Hecke algebras, and extensions of these relations to representations of W-algebras. This is based on joint work with Dan Xie and Wenbin Yan.

Ziqing Xiang: Quantum wreath product

28th July, 2023

The classical wreath product G ≀ Σd is a semidirect product Gd ⋊ Σd with Σd acting on Gd by permutations. We deform this classical wreath product by deforming G into an associative algebra B, deforming Σd into a Hecke algebra, and deforming the action. The result is called a quantum wreath product B ≀ H(d). Many variants of Hecke algebras can be viewed as quantum wreath products, hence could be treated in a unified manner.

In this talk, we will discuss necessary and sufficient conditions for quantum wreath products to have a basis of suitable size. We will also discuss some other structural results, the Schur algebras of these quantum wreath products, and their representations.

Hiraku Nakajima: Coulomb branches and DAHA

21st March, 2023

The geometric construction of DAHA by the equivariant K-theory of the Steinberg-type variety for an affine flag variety by Vasserot, Varagnolo-Vasserot is a precursor of the Coulomb branch construction. It has been generalized to versions of DAHA, such as cyclotomic DAHA naturally in view of Coulomb branches. I would like to recall these results, and then add one new example which seems not be known before.

David Jordan: Skeins, tori and DAHA

20th March, 2023

I will present some recent works with Gunningham, Safronov, Vazi-rani, and Yang (in various combinations) and which compute GLN-, SLN- and PGLN-skein modules for the 3-torus T 3, and related work of Kinnear which generalizes this to mapping tori T 2 ×γ S1, for γ ∈ SL2(ℤ).

The proofs for GLN and SLN start with a description of the skein category of T 2 via the representation theory of double affine Hecke algebras, while for PGLN they rely on an instance of electric-magnetic duality.