“Hendoff” for inaugural SMRI Exec Director Anthony Henderson: 19–22 April 2022
The Hendoff is a celebration of Anthony Henderson’s manifold contributions to mathematics in Australia, through both his research and service to the community. After over two years at the helm of SMRI and more than twenty years at Sydney, Anthony is stepping down. He was sent off with style at the Hendoff, which took place over 19–22 April at the University of Sydney.
Anthony began his distinguished career at the University of Sydney as a postdoctoral scholar in 2001, and it culminated with his promotion to Professor in 2016 and becoming the inaugural Executive Director of SMRI in 2018. SMRI was established by SMRI Director Geordie Williamson and Anthony and is the first institute of its kind in Australia.
Both as a Professor and at his role in SMRI, Anthony has made deep and important contributions to pure mathematics, particularly in studies related to the geometry of nilpotent varieties and Springer fibres, and their generalisations. His many awards and distinctions include the Australian Mathematical Society Medal (2012) and the Christopher Heyde Medal (2011). Anthony has also been an important mentor to many early career mathematicians in Australia, several of whom spoke at this conference.
The Hendoff was organised by Gus Lehrer, Andrew Mathas, Geordie Williamson and Oded Yacobi. The following speakers presented at the conference:
Speaker  Institution  Presentation  

Amnon Neeman  ANU 
Abstract: A metric on a category assigns lengths to morphisms, with the triangle inequality holding. This notion goes back to a 1974 article by Lawvere. We’ll begin with a quick review of some basic constructions, like forming the Cauchy completion of a category with respect to a metric. And then will begin a string of surprising new results. It turns out that, in a triangulated category with a metric, there is a reasonable notion of Fourier series, and an approximable triangulated category can be thought of as a category where many objects are the limits of their Fourier expansions. And then come two types of theorems: (1) theorems providing examples, meaning showing that some category you might naturally want to look at is approximable, and (2) general structure theorems about approximable triangulated categories. And what makes it all interesting is (3) applications. These turn out to include the proof of a conjecture by Bondal and Van den Bergh, a major generalization of a theorem of Rouquier’s, a short, sweet proof of Serre’s GAGA theorem and a proof of a conjecture by Antieau, Gepner and Heller. 

Anna Romanov  UNSW 
Abstract: Admissible representations of real reductive groups are a key player in the world of unitary representation theory. The characters of irreducible admissible representations were described by Lustig—Vogan in the 80’s in terms of a geometricallydefined module over the associated Hecke algebra. In this talk, I’ll describe a categorification of a block of the LV module using Soergel bimodules. 

Anne Thomas  University of Sydney 
Abstract: It is well known that the geometry of BruhatTits buildings encodes many interesting algebraic properties of the corresponding groups. Retractions play a key role in this. We introduce chimney retractions for arbitrary affine buildings and give a recursive description of the shadows which they induce. We then explain how these shadows can be used to study subvarieties of and orbits in (partial) affine flag varieties. This is joint work with Elizabeth Milićević, Yusra Naqvi and Petra Schwer. 

Jie Du  UNSW 
Abstract: We investigate the 𝑖quantum groups 𝑈^{𝐽}(n) and 𝑈^{𝑖}(n) and their associated qSchur algebras 𝑺^{𝐽}(n,r) and 𝑺^{𝑖}(n,r) of types B and C, respectively. This includes short (element) multiplication formulas, long (element) multiplication formulas, and triangular relations in 𝑺^{𝐽}(n,r) and 𝑺^{𝑖}(n,r). We will also give new realisations of Beilinson–Lusztig–MacPherson type for both 𝑈^{𝐽}(n) and 𝑈^{𝑖}(n) and discuss their Lusztig forms. This allows us to link representations of 𝑈^{𝐽}(n) and 𝑈^{𝑖}(n) with those of finite orthogonal and symplectic groups. This is joint work with Yadi Wu. 

Kari Vilonen  University of Melbourne 
Abstract: I will explain how mixed Hodge modules can be utilized to understand representation theory of real groups. In particular, we obtain a refinement of the LusztigVogan polynomials in this setting. Adams, van Leeuwen, Trapa, and Vogan (ALTV) have given an algorithm to determine the unitary dual of a real reductive group. As a corollary of our results we obtain a proof of a key result of (ALTV) on signature polynomials. This is joint work with Dougal Davis. 

Kevin Coulembier  University of Sydney 
Abstract: Recent results in algebraic geometry, as well as in the theory of tensor categories, motivate studying the process of taking the inverse limit of an affine group scheme (over a field of positive characteristic) along the Frobenius homomorphism. This is the ‘perfection’ of the group scheme. 

Masoud Kamgarpour  University of Queensland 
Abstract: Character variety of surface groups plays a central role in diverse areas of mathematics such as Geometric Langlands program and nonabelian Hodge theory. Determining cohomology of the character variety has been a subject active research for decades. In this talk, I will report an ongoing project to count points on character varieties over finite fields. The main goal is to generalise the work of Hausel—LetellierVillegas from type A to arbitrary type. A key role is played by representation theory of finite reductive groups (Deligne—Lusztig theory, Lusztig’s Jordan decomposition, etc.) 

Neil Saunders  University of Greenwich 
Abstract: The Exotic Springer correspondence as defined by Kato provides a bijection between $Sp$orbits on the Hilbert nullcone of a certain $Sp_{2n}$module and irreducible representations of the Weyl group of type C, $W(C_n)$. This particular nullcone is known as the Exotic nilpotent cone. With V. Nandakumar and D. Rosso, we demonstrated an explicit bijection between irreducible components of the fibres of a resolution of the exotic nilpotent cone and standard Young bitableaux, which label the irreducible representations of $W(C_n)$ . In this talk, we will detail some results that follow from this bijection and report on ongoing work with D. Rosso in extending these results to partial flag varieties in type C. 

Peter McNamara  University of Melbourne 
Abstract: I will talk about the folding process in categorification, which allows one to extend results from simplylaced Lie algebras to general 

Peter Trapa  University of Utah 


Pramod Achar  Louisiana State University 
Abstract: Let $G$ be a connected reductive group over an algebraically closed field, and let $C$ be a nilpotent orbit for $G$. If $L$ is an irreducible $G$equivariant vector bundle on $C$, then by work of Deligne, Bezrukavnikov, Arinkin, one can define a “coherent intersection cohomology complex” $IC(C,L)$. These objects play an important role in various results related to the local geometric Langlands program. 

Ting Xue  University of Melbourne 
Abstract: We discuss character sheaves in the setting of graded Lie algebras, concentrating on the construction of cuspidal character sheaves. Irreducible representations of Hecke algebras of complex reflection groups at roots of unity enter the description of character sheaves. We will explain the connection between our work and the recent work of Lusztig and Yun, where irreducible representations of trigonometric double affine Hecke algebras appear in the picture. This is based on joint work with Kari Vilonen and partly with Misha Grinberg. 

Tony Licata  ANU 
Abstract: The 2CalabiYau category of a quiver is a basic object of interest in study of braid groups and their higher representation theory. In this talk I’ll describe how to extract some geometry from this object, by endowing the set of spherical objects of the category with a topology. Conjectures (and, less prominently, theorems) will appear. 

Yang Zhang  University of Sydney 
Abstract: In this talk, I will introduce a finite dimensional algebra, called noncrossing algebra, associated with the noncrossing partition lattice of a finite Coxeter group. This new algebra has some similarities with the OrlikSomolon algebra, but involves new combinatorics in its construction. I will show how this algebra can be used to compute the cohomology of the Milnor fibre of the corresponding Coxeter arrangement. This is joint work with Gus Lehrer. 

Zsuzsi Dancso  University of Sydney 
Abstract: The BakerCampbell Hausdorff Theorem gives a formula for log(e^xe^y) where x and y don’t commute. The KashiwaraVergne theorem is a refinement of this fact, and has wide implications in harmonic analysis and Lie theory: in particular, it implies the multiplicativity of the Duflo isomorphism. There are two different topological approaches to the KashiwaraVergne theorem and its cousins: one via fourdimensional knot theory (joint works with Dror BarNatan, Iva Halacheva, Tamara Hogan, Marcy Robertson and Nancy Scherich), and one via homotopy curves on surfaces due to Alekseev, Kawazumi, Kuno and Naef. 