Research Statement Sam Gunningham
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Research Statement Sam Gunningham My research is centered around the field of Geometric Representation Theory. Much of my work involves the study of algebraic systems of differential equations (D-modules) in the presence of symmetry, drawing on concepts and tools from category theory, homotopy theory, derived algebraic geometry, and mathematical physics. The first section of this statement gives an informal overview of my research, with a more detailed account following in the subsequent sections. 1 Overview: Categorical Harmonic Analysis Briefly, the goal of harmonic analysis is to express functions as superpositions of “basic waves”. In categorical harmonic analysis, the functions are replaced by differential equa- tions (or alternatively their sheaves of solutions). This is motivated by the observation that the “basic waves” are typically solutions of “basic eigenproblems” (for example, the exponential function upxq “ eiξx is an eigenfunction for the polynomial differential opera- tor B{Bx). The main object of study is typically the category DpXq of D-modules (algebraic incarnations of differential equations) on a space X. The theory of reductive groups has long been a fertile setting for harmonic analysis (after pioneering work of Harish-Chandra, Langlands, and others) and much of my own work is set in this world. For example, many of my projects are centered around the category DpGqG of class D-modules on a reductive group G (D-module analogues of class functions). This rich category is the home of Harish-Chandra’s system of differential equations (recast is the language of D-modules by Hotta and Kashiwara), a setting for Springer Theory, and for Lusztig’s character sheaves (in their D-module interpretation due to Ginzburg). My on- going Springer theory project [7, 10, 11] utilizes categorical and geometric techniques to bring surprising new results and a fresh understanding of these important works, shifting focus from the classical study of character sheaves to combinatorial descriptions of entire categories of D-modules. This work is set in the wider context of geometric Langlands in genus 1 and there are exciting applications to the geometric Langlands conjectures for elliptic curves [5], as well as Cherednik algebras and Hilbert schemes. Class D-modules also arise as the Drinfeld center of Harish-Chandra bimodules– a central player in the classical and categorical representation theory of reductive Lie groups. My work [1] on the quantum Ngoˆ action (which we show appears naturally in the context of Langlands dual Coulomb branches) explores this monoidal category from an entirely new perspective, with wide-ranging applications in categorical representation theory and mathematical physics. Topological field theory is a key tool in my work; we are able to extract hidden structure and symmetries from classical representation theoretic objects (for example in my work on the homology of character stacks [2], and on quantum D-modules). 1 In the remainder of this section, we will expand upon each of these themes in my research. 1.1 Parabolic Induction and Generalized Springer Theory. In Harish-Chandra’s phi- losophy of cusp forms, one studies objects (e.g. representations, class functions, sheaves) associated to a reductive group G by parabolic induction from analogous objects associ- ated to certain smaller reductive groups (the Levi subgroups). Those objects which cannot be induced from any smaller Levi are called cuspidal. In my work [3] with David Ben-Zvi and Hendrik Orem we show (see Theorem 16) that there are no cuspidals for categorical representations of reductive groups: every (de Rham) G-category is contained in a principal series. On the other hand, in his remark- able 1984 paper [Lus84], Lusztig found (and classified) so-called cuspidal local systems, implying that the corresponding result fails for class D-modules (the home for characters of categorical representations). Motivated by Lusztig’s work, in my paper [11] I give a complete description of the abelian category of class D-modules on the Lie algebra (see Theorem 2), greatly extending Lusztig’s description for the nilpotent cone (the so-called generalized Springer correspondence).1 A number of mysteries remain surrounding the nature of the generalized Springer correspondence and the classification of cuspidals. I am currently looking at generalized Springer theory from a new angle [6]: that of the un- derlying G-representation on global sections of the underlying D-module (inspired by the observation (see Section 2.4) that Hotta-Kashiwara’s results on the Harish-Chandra sys- tem characterize the “Springer block” as consisting of objects with an invariant vector). Beyond class D-modules on the group and Lie algebra already discussed, one may consider the following variants: • Elliptic: replace G by the “elliptic group”, GE of framed semistable G-bundles on an elliptic curve E (see Section 2.7). • Quantum: replace G with the quantum group, differential operators with multi- plicative difference operators (see Section 5.2). • Mirabolic: In the case G “ GLn, adjoin a framing vector, and consider twisted differ- ential operators (see Section 2.6). In each of these settings, one expects to have a generalized Springer theory package, with induction/restriction functors, character sheaves, cuspidal data, etc., and in each case there are a variety of applications. For example, mirabolic character sheaves give rise to modules for the spherical subalgebra of a Cherednik algebra via Hamiltonian reduction; a better understanding of generalized Springer theory in this setting would allow one to see parts of the category which are invisible to Hamiltonian reduction. 1Interestingly, as shown in my paper [10] while the block decomposition indexed by cuspidal data contin- ues to hold for the derived category, the Springer-theoretic description of the blocks in terms of relative Weyl groups fails (see Theorem 4). This forms an interesting counterpoint with the work of Rider who proves a derived Springer theorem for the nilpotent cone. 2 . 1.2 A Categorical Plancherel Theorem and the quantum Ngoˆ action. The classical Plancherel theorem of Harish-Chandra is a central result in harmonic analysis; it ex- presses a class function on G as an integral over the unitary spectrum G. In my paper [1] with David Ben-Zvi (as part of an ongoing joint project; see Section 3.4 below) we up- grade this result to the categorical level: expressing a class D-module onp G as a sheaf on the categorical spectrum (a certain form of the dual maximal torus modulo Weyl group symmetries T _{W ). Following this guiding principle has lead us through an extraordinary diversity of mathematics! The central player in our work the universal centralizer J, and its quantization, the category h of Whittaker D-modules. This is a fundamental object in representation W theory and gauge theory; in particular, it is the phase space for the Toda lattice and the Coulomb branch for 3-d pure “ 4 gauge theory. The universal centralizer also carries N a universal action on Hamiltonian G-spaces, as described in work of Knop and used in Ngo’sˆ proof of the fundamental lemma. As part of our work we show (see Theorem 17) that the convolution of bi-Whittaker D-modules is commutative, and construct a central action of h on G-categories: a quantum form of the universal action of J. Moreover, W the spectrum of the categorical commutative algebra h is the expected form of T _{W , W giving precise meaning to the categorical Plancherel theorem described above. Neither the commutativity nor the braided functor are manifest in the definition of Whittaker D-modules. To prove our results in [1], we utilize Langlands duality (namely, the derived geometric Satake theorem of Bezrukavnikov and Finkelberg). In this way we are able to express the various players described above in terms of the topology of the Langlands dual affine Grassmannian, where the desired structures are relatively appar- ent. The universal centralizer J and its quantum form h have appeared prominently in W recent work of Braverman-Finkelberg-Nakajima, Teleman, and Ginzburg, amongst other places, and the Ngoˆ homomorphism and its quantum form appears to be a key aspect of these stories which has not yet been fully explored. This work is still ongoing and there are many directions for future research. For example, we are thinking about: applications of the Ngoˆ action to the theory of G-categories, the role this structure plays in gauge theory, and potential ramifications in the more general setting of Knop’s work on spherical varieties. 1.3 Topological Field Theory and (Quantum) Character Varieties. From a mathemat- ical perspective a topological field theory (TFT) is an assignment of algebraic invariants (e.g. categories, vector spaces, numbers) to smooth manifolds which satisfies certain rules with respect to cutting or gluing of manifolds. The motivating example is 2d gauge theory ZΓ for a finite group Γ ; this TFT assigns 1 Γ to a circle the space of class functions ZΓ pS q “ CrΓ s , and to a closed 2-manifold S, the 3 number ZΓ pSq of Γ -Galois covers of S (counted with multiplicity given by the reciprocal of the number of automorphisms). Unwinding the cut-and-paste laws for the TFT leads to a formula (originally due to Frobenius) for ZΓ pSq in terms of the characters of Γ . The case when the gauge group Γ is the symmetric group Sn is of particular interest; in this setting the numbers ZSn pSq (and their variants for surfaces with punctures) are called Hurwitz numbers. In my paper [9] I construct an analogous TFT in order to extract a formula for so-called spin Hurwitz numbers (see Section 4.2), which compute Gromov- Witten invariants of certain complex surfaces amongst other things. In my recent work with Arun Debray [4], we construct an extension of the invertible “Arf theory” underlying this story to pin´-manifolds.