Last updated April 2018

The problems that interest me the most concern the interactions between low dimensional topology, non-commutative algebra, and representation theory, especially as these interactions arise in mathematical physics and gauge theory.

My research output falls roughly into three distinct areas: quantum geometric representation theory and its relationship with low-dimensional topology; the structure theory, representation theory, and classification of tensor categories in various contexts; and finally the development of "non-commutative algebraic geometry", i.e. studying non-commutative algebras via differential geometry of their abelianizations.

Together with my collaborators David Ben-Zvi and Adrien Brochier, I am currently up to my neck in understanding 4D topological field theories related to N=4 super Yang-Mills, and its 3-dimensional cousin Chern-Simons theory. These fundamental physical field theories have implications for pure mathematics, through their relationship with quantum groups, character varieties, and mapping class groups and braid groups.

## Quantization and quantum geometric representation theory

Much of geometric representation theory is concerned with realizing representations of algebraic groups, their Lie algebras, Weyl groups, and associated Hecke algebras in geometric terms, i.e. in terms of various categories of sheaves on algebraic spaces constructed from the group. Quantum geometric representation theory is a relatively young field, in which one first $q$-deforms the classical geometric constructions, in a way compatible with the deformation to the quantum group, and studies the resulting categories.

### Finished Papers

• The quantum Frobenius for character varieties and multiplicative quiver varieties
w/ I. Ganev and P. Safronov arXiv:1901.11450

We develop a general mechanism for constructing sheaves of Azumaya algebras on moduli spaces obtained by Hamiltonian reduction, via their quantizations at roots of unity. We achieve this by exploiting a strong compatibility between quantum Hamiltonian reduction and the quantum Frobenius homomorphism. We therefore introduce the concepts of Frobenius quantum moment maps and their Hamiltonian reduction, and of Frobenius Poisson orders. We use these tools to construct canonical central subalgebras of quantum algebras, and explicitly compute the resulting Azumaya loci we encounter, using a natural nondegeneracy assumption. As our main applications, we prove that quantized multiplicative quiver varieties and quantum character varieties define sheaves of Azumaya algebras over the corresponding classical moduli spaces, and finally we compute the Azumaya locus of the Kauffman bracket skein algebras, proving a strong form of the Unicity Conjecture of Bonahon and Wong.

• The center of the reflection equation via quantum minors
w/ N. White arXiv:1709.09149

We give simple formulas for the elements $c_k$ appearing in a quantum Cayley-Hamilton formula for the reflection equation algebra (REA) associated to the quantum group $U_q(\mathfrak{gl}_N)$, answering a question of Kolb and Stokman. The $c_k$'s are certain canonical generators of the center of the REA, and hence of $U_q(\mathfrak{gl}_N)$ itself; they have been described by Reshetikhin using graphical calculus, by Nazarov-Tarasov using quantum Yangians, and by Gurevich, Pyatov and Saponov using quantum Schur functions; however no explicit formulas for these elements were previously known.
As byproducts, we prove a quantum Girard-Newton identity relating the $c_k$'s to the so-called quantum power traces, and we give a new presentation for the quantum group $U_q(\mathfrak{gl}_N)$, as a localization of the REA along certain principal minors.

• The rectangular representation of the double affine Hecke algebra via elliptic Schur-Weyl duality
w/ M. Vazirani arXiv:1708.06024

Given a module $M$ for the algebra $\mathcal{D}_{\mathtt{q}}(G)$ of quantum differential operators on $G$, and a positive integer $n$, we may equip the space $F_n^G(M)$ of invariant tensors in $V^{\otimes n}\otimes M$, with an action of the double affine Hecke algebra of type $A_{n-1}$. Here $G= SL_N$ or $GL_N$, and $V$ is the $N$-dimensional defining representation of $G$.
In this paper we take $M$ to be the basic $\mathcal{D}_{\mathtt{q}}(G)$-module, i.e. the quantized coordinate algebra $M= \mathcal{O}_{\mathtt{q}}(G)$. We describe a weight basis for $F_n^G(\mathcal{O}_{\mathtt{q}}(G))$ combinatorially in terms of walks in the type $A$ weight lattice, and standard periodic tableaux, and subsequently identify $F_n^G(\mathcal{O}_{\mathtt{q}}(G))$ with the irreducible "rectangular representation" of height $N$ of the double affine Hecke algebra.

• Quantum character varieties and braided module categories
w/ A. Brochier and D. Ben-Zvi arXiv:1606.04769

We study a categorical invariant of surfaces associated to a braided tensor category $\mathcal A$, the factorization homology $\int_S \mathcal A$ or quantum character variety of $S$. In our previous paper arXiv:1501.04652 we introduced these invariants and computed them for a punctured surface $S$ as a category of modules for a canonical and explicit algebra, quantizing functions on the classical character variety. In this paper we compute quantum character varieties of arbitrary closed surfaces with boundaries and marked points.
We identify the algebraic data governing marked points and boundary components with the notion of a {\em braided module category} for $\mathcal A$, and we describe braided module categories with a generator in terms of certain explicit algebra homomorphisms called {\em quantum moment maps}. We then show that the quantum character variety of a decorated surface is obtained from that of the corresponding punctured surface as a quantum Hamiltonian reduction.
Characters of braided $\mathcal A$-modules are objects of the torus integral $\int_{T^2}\mathcal A$. We initiate a theory of character sheaves for quantum groups by identifying the torus integral of $\mathcal A=\operatorname{Rep}_q G$ with the category $\mathcal D_q(G/G)$-mod of equivariant quantum $\mathcal D$-modules. When $G=GL_n$, we relate the mirabolic version of this category to the representations of the spherical double affine Hecke algebra (DAHA) $\mathbb{SH}_{q,t}$.

• The Harish-Chandra isomorphism for quantum $GL_2$
w/ M. Balagovic arXiv:1603.09218

We construct an explicit Harish-Chandra isomorphism, from the quantum Hamiltonian reduction of the algebra $\mathcal D_q(GL_2)$ of quantum differential operators on $GL_2$, to the spherical double affine Hecke algebra associated to $GL_2$. The isomorphism holds for all deformation parameters non-zero $q, t,$ such that $t$ does not equal to $\pm i$, and $q$ is not a non-trivial root of unity. We also discuss its extension to the root of unity case.

• Integrating quantum groups over surfaces
w/ A. Brochier and D. Ben-Zvi arXiv:1501.04652
Journal of Topology, to appear.

We apply the mechanism of factorization homology to construct and compute category-valued two-dimensional topological field theories associated to braided tensor categories, generalizing the $(0,1,2)$-dimensional part of Crane-Yetter-Kauffman 4D TFTs associated to modular categories. Starting from modules for the Drinfeld-Jimbo quantum group $U_q(\mathfrak g)$ we obtain in this way an aspect of topologically twisted 4-dimensional ${\mathcal N}=4$ super Yang-Mills theory, the setting introduced by Kapustin-Witten for the geometric Langlands program.
For punctured surfaces, in particular, we produce explicit categories which quantize character varieties (moduli of $G$-local systems) on the surface; these give uniform constructions of a variety of well-known algebras in quantum group theory. From the annulus, we recover the reflection equation algebra associated to $U_q(\mathfrak g)$, and from the punctured torus we recover the algebra of quantum differential operators associated to $U_q(\mathfrak g)$. From an arbitrary surface we recover Alekseev's moduli algebras. Our construction gives an intrinsically topological explanation for well-known mapping class group symmetries and braid group actions associated to these algebras, in particular the elliptic modular symmetry (difference Fourier transform) of quantum $\mathcal D$-modules.

• Fourier transform for quantum D-modules via the punctured torus mapping class group.
w/ A. Brochier arXiv:1403.1841.
Quantum Topol. 8 (2017), 361-379

We construct a certain cross product of two copies of the braided dual $\tilde H$ of a quasitriangular Hopf algebra $H$, which we call the elliptic double $E_H$, and which we use to construct representations of the punctured elliptic braid group extending the well-known representations of the planar braid group attached to $H$. We show that the elliptic double is the universal source of such representations. We recover the representations of the punctured torus braid group obtained in arXiv:0805.2766, and hence construct a homomorphism to the Heisenberg double $D_H$, which is an isomorphism if $H$ is factorizable.
The universal property of $E_H$ endows it with an action by algebra automorphisms of the mapping class group $\widetilde{SL_2(\mathbb{Z})}$ of the punctured torus. One such automorphism we call the quantum Fourier transform; we show that when $H=U_q(\mathfrak{g})$, the quantum Fourier transform degenerates to the classical Fourier transform on $D(\mathfrak{g})$ as $q\to 1$.

• Quantized Multiplicative Quiver Varieties.
Adv Math. Vol 250 (2014). arXiv:1010.4076

Beginning with the data of a quiver $Q$, and its dimension vector $\mathbf{d}$, we construct an algebra $D_q=D_q(Mat_d(Q))$, which is a flat $q$-deformation of the algebra of differential operators on the affine space $Mat_d(Q)$. The algebra $D_q$ is equivariant for an action by a product of quantum general linear groups, acting by conjugation at each vertex. We construct a quantum moment map for this action, and subsequently define the Hamiltonian reduction $A^\lambda_\mathbf{d}(Q)$ of $D_q$ with moment parameter $\lambda$. We show that $A^\lambda_d(Q)$ is a flat formal deformation of Lusztig's quiver varieties, and their multiplicative counterparts, for all dimension vectors satisfying a flatness condition of Crawley-Boevey: indeed the product on $A^\lambda_d(Q)$ yields a Fedosov quantization the of symplectic structure on multiplicative quiver varieties. As an application, we give a description of the category of representations of the spherical double affine Hecke algebra of type $A_{n-1}$, and its generalization constructed by Etingof, Oblomkov, and Rains, in terms of a quotient of the category of equivariant $D_q$-modules by a Serre sub-category of aspherical modules.

• Quantum symmetric pairs and representations of double affine Hecke algebras of type $C^\vee C_n$.
w/ X. Ma. Selecta Mathematica, Vol. 17, No. 1 (2011). arXiv:0908.3013.

We build representations of the affine and double affine braid groups and Hecke algebras of type $C^\vee C_n$, based upon the theory of quantum symmetric pairs $(U,B)$. In the case $U=U_q(gl_N)$, our constructions provide a quantization of the representations constructed by Etingof, Freund and Ma in arXiv:0801.1530, and also a type $BC$ generalization of the results in arXiv:0805.2766.

• Quantum D-modules, elliptic braid groups, and double affine Hecke algebras.
Int. Math. Res. Not. (2009). arXiv:0805.2766

We build representations of the elliptic braid group from the data of a quantum $D$-module $M$ over a ribbon Hopf algebra $U$. The construction is modelled on, and generalizes, similar constructions by Lyubashenko and Majid, and also certain geometric constructions of Calaque, Enriquez, and Etingof concerning trigonometric Cherednik algebras. In this context, the former construction is the special case where M is the basic representation, while the latter construction can be recovered as a quasi-classical limit of $U=U_t(sl_N)$, as $t$ limits to $1$. In the latter case, we produce representations of the double affine Hecke algebra of type $A_{n-1}$, for each $n$.

### Works in Progress

• Quantum Springer theory. w/ D. Ben-Zvi and S. Gunningham. The aim of this project is to study natural domain walls between reductive $G$ and its Levi subgroups $L$ corresponding to parabolic subgroups $P$, and develop their implications to the respective QGL theories of $G$ and $L$.
• Quantum cluster structures via Factorization homology. w/ Ian Le, Gus Schrader, and Sasha Shapiro. The aim of this project is to extract the beuatiful combinatorics of the Fock-Goncharov cluster quantization of decorated character varieties from the mechanism of stratified factorization homology.
• The DAHA of type C from factorization homology. w/ T. Weelinck. The idea of this project is to build on Tim's thesis work pinpointing quantum symmetric pairs as the basic input for Z/2 orbifold factorization homology, and to recover the results of my paper with Xiaoguang Ma in this framework.

## Tensor Categories: structure theory, classification and representation theory

I study tensor categories in all their various flavours: fusion, braided, symmetric, stable $\infty$-. I am especially interested in understanding tensor categories as categorifications of Frobenius algebras; this analogy is espeically strong when the category is moreover pivotal. Studying these structures typically involves higher categorical and homotopical techniques, but it has the attractive feature of being very complete, thus bringings these often abstract tools down to earth.

Fusion categories possess a host of finiteness properties, which make classification schemes (by global dimension, by number of simple objects, by dimensions of generators, etc...) feasible; their structure and representation theory mimics that of finite dimensional Frobenius algebras. Braided tensor categories play an important role in quantum computation, and their structure and representation theory mimics that of commutative Frobenius algebras (especially if when they are ribbon).

### Collaborators

Pinhas Grossman, Pavel Etingof, Eric Larson, Scott Morrison, Noah Snyder

### Finished Papers

• On dualizability of braided tensor categories
w/ A. Brochier and N. Snyder. arXiv:1804.07538

We study the question of dualizability in higher Morita categories of locally presentable tensor categories and braided tensor categories. Our main results are that the 3-category of rigid tensor categories with enough compact projectives is 2-dualizable, that the 4-category of rigid braided tensor categories with enough compact projectives is 3-dualizable, and that (in characteristic zero) the 4-category of braided fusion categories is 4-dualizable. Via the cobordism hypothesis, this produces respectively 2, 3 and 4-dimensional framed local topological field theories. In particular, we produce a framed 3-dimensional local TFT attached to the category of representations of a quantum group at any value of q.

• Cyclic extensions of fusion categories via the Brauer-Picard groupoid
w/ P. Grossman and N. Snyder. to appear in Quantum Topology, arXiv:1211.6414.

We construct a long exact sequence computing the obstruction space, $\pi_1(BrPic(\mathcal{C}_0))$, to $G$-graded extensions of a fusion category $\mathcal{C}_0$. The other terms in the sequence can be computed directly from the fusion ring of $\mathcal{C}_0$. We apply our result to several examples coming from small index subfactors, thereby constructing several new fusion categories as $G$-extensions. The most striking of these is a $\mathbb{Z}/2\mathbb{Z}$-extension of one of the Asaeda-Haagerup fusion categories, which is one of only two known 3-supertransitive fusion categories outside the ADE series.

In another direction, we show that our long exact sequence appears in exactly the way one expects: it is part of a long exact sequence of homotopy groups associated to a naturally occuring fibration. This motivates our constructions, and gives another example of the increasing interplay between fusion categories and algebraic topology.

• On the classification of certain fusion categories
w/ Eric Larson. J. Noncommutative Geometry Vol 3, Issue 3 (2009). arXiv:0812.1603

We advance the classification of fusion categories in two directions. Firstly, we completely classify integral fusion categories -- and consequently, semi-simple Hopf algebras -- of dimension $pq^2$, where $p$ and $q$ are distinct primes. This case is especially interesting because it is the simplest class of dimensions where not all integral fusion categories are group-theoretical. Secondly, we classify a certain family of $\mathbb{Z}/3\mathbb{Z}$-graded fusion categories, which are generalizations of the $\mathbb{Z}/2\mathbb{Z}$-graded Tambara-Yamagami categories. Our proofs are based on the recently developed theory of extensions of fusion categories.

## Non-commutative algebra and differential geometry

Many approaches to non-commutative algebraic geometry follow one of two tracks: either (1) to study non-commutative algebras which are "close to" commutative algebras, i.e. which depend on a parameter and such that at special values of the parameter they become commtuative -- this is the spirit of "quantum geometric representation theory" above, or (2) to develop fundamental results about representation theory of general non-commutative rings by analogy with Groethendieck-style algebraic geometry.

The line of research below was pioneered by Kapronov, and Feigin-Shoikhet, and instead studies those non-commutative algebras which are complete in their commutator filtration. Such "Lie-nilpotent" algebras can be imagined to lie on the normal cone to the space of commutative algebras within the space of all algebras, viewing the commutator ideal as the defining ideal of commutative algebras. The papers below each take this idea seriously, and relate basic algebraic invariants of the non-commutative algebra in terms of algebro-geometric constructions on its abelianization.

### Collaborators

N. Arbesfeld, A. Bapat, S. Bhupatiraju, Y. Chen, P. Etingof, W. Kuszmaul, J. Li, H. Orem, M. Zhang

### Papers

• An algebro-geometric construction of lower central series of associative algebras
w/ H. Orem. To appear in Int. Math. Res. Not. arXiv:1302.2992

The lower central series invariants $M_k$ of an associative algebra $A$ are the two-sided ideals generated by $k$-fold iterated commutators; the $M_k$ provide a filtration of $A$. We study the relationship between the geometry of $X = Spec A_{ab}$ and the associated graded components $N_k$ of this filtration. We show that the $N_k$ form coherent sheaves on a certain nilpotent thickening of $X$, and that Zariski localization on $X$ coincides with noncommutative localization of $A$. Under certain freeness assumptions on $A$, we give an alternative construction of $N_k$ purely in terms of the geometry of $X$ (and in particular, independent of $A$). Applying a construction of Kapranov, we exhibit the $N_k$ as natural vector bundles on the category of smooth schemes.

• Lower central series of a free associative algebra over the integers and finite fields
w/ S. Bhupatiraju, P. Etingof, D. Jordan, W. Kuszmaul, J. Li. J. Algebra 2012. arXiv:1203.1893.

Consider the free algebra $A_n$ generated over $\mathbb{Q}$ by $n$ generators $x_1, \ldots, x_n$. Interesting objects attached to $A = A_n$ are members of its lower central series, $L_i = L_i(A)$, defined inductively by $L_1 = A, L_{i+1} = [A,L_{i}]$, and their associated graded components $B_i = B_i(A)$ defined as $B_i=L_i/L_{i+1}$. These quotients B_i, for i at least 2, as well as the reduced quotient $\bar{B}_1=A/(L_2+A L_3)$, exhibit a rich geometric structure, as shown by Feigin and Shoikhet and later authors, (Dobrovolska-Kim-Ma,Dobrovolska-Etingof,Arbesfeld-Jordan,Bapat-Jordan). We study the same problem over the integers $\mathbb{Z}$ and finite fields $\mathbb{F}_p$. New phenomena arise, namely, torsion in $B_i$ over $\mathbb{Z}$, and jumps in dimension over $\mathbb{F}_p$. We describe the torsion in the reduced quotient $\bar{B}_1$ and $B_2$ geometrically in terms of the De Rham cohomology of $\mathbb{Z}^n$. As a corollary we obtain a complete description of $\bar{B}_1(A_n(\mathbb{Z}))$ and $\bar{B}_1(A_n(\mathbb{F}_p))$, as well as of $B_2(A_n(\mathbb{Z}[\frac{1}{2}]))$ and $B_2(A_n(\mathbb{F}_p))$, $p>2$. We also give theoretical and experimental results for $B_i$ with $i>2$, formulating a number of conjectures and questions based on them. Finally, we discuss the supercase, when some of the generators are odd (fermionic) and some are even (bosonic), and provide some theoretical results and experimental data in this case.

• The lower central series of the symplectic quotient of a free associative algebra
w/ B. Bond. J. Pure & Appl. Alg. arXiv:1111.2316.

We study the lower central series filtration $L_k$ for a symplectic quotient $A=A_{2n}/\lt w \gt$; of the free algebra $A_{2n}$ on $2n$ generators, where $w=\sum [x_i,x_{i+n}]$. We construct an action of the Lie algebra $H_{2n}$ of Hamiltonian vector fields on the associated graded components of the filtration, and use this action to give a complete description of the reduced first component $\bar{B}_1(A)= A/(L_2 + AL_3)$ and the second component $B_2=L_2/L_3$, and we conjecture a description for the third component $B_3=L_3/L_4$.

• Lower central series of free algebras in symmetric tensor categories
w/ A. Bapat. J. Alg, 2012. arXiv:1001.1375.

We continue the study of the lower central series of a free associative algebra, initiated by B. Feigin and B. Shoikhet (arXiv:math/0610410). We generalize via Schur functors the constructions of the lower central series to any symmetric tensor category; specifically we compute the modified first quotient $\bar{B}_1$, and second and third quotients $B_2$, and $B_3$ of the series for a free algebra $T(V)$ in any symmetric tensor category, generalizing the main results of arXiv:math/0610410 and arXiv:0902.4899. In the case $A_{m|n}:=T(\mathbb{C}^{m|n})$, we use these results to compute the explicit Hilbert series. Finally, we prove a result relating the lower central series to the corresponding filtration by two-sided associative ideals, confirming a conjecture from arXiv:0805.1909, and another one from arXiv:0902.4899, as corollaries.

• New results on the lower central series of an free associative algebra
w/ N. Arbesfeld. J. Alg, 2010. arXiv:0902.4899

We continue the study of the lower central series and its associated graded components for a free associative algebra with n generators, as initiated by B. Feigin and B. Shoikhet. We establish a linear bound on the degree of tensor field modules appearing in the Jordan-Hoelder series of each graded component, which is conjecturally tight. We also bound the leading coefficient of the Hilbert polynomial of each graded component. As applications, we confirm conjectures of P. Etingof and B. Shoikhet concerning the structure of the third graded component.

• Poisson traces in positive characteristic
w/ Y. Chen, P. Etingof, M. Zhang. arXiv:1112.6385.

We study Poisson traces of the structure algebra $A$ of an affine Poisson variety $X$ defined over a field of characteristic p. According to arXiv:0908.3868v4, the dual space $HP_0(A)$ to the space of Poisson traces arises as the space of coinvariants associated to a certain D-module $M(X)$ on $X$. If $X$ has finitely many symplectic leaves and the ground field has characteristic zero, then $M(X)$ is holonomic, and thus $HP_0(A)$ is finite dimensional. However, in characteristic $p$, the dimension of $HP_0(A)$ is typically infinite. Our main results are complete computations of $HP_0(A)$ for sufficiently large $p$ when $X$ is either 1) a quasi-homogeneous isolated surface singularity in the three-dimensional space, 2) a quotient singularity $V/G$, for a symplectic vector space V by a finite subgroup $G \subset Sp(V)$, and 3) a symmetric power of a symplectic vector space or a Kleinian singularity. In each case, there is a finite nonnegative grading, and we compute explicitly the Hilbert series. The proofs are based on the theory of D-modules in positive characteristic.