# Alistair SavageUniversity of Ottawa · Department of Mathematics and Statistics

Alistair Savage

Doctor of Philosophy

## About

90

Publications

3,328

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Introduction

Alistair Savage currently works at the Department of Mathematics and Statistics, University of Ottawa. His research involves the use of geometric, combinatorial, and categorical methods to study various topics in representation theory.

Additional affiliations

July 2006 - present

January 2004 - June 2006

Education

September 1998 - May 2003

September 1994 - May 1998

## Publications

Publications (90)

Suppose a finite group acts on a scheme X and a finite-dimensional Lie
algebra g. The corresponding equivariant map algebra is the Lie algebra M of
equivariant regular maps from X to g. We classify the irreducible
finite-dimensional representations of these algebras. In particular, we show
that all such representations are tensor products of evalua...

Suppose a group Gamma acts on a scheme X and a Lie superalgebra g. The
corresponding equivariant map superalgebra is the Lie superalgebra of
equivariant regular maps from X to g. We classify the irreducible finite
dimensional modules for these superalgebras under the assumptions that the
coordinate ring of X is finitely generated, Gamma is finite a...

The Grothendieck groups of the categories of finitely generated modules and
finitely generated projective modules over a tower of algebras can be endowed
with (co)algebra structures that, in many cases of interest, give rise to a
dual pair of Hopf algebras. Moreover, given a dual pair of Hopf algebras, one
can construct an algebra called the Heisen...

We show that the Grothendieck groups of the categories of finitely-generated
graded supermodules and finitely-generated projective graded supermodules over
a tower of graded superalgebras satisfying certain natural conditions give rise
to twisted Hopf algebras that are twisted dual. Then, using induction and
restriction functors coming from such to...

In the present paper we generalize the construction of the nil Hecke ring of
Kostant-Kumar to the context of an arbitrary algebraic oriented cohomology
theory of Levine-Morel and Panin-Smirnov, e.g. to Chow groups, Grothendieck's
K_0, connective K-theory, elliptic cohomology, and algebraic cobordism. The
resulting object, which we call a formal (af...

We introduce the quantum isomeric supercategory and the quantum affine isomeric supercategory. These diagrammatically defined supercategories, which can be viewed as isomeric analogues of the HOMFLYPT skein category and its affinisation, provide powerful categorical tools for studying the representation theory of the quantum isomeric superalgebras...

We introduce two families of diagrammatic monoidal supercategories. The first family, depending on an associative superalgebra, generalizes the oriented Brauer category. The second, depending on an involutive superalgebra, generalizes the unoriented Brauer category. These two families of supercategories admit natural superfunctors to supercategorie...

To any Frobenius superalgebra A we associate an oriented Frobenius Brauer category and an affine oriented Frobenius Brauer category. We define natural actions of these categories on categories of supermodules for general linear Lie superalgebras glm|n(A) with entries in A. These actions generalize those on module categories for general linear Lie s...

We introduce the quantum isomeric supercategory and the quantum affine isomeric supercategory. These diagrammatically defined supercategories, which can be viewed as isomeric analogues of the HOMFLYPT skein category and its affinization, provide powerful categorical tools for studying the representation theory of the quantum isomeric superalgebras...

We introduce a graphical calculus for the representation theory of the quantized enveloping algebra of type $F_4$. We do this by giving a diagrammatic description of the category of invariant tensors on the 26-dimensional fundamental representation.

We define a diagrammatic monoidal category, together with a full and essentially surjective monoidal functor from this category to the category of modules over the exceptional Lie algebra of type $F_4$. In this way, we obtain a set of diagrammatic tools for studying type $F_4$ representation theory that are analogous to those of the oriented and un...

We associate a diagrammatic monoidal category Heisk(A;z,t), which we call the quantum Frobenius Heisenberg category, to a symmetric Frobenius superalgebra A, a central charge k∈Z, and invertible parameters z,t in some ground ring. When A is trivial, i.e. it equals the ground ring, these categories recover the quantum Heisenberg categories introduce...

We show that the central charge $k$ reduction of the universal central extension of the elliptic Hall algebra is isomorphic to the trace, or zeroth Hochschild homology, of the quantum Heisenberg category of central charge $k$. As an application, we construct large families of representations of the universal extension of the elliptic Hall algebra.

We describe bases for the morphism spaces of the Frobenius Heisenberg categories associated to a symmetric graded Frobenius algebra, proving several open conjectures. Our proof uses a categorical comultiplication and generalized cyclotomic quotients of the category. We use our basis theorem to prove that the Grothendieck ring of the Karoubi envelop...

To any Frobenius superalgebra $A$ we associate an oriented Frobenius Brauer category and an affine oriented Frobenius Brauer categeory. We define natural actions of these categories on categories of supermodules for general linear Lie superalgebras $\mathfrak{gl}_{m|n}(A)$ with entries in $A$. These actions generalize those on module categories for...

These are lecture notes for a mini-course given at the conference Interactions of Quantum Affine Algebras with Cluster Algebras, Current Algebras, and Categorification in June 2018. The goal is to introduce the reader to string diagram techniques for monoidal categories, with an emphasis on their role in categorification.

We show that any Abelian module category over the (degenerate or quantum) Heisenberg category satisfying suitable finiteness conditions may be viewed as a 2-representation over a corresponding Kac–Moody 2-category (and vice versa). This gives a way to construct Kac–Moody actions in many representation-theoretic examples which is independent of Rouq...

We define the affinization of an arbitrary monoidal category $\mathcal{C}$, corresponding to the category of $\mathcal{C}$-diagrams on the cylinder. We also give an alternative characterization in terms of adjoining dot generators to $\mathcal{C}$. The affinization formalizes and unifies many constructions appearing in the literature. In particular...

To each symmetric graded Frobenius superalgebra we associate a W-algebra. We then define a linear isomorphism between the trace of the Frobenius Heisenberg category and a central reduction of this W-algebra. We conjecture that this is an isomorphism of graded superalgebras.

We associate a diagrammatic monoidal category $\mathcal{H}\textit{eis}_k(A;z,t)$, which we call the quantum Frobenius Heisenberg category, to a symmetric Frobenius superalgebra $A$, a central charge $k \in \mathbb{Z}$, and invertible parameters $z,t$ in some ground ring. When $A$ is trivial, i.e. it equals the ground ring, these categories recover...

To any Frobenius superalgebra $A$ we associate towers of Frobenius nilCoxeter algebras and Frobenius nilHecke algebras. These act naturally, via Frobeinus divided difference operators, on Frobenius polynomial algebras. When $A$ is the ground ring, our algebras recover the classical nilCoxeter and nilHecke algebras. When $A$ is the two-dimensional C...

To each symmetric graded Frobenius superalgebra we associate a W-algebra. We then define a linear isomorphism between the trace of the Frobenius Heisenberg category and a central reduction of this W-algebra. We conjecture that this is an isomorphism of graded superalgebras.

To every group $G$ we associate a linear monoidal category $\mathcal{P}\mathit{ar}(G)$ that we call a group partition category. We give explicit bases for the morphism spaces and also an efficient presentation of the category in terms of generators and relations. We then define an embedding of $\mathcal{P}\mathit{ar}(G)$ into the group Heisenberg c...

We describe bases for the morphism spaces of the Frobenius Heisenberg categories associated to a symmetric graded Frobenius algebra, proving several open conjectures. Our proof uses a categorical comultiplication and generalized cyclotomic quotients of the category. We use our basis theorem to prove that the Grothendieck ring of the Karoubi envelop...

We show that any Abelian module category over the (degenerate or quantum) Heisenberg category satisfying suitable finiteness conditions may be viewed as a 2-representation over a corresponding Kac-Moody 2-category (and vice versa). This gives a way to construct Kac-Moody actions in many representation-theoretic examples which is independent of Rouq...

We define a faithful linear monoidal functor from the partition category, and hence from Deligne's category $\mathrm{Rep}(S_t)$, to the Heisenberg category. We show that the induced map on Grothendieck rings is injective and corresponds to the Kronecker coproduct on symmetric functions.

To each symmetric algebra we associate a family of algebras that we call quantum affine wreath algebras. These can be viewed both as symmetric algebra deformations of affine Hecke algebras of type $A$ and as quantum deformations of affine wreath algebras. We study the structure theory of these new algebras and their natural cyclotomic quotients.

We introduce a diagrammatic monoidal category $\mathcal{H}eis_k(z,t)$ which we call the {\em quantum Heisenberg category}, here, $k \in \Z$ is "central charge" and $z$ and $t$ are invertible parameters. Special cases were known before: for central charge $k=-1$ and parameters $z = q-q^{-1}$ and $t = -z^{-1}$ our quantum Heisenberg category may be o...

The degenerate Heisenberg category $\mathcal{H}eis_k$ is a strict monoidal category which was originally introduced in the special case $k=-1$ by Khovanov in 2010. Khovanov conjectured that the Grothendieck ring of the additive Karoubi envelope of his category is isomorphic to a certain $\mathbb{Z}$-form for the universal enveloping algebra of the...

These are lectures notes for a mini-course given at the conference Interactions of Quantum Affine Algebras with Cluster Algebras, Current Algebras, and Categorification in June 2018. The goal is to introduce the reader to string diagram techniques for monoidal categories, with an emphasis on their role in categorification.

We associate a graded monoidal supercategory $\mathcal{H}\mathit{eis}_{F,\xi}$ to every graded Frobenius superalgebra $F$ and integer $\xi$. These categories, which categorify a broad range of lattice Heisenberg algebras, recover many previously defined Heisenberg categories as special cases. In this way, the categories $\mathcal{H}\mathit{eis}_{F,...

We study the structure and representation theory of affine wreath product algebras and their cyclotomic quotients. These algebras, which appear naturally in Heisenberg categorification, simultaneously unify and generalize many important algebras appearing in the literature. In particular, special cases include degenerate affine Hecke algebras, affi...

We associate a monoidal category $\mathcal{H}_B$, defined in terms of planar
diagrams, to any graded Frobenius superalgebra $B$. This category acts
naturally on modules over the wreath product algebras associated to $B$. To $B$
we also associate a (quantum) lattice Heisenberg algebra $\mathfrak{h}_B$. We
show that, provided $B$ is not concentrated...

We associate a monoidal category $\mathcal{H}^\lambda$ to each dominant integral weight $\lambda$ of $\widehat{\mathfrak{sl}}_p$ or $\mathfrak{sl}_\infty$. These categories, defined in terms of planar diagrams, act naturally on categories of modules for the degenerate cyclotomic Hecke algebras associated to $\lambda$. We show that, in the $\mathfra...

We introduce a simple diagrammatic 2-category $\mathscr{A}$ that categorifies the image of the Fock space representation of the Heisenberg algebra and the basic representation of $\mathfrak{sl}_\infty$. We show that $\mathscr{A}$ is equivalent to a truncation of the Khovanov--Lauda categorified quantum group $\mathscr{U}$ of type $A_\infty$, and al...

Starting from a graded Frobenius superalgebra $B$, we consider a graphical calculus of $B$-decorated string diagrams. From this calculus we produce algebras consisting of closed planar diagrams and of closed annular diagrams. The action of annular diagrams on planar diagrams can be used to make clockwise (or counterclockwise) annular diagrams into...

We construct invariant polynomials on truncated multicurrent algebras, which are Lie algebras of the form $\mathfrak{g} \otimes_\mathbb{F} \mathbb{F}[t_1,\dotsc,t_\ell]/I$, where $\mathfrak{g}$ is a finite-dimensional Lie algebra over a field $\mathbb{F}$ of characteristic zero, and $I$ is a finite-codimensional ideal of $\mathbb{F}[t_1,\dotsc,t_\e...

This paper is the result of a research project completed in the context of the first author's Undergraduate Student Research Award from the Natural Sciences and Engineering Research Council of Canada (NSERC). We prove a nesting phenomenon for twisted Frobenius extensions. Namely, suppose $R \subseteq B \subseteq A$ are graded superrings such that $...

For quivers not of finite or affine type, certain isomorphisms asserted in the article under correction do not hold, as pointed out by Sarah Scherotzke. This note describes the affected results briefly. A corrected version of the paper can be found at arXiv 0909.3746.

We define global and local Weyl modules for Lie superalgebras of the form
$\mathfrak{g} \otimes A$, where $A$ is an associative commutative unital
$\mathbb{C}$-algebra and $\mathfrak{g}$ is a basic Lie superalgebra or
$\mathfrak{sl}(n,n)$, $n \ge 2$. Under some mild assumptions, we prove
universality, finite-dimensionality, and tensor product decom...

We define twisted Frobenius extensions of graded superrings. We develop
equivalent definitions in terms of bimodule isomorphisms, trace maps, bilinear
forms, and dual sets of generators. The motivation for our study comes from
categorification, where one is often interested in the adjointness properties
of induction and restriction functors. We sho...

An equivariant map queer Lie superalgebra is the Lie superalgebra of regular
maps from an algebraic variety (or scheme) $X$ to a queer Lie superalgebra
$\mathfrak{q}$ that are equivariant with respect to the action of a finite
group $\Gamma$ acting on $X$ and $\mathfrak{q}$. In this paper, we classify all
irreducible finite-dimensional representati...

We introduce a twisted version of the Heisenberg double, constructed from a
twisted Hopf algebra and a twisted Hopf pairing. We state a Stone--von Neumann
type theorem for a natural Fock space representation of this twisted Heisenberg
double and deduce the effect on the algebra of shifting the product and
coproduct of the original twisted Hopf alge...

These are the notes for a two-week mini-course given at a winter school in
January 2014 as part of the thematic semester New Directions in Lie Theory at
the Centre de Recherches Math\'ematiques in Montr\'eal. The goal of the course
was to give an overview of the idea of categorification, with an emphasis on
some examples where explicit computation...

Equivariant map algebras are Lie algebras of algebraic maps from a scheme (or
algebraic variety) to a target finite-dimensional Lie algebra (in the case of
the current paper, we assume the latter is a simple Lie algebra) that are
equivariant with respect to the action of a finite group. In the first part of
this paper, we define global Weyl modules...

This paper presents an overview of the current state of knowledge in the
field of equivariant map algebras and discusses some open problems in this
area.

We give a complete classification of the irreducible quasifinite modules for
algebras of the form Vir \otimes A, where Vir is the Virasoro algebra and A is
a Noetherian commutative associative unital algebra over the complex numbers.
It is shown that all such modules are tensor products of generalized evaluation
modules. We also give an explicit su...

In this expository paper we present an overview of various graphical categorifications of the Heisenberg algebra and its Fock space representation. We begin with a discussion of “weak” categorifications via modules for Hecke algebras and “geometrizations” in terms of the cohomology of the Hilbert scheme of points on the resolution of a simple singu...

Suppose a finite group acts on a scheme X and a finite-dimensional Lie
algebra g. The associated equivariant map algebra is the Lie algebra of
equivariant regular maps from X to g. Examples include generalized current
algebras and (twisted) multiloop algebras. Local Weyl modules play an important
role in the theory of finite-dimensional representat...

Suppose a finite group acts on a scheme X and a finite-dimensional Lie
algebra g. The associated equivariant map algebra is the Lie algebra of
equivariant regular maps from X to g. The irreducible finite-dimensional
representations of these algebras were classified in previous work with P.
Senesi, where it was shown that they are all tensor product...

We define a category of planar diagrams whose Grothendieck group contains an
integral version of the infinite rank Heisenberg algebra, thus yielding a
categorification of this algebra. Our category, which is a q-deformation of one
defined by Khovanov, acts naturally on the categories of modules for Hecke
algebras of type A and finite general linear...

Using subvarieties, which we call Demazure quiver varieties, of the quiver varieties of Nakajima, we give a geometric realization of Demazure modules of Kac-Moody algebras with symmetric Cartan data. We give a natural geometric characterization of the extremal weights of a representation and show that Lusztig's semicanonical basis is compatible wit...

These are notes for a lecture series given at the Fields Institute Summer
School in Geometric Representation Theory and Extended Affine Lie Algebras,
held at the University of Ottawa in June 2009. We give an introduction to the
geometric realization of crystal graphs via the quiver varieties of Lusztig and
Nakajima. The emphasis is on motivating th...

Quivers play an important role in the representation theory of algebras, with
a key ingredient being the path algebra and the preprojective algebra. Quiver
grassmannians are varieties of submodules of a fixed module of the path or
preprojective algebra. In the current paper, we study these objects in detail.
We show that the quiver grassmannians co...

Henriques and Kamnitzer have defined a commutor for the category of crystals of a finite-dimensional complex reductive Lie algebra that gives it the structure of a coboundary category (somewhat analogous to a braided monoidal category). Kamnitzer and Tingley then gave an alternative definition of the crystal commutor, using Kashiwara's involution o...

We define complexes of vector bundles on products of moduli spaces of framed
rank r torsion-free sheaves on the complex projective plane. The top
non-vanishing Chern classes of the cohomology of these complexes yield actions
of the r-colored Heisenberg and Clifford algebras on the equivariant cohomology
of the moduli spaces. In this way we obtain a...

In this expository paper, we discuss and compare the notions of braided and coboundary monoidal categories. Coboundary monoidal categories are analogues of braided monoidal categories in which the role of the braid group is replaced by the cactus group. We focus on the categories of representations of quantum groups and crystals and explain how whi...

We show that the category of representations of the Euclidean group E(2) is equivalent to the category of representations of the preprojective algebra of the quiver of type A1. Furthermore, we consider the moduli space of E(2)-modules along with a set of generators. We show that these moduli spaces are quiver varieties of the type considered by Nak...

International audience
We show that the category of representations of the Euclidean group $E(2)$ is equivalent to the category of representations of the preprojective algebra of the quiver of type $A_{\infty}$. Furthermore, we consider the moduli space of $E(2)$-modules along with a set of generators. We show that these moduli spaces are quiver va...

We show that the category of representations of the Euclidean group of orientation-preserving isometries of two-dimensional Euclidean space is equivalent to the category of representations of the preprojective algebra of infinite type A. We also consider the moduli space of representations of the Euclidean group along with a set of generators. We s...

Ginzburg and Nakajima have given two different geometric constructions of quotients of the universal enveloping algebra of sln and its irreducible finite-dimensional highest weight representations using the convolution product in the Borel–Moore homology of flag varieties and quiver varieties, respectively. The purpose of this paper is to explain t...

Kashiwara and Saito have defined a crystal structure on the set of irreducible components of Lusztig's quiver varieties. This gives a geometric realization of the crystal graph of the lower half of the quantum group associated to a simply-laced Kac-Moody algebra. Using an enumeration of the irreducible components of Lusztig's quiver varieties in fi...

The fixed points of a natural torus action on the Hilbert schemes of points
in C^2 are quiver varieties of infinite type A. The equivariant cohomology of
the Hilbert schemes and quiver varieties can be given the structure of bosonic
and fermionic Fock spaces respectively. Then the localization theorem, which
relates the equivariant cohomology of a...

This is an overview article on finite-dimensional algebras and quivers, written for the Encyclopedia of Mathematical Physics. We cover path algebras, Ringel-Hall algebras and the quiver varieties of Lusztig and Nakajima.

Ginzburg and Nakajima have given two different geometric constructions
of quotients of the universal enveloping algebra of sl_n and its
irreducible finite-dimensional highest weight representations using the
convolution product in the Borel-Moore homology of flag varieties and
quiver varieties respectively. The purpose of this paper is to explain
t...

Using subvarieties, which we call Demazure quiver varieties, of the quiver
varieties of Nakajima, we give a geometric realization of Demazure modules of
Kac-Moody algebras with symmetric Cartan data. We give a natural geometric
characterization of the extremal weights of a representation and show that
Lusztig's semicanonical basis is compatible wit...

We consider a generalization of the quiver varieties of Lusztig and Nakajima to the case of all symmetrizable Kac-Moody Lie algebras. To deal with the non-simply laced case one considers admissible automorphisms of a quiver and the irreducible components of the quiver varieties fixed by this automorphism. We define a crystal structure on these irre...

For irreducible integrable highest weight modules of the finite and affine
Lie algebras of type A and D, we define an isomorphism between the geometric
realization of the crystal graphs in terms of irreducible components of
Nakajima quiver varieties and the combinatorial realizations in terms of Young
tableaux and Young walls. For affine type A, we...

Using the tensor product variety introduced in Malkin (Duke Math. J., to appear) and Nakajima (Invent. Math. 146 (2001) 399), the complete structure of the tensor product of a finite number of integrable highest weight modules of Uq(sl2) is recovered. In particular, the elementary basis, Lusztig's canonical basis, and the basis adapted to the decom...

Applying the techniques of an earlier paper with Frenkel, we develop a
geometric realization of spin representations and Clifford algebras. In doing
so, we give an explicit parametrization of the irreducible components of
Nakajima varieties of type D in terms of Young diagrams. We explicity compute
the geometric action of the Lie algebra and are ab...

We relate two apparently different bases in the representations of affine Lie
algebras of type A: one arising from statistical mechanics, the other from
gauge theory. We show that the two are governed by the same combinatorics and
therefore can be viewed as identical. In particular, we are able to give an
alternative and much simpler geometric proo...

We construct the fusion product of finite-dimensional sl_2-modules in the
homology of (or in the space of constructible functions on) a certain
subvariety L_l(w_1, ..., w_r) of Nakajima's tensor product variety L(w_1,...,
w_r). We also give a combinatorial description of the irreducible components of
this subvariety using the notions of graphical c...

A large-n approximation to the S=1 antiferromagnetic chain, using the
symmetric tensor representation and its conjugate, is developed to order 1/n in
order to calculate the magnon wave-function and to study the effect of
modifying the exchange coupling from J to J' on a single link. It is shown that
a magnon boundstate exists below the Haldane gap...

In this expository talk, we describe a construction due to Ginzburg which endows the homology of flag varieties and certain other, closely related, varieties with the structure of representations of sln. The action of sln is described in terms of natural geometric operations and the construction yields bases in the representations with nice propert...

University Microfilms order no. 3084364. Thesis (Ph. D.)--Yale University, 2003. Includes bibliographical references.