Preface.- Chapter 0. Introduction.- Chapter 1. Symplectic Geometry.- Chapter 2. Mosaic.- Chapter 3. Complex Semisimple Groups.- Chapter 4. Springer Theory.- Chapter 5. Equivariant K-Theory.- Chapter 6. Flag Varieties, K-Theory, and Harmonic Polynomials.- Chapter 7. Hecke Algebras and K-Theory.- Chapter 8. Representations of Convolution Algebras.- Bibliography.

Representation theory and complex geometry

The aim of this paper is to work out a concrete example as well as to provide the general pattern of applications of Koszul duality to repre- sentation theory. The paper consists of three parts relatively independent of each other. The first part gives a reasonably selfcontained introduction to Koszul rings and Koszul duality. Koszul rings are certain Z-graded rings with particularly nice homological properties which involve a kind of duality. Thus, to a Koszul ring one associates naturally the dual Koszul ring. The second part is devoted to an application to representation theory of semisimple Lie algebras. We show License or copyright restrictions may apply to redistribution; see http://www.ams.org/journal-terms-of-use KOSZUL DUALITY PATTERNS 527 that the block of the Bernstein-Gelfand-Gelfand category O that corresponds to any fixed central character is governed by the Koszul ring. Moreover, the dual of that ring governs a certain subcategory of the category O again. This generalizes the selfduality theorem conjectured by Beilinson and Ginsburg in 1986 and proved by Soergel in 1990. In the third part we study certain cate- gories of mixed perverse sheaves on a variety stratified by affine linear spaces. We provide a general criterion for such a category to be governed by a Koszul ring. In the flag variety case this reduces to the setup of part two. In the more general case of affine flag manifolds and affine Grassmannians the criterion should yield interesting results about representations of quantum groups and affine Lie algebras. Department of Mathematics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 E-mail address: sasha@math.mit.edu Department of Mathematics, The University of Chicago, Chicago, Illinois 60637 E-mail address: ginzburg@math.uchicago.edu Max-Planck-Institut fur Mathematik, Gottfried-Claren-Strase 26, D-53 Bonn 3, Germany Current address: Mathematisches Institut, Universitat Freiburg, Albertstrase 23b, D-79104 Freiburg, Germany E-mail address: soergel@sun1.mathematik.uni-freiburg.de License or copyright restrictions may apply to redistribution; see http://www.ams.org/journal-terms-of-use

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Koszul Duality Patterns in Representation Theory

The geometric Langlands program can be described in a natural way by compactifying on a Riemann surface C a twisted version of N=4 super Yang-Mills theory in four dimensions. The key ingredients are electric-magnetic duality of gauge theory, mirror symmetry of sigma-models, branes, Wilson and 't Hooft operators, and topological field theory. Seemingly esoteric notions of the geometric Langlands program, such as Hecke eigensheaves and D-modules, arise naturally from the physics.

https://www.intlpress.com/site/pub/files/_fulltext/journals/cntp/2007/0001/0001/CNTP-2007-0001-0001-a001.pdf

Electric-Magnetic Duality And The Geometric Langlands Program

These lecture notes are based on the master class given at the Center for the Topology and Quantization of Moduli Spaces, University of Aarhus, August 2007. I provide an introduction to the recent work on the Montonen-Olive duality of N = 4 super-Yang-Mills theory and the Geometric Langlands Program.

http://www.ctqm.au.dk/research/MCS/LectureNotesKapustin.pdf

Lectures on Electric-Magnetic Duality and the Geometric Langlands Program

Introduction 221 § 1. Relative Monodromy Filtration 227 §2. Mixed Hodge Modules on Complex Spaces (2. a) Vanishing Cycle Functors and Specializations (Divisor Case) 236 (2.b) Extensions over Locally Principal Divisors 243 (2.c) Direct Images 251 (2.d) Mixed Hodge Modules 256 (2.e) Beilinson's Functor and its Applications 261 (2.f) Extensions over Closed Subspaces 265 (2.g) Kodaira Vanishing 273 §3. Mixed Hodge Modules of Normal Crossing Type (3.a) Constant Case 277 (3. b) Pure Case 288 (3.c) Mixed Case 304 §4. Algebraic Case 314

Mixed hodge modules

As such, it can be viewed as a first step in the geometric Langlands program. The connected complex reductive groups have a combinatorial classification by their root data. In the root datum the roots and the co-roots appear in a symmetric manner and so the connected reductive algebraic groups come in pairs. If G is a reductive group, we write G for its companion and call it the dual group G. The notion of the dual group itself does not appear in Satake's paper, but was introduced by Langlands, together with its various elaborations, in [LI], [L2] and is a cornerstone of the Langlands program. It also appeared later in physics [MO], [GNO]. In this paper we discuss the basic relationship between G and G. We begin with a reductive G and consider the affine Grassmannian Qx, the Grassmannian for the loop group of G. For technical reasons we work with formal algebraic loops. The affine Grassmannian is an infinite dimen sional complex space. We consider a certain category of sheaves, the spherical perverse sheaves, on ?r. These sheaves can be multiplied using a convolution product and this leads to a rather explicit construction of a Hopf algebra, by what has come to be known as Tannakian formalism. The resulting Hopf algebra turns out to be the ring of functions on G. In this interpretation, the spherical perverse sheaves on the affine Grassman nian correspond to finite dimensional complex representations of G. Thus, instead of defining G in terms of the classification of reductive groups, we pro vide a canonical construction of G, starting from G. We can carry out our construction over the integers. The spherical perverse sheaves are then those with integral coefficients, but the Grassmannian remains a complex algebraic object.

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Geometric Langlands duality and representations of algebraic groups over commutative rings

Elementary construction of perverse sheaves

This is an expanded version of the text ``Perverse Sheaves on Loop Grassmannians and Langlands Duality'', AG/9703010. The main new result is a topological realization of algebraic representations of reductive groups over arbitrary rings. 
We outline a proof of a geometric version of the Satake isomorphism. Given a connected, complex algebraic reductive group G we show that the tensor category of representations of the Langlands dual group is naturally equivalent to a certain category of perverse sheaves on the loop Grassmannian of G. The above result has been announced by Ginsburg in and some of the arguments are borrowed from his approach. However, we use a more "natural" commutativity constraint for the convolution product, due to Drinfeld. Secondly, we give a direct proof that the global cohomology functor is exact and decompose this cohomology functor into a direct sum of weights. The geometry underlying our arguments leads to a construction of a canonical basis of Weyl modules given by algebraic cycles and an explicit construction of the group algebra of the dual group in terms of the affine Grassmannian. We completely avoid the use of the decomposition theorem which makes our techniques applicable to perverse sheaves with coefficients over an arbitrary commutative ring. We deduce the classical Satake isomorphism using the affine Grassmannian defined over a finite field. This note contains indications of proofs of some of the results. The details will appear elsewhere.

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Perverse Sheaves on affine Grassmannians and Langlands Duality

Let G be a split connected reductive group over a finite field F_q, and N its maximal unipotent subgroup. V. Drinfeld has introduced a remarkable partial compactification of the moduli stack of N-bundles on a smooth projective curve X over F_q. In this paper we study Drinfeld's moduli space and a certain category of perverse sheaves on it. The definition of this category is motivated by the study of the Whittaker functions on the group G(K), where K=F_q((t)). We prove that our category is semi-simple, and that irreducible objects of this category are "clean", i.e., they are extenstions by 0 of local systems supported on the strata. As an application of these results, we obtain a purely geometric proof of the Casselman-Shalika formula for the Whittaker functions.

Whittaker patterns in the geometry of moduli spaces of bundles on curves

Let X be a (smooth, projective) curve and G be a reductive group over a finite field Fq. The field KX of rational functions on X is what number theorists call a global field and we can do the theory of automorphic functions over it. Namely, we consider the quotient G(KX)\G(A) (here A is the ring of adeles corresponding to KX) and study the space of functions on it, as a representation of the adele group G(A). It was essentially an observation of A.Weil that the quotient G(KX)\G(A) is closely related to the set of isomorphism classes of principal G-bundles on our curve X. However, G-bundles on X possess a richer structure: one can view the above set as a set of Fq-points of an algebraic variety (or, rather, a stack), denoted BunG. Moreover, instead of studying the vector space of functions on the “discrete” set of points of BunG, we will consider the category of `-adic sheaves on it. Basically, what people call the geometric Langlands program is an attempt to understand a spectral decomposition of the above category under the action of the so-called Hecke functors (the latter will be defined in the talk). The answer predicted by the Geometric Langlands Conjecture links this spectral decomposition to the moduli stack of local systems on X with respect to the Langlands dual group Ǧ.

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Kari Vilonen

Papers

Geometric Langlands duality and representations of algebraic groups over commutative rings

Elementary construction of perverse sheaves

Perverse Sheaves on affine Grassmannians and Langlands Duality

Whittaker patterns in the geometry of moduli spaces of bundles on curves

On the geometric Langlands conjecture