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

We define new invariants of 3d Calabi-Yau categories endowed with a stability structure. Intuitively, they count the number of semistable objects with fixed class in the K-theory of the category ('number of BPS states with given charge' in physics language). Formally, our motivic DT-invariants are elements of quantum tori over a version of the Grothendieck ring of varieties over the ground field. Via the quasi-classical limit 'as the motive of affine line approaches to 1' we obtain numerical DT-invariants which are closely related to those introduced by Behrend. We study some properties of both motivic and numerical DT-invariants including the wall-crossing formulas and integrality. We discuss the relationship with the mathematical works (in the non-triangulated case) of Joyce, Bridgeland and Toledano-Laredo, as well as with works of physicists on Seiberg-Witten model (string junctions), classification of N=2 supersymmetric theories (Cecotti-Vafa) and structure of the moduli space of vector multiplets. Relating the theory of 3d Calabi-Yau categories with distinguished set of generators (called cluster collection) with the theory of quivers with potential we found the connection with cluster transformations and cluster varieties (both classical and quantum).

Stability structures, motivic Donaldson-Thomas invariants and cluster transformations

Let G be an inner form of a general linear group over a non-archimedean local field. We prove that the local Langlands correspondence for G preserves depths. We also show that the local Langlands correspondence for inner forms of special linear groups preserves the depths of essentially tame Langlands parameters.

Progress in Mathematics

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

Topological-anti-topological fusion

Dans [7], Deligne a introduit la notion de complexe pur, et démontré la stabilité par image directe par morphismes propres (i.e. la version relative de la conjecture de Weil). Cette théorie, combinée avec celle de faisceaux pervers, a été ensuite développée par Beilinson-Bernstein-Deligne-Gabber[l]: ils ont démontré le théorème de décomposition, et la stabilité des faisceaux pervers purs par images directes intermédiaires (en particulier, la pureté des complexes d'intersection) [loc. cit]. D'après la philosophie de Deligne [6, 7], on a conjecturé qu'il existerait des objets en car. 0, correspondant aux complexes purs, cf. [2,3,4] etc. Le but de cet article est de donner une réponse positive: en utilisant la théorie de £)-Modules filtrés, la théorie de la filtration V [17, 22] et la théorie de faisceaux pervers, on définit la notion de Module de Hodge polarisable (cf. 5.1-2), qui doit correspondre à celle de faisceau pervers pur, et on démontre la stabilité par images directes (perverses) par un morphisme projectif avec un théorème de Lefschetz fort relatif et une polarisation induite sur les parties primitives (i.e. une théorie de Hodge-Lefschetz relative), cf. 5.3.1. Soient X une variété analytique complexe lisse, 3)x le faisceau des opérateurs différentiels analytiques muni de la filtration F par degré d'opérateur, et MF(â)x} la catégorie des .^-Modules filtrés. On dit que (M, F)^MF(3)X} est holonome, si M est un .â^-Module holonome [16, 18] et GrM est un Gr£Dx-ModulQ cohérent (i.e. F est une bonne filtration de M[loc. cit]). Soit MFh(3)x} la sous-catégorie pleine de MF(3)X) des .â^-Modules filtrés holonomes. D'après Kashiwara [16] on a le foncteur

Modules de Hodge Polarisables

the filtration F on Ox' The right hand side of (*) was first studied J f by Brieskorn [B] and we call it the Brieskorn lattice of M, and denote it by Mo. In fact, he defined the regular singular connection (called the Gauss-Manin connection) on Mo, which calculates the Milnor monodromy. We can easily verify that the connection V is compatible with the action of 9f on M as left Z>s,o-module, where t is the coordinate of S'. More precisely, the inverse of the action of Va/^resp.c^) is well-defined as a C-endomorphism of Mo(resp.M) and they coincide on F-nM by the isomorphism (*)

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On the structure of Brieskorn lattice

by duality [18], because R~Tr,CCy, = 0 for i > 0 by [6, 31] (this follows also from [13, 21, 23]) where rc is assumed projective. Here COy denotes the dualizing sheaf (i.e., the dualizing complex [18] shifted by the dimension to the right). The trace morphism (0.2) is injective, and its image is independent of the choice of resolution, because (0.2) is an isomorphism if Y is smooth. We will denote by CSy the image of (0.2). See (2.4.8) below. Now assume Y is a reduced divisor D on a complex manifold X of dimension n. Let f be a reduced defining equation of D on a neighborhood of a: C D. The b-function (i.e., Bernstein polynomial) of f is a monic polynomial bf(s) in s with rational coefficients, and is a generator of the ideal whose element b(s) satisfies the relation

On b-function, spectrum and rational singularity

Using the theory of mixed Hodge Modules, we introduce the notion of mixed Hodge complex on an algebraic variety, and establish the relation between the filtered complex of Du Bois and the corresponding complex of mixed Hodge Modules. Some application to the Du Bois singularity is given.

Morihiko Saito

Papers

Mixed hodge modules

Modules de Hodge Polarisables

On the structure of Brieskorn lattice

On b-function, spectrum and rational singularity

Mixed Hodge complexes on algebraic varieties