Showing papers on "Representation theory published in 2003"

Lectures on Invariant Theory

Abstract: The primary goal of this 2003 book is to give a brief introduction to the main ideas of algebraic and geometric invariant theory. It assumes only a minimal background in algebraic geometry, algebra and representation theory. Topics covered include the symbolic method for computation of invariants on the space of homogeneous forms, the problem of finite-generatedness of the algebra of invariants, the theory of covariants and constructions of categorical and geometric quotients. Throughout, the emphasis is on concrete examples which originate in classical algebraic geometry. Based on lectures given at University of Michigan, Harvard University and Seoul National University, the book is written in an accessible style and contains many examples and exercises. A novel feature of the book is a discussion of possible linearizations of actions and the variation of quotients under the change of linearization. Also includes the construction of toric varieties as torus quotients of affine spaces.

Algebras of p-adic distributions and admissible representations

Abstract: Let G be a compact, locally L-analytic group, where L is a finite extension of Qp. Let K be a discretely valued extension field of L. We study the algebra D(G,K) of K-valued locally analytic distributions on G, and apply our results to the locally analytic representation theory of G in vector spaces over K. Our objective is to lay a useful and powerful foundation for the further study of such representations. We show that the noncommutative, nonnoetherian ring D(G,K) "behaves" like the ring of functions on a rigid Stein space, and that (at least when G is Qp-analytic) it is a faithfully flat extension of its subring K\otimes Zp[[G]], where Zp[[G]] is the completed group ring of G. We use this point of view to describe an abelian subcategory of D(G,K) modules that we call coadmissible. We say that a locally analytic representation V of G is admissible if its strong dual is coadmissible as D(G,K)-module. For noncompact G, we say V is admissible if its strong dual is coadmissible as D(H,K) module for some compact open subgroup H. In this way we obtain an abelian category of admissible locally analytic representations. These methods allow us to answer a number of questions raised in our earlier papers on p-adic representations; for example we show the existence of analytic vectors in the admissible Banach space representations of G that we studied in "Banach space representations ...", Israel J. Math. 127, 359-380 (2002). Finally we construct a dimension theory for D(G,K), which behaves for coadmissible modules like a regular ring, and show that smooth admissible representations are zero dimensional.

Cohomology of Vector Bundles and Syzygies

Abstract: The central theme of this book is an exposition of the geometric technique of calculating syzygies. It is written from a point of view of commutative algebra, and without assuming any knowledge of representation theory the calculation of syzygies of determinantal varieties is explained. The starting point is a definition of Schur functors, and these are discussed from both an algebraic and geometric point of view. Then a chapter on various versions of Bott's Theorem leads on to a careful explanation of the technique itself, based on a description of the direct image of a Koszul complex. Applications to determinantal varieties follow, plus there are also chapters on applications of the technique to rank varieties for symmetric and skew symmetric tensors of arbitrary degree, closures of conjugacy classes of nilpotent matrices, discriminants and resultants. Numerous exercises are included to give the reader insight into how to apply this important method.

From fractal groups to fractal sets

Abstract: The idea of self-similarity is one of the most fundamental in the modern mathematics. The notion of “renormalization group”, which plays an essential role in quantum field theory, statistical physics and dynamical systems, is related to it. The notions of fractal and multi-fractal, playing an important role in singular geometry, measure theory and holomorphic dynamics, are also related. Self-similarity also appears in the theory of C*-algebras (for example in the representation theory of the Cuntz algebras) and in many other branches of mathematics. Starting from 1980 the idea of self-similarity entered algebra and began to exert great influence on asymptotic and geometric group theory.

Quantum Reduction for Affine Superalgebras

Abstract: We extend the homological method of quantization of generalized Drinfeld--Sokolov reductions to affine superalgebras. This leads, in particular, to a unified representation theory of superconformal algebras.

Realizations of real low-dimensional Lie algebras

Abstract: Using a new powerful technique based on the notion of megaideal, we construct a complete set of inequivalent realizations of real Lie algebras of dimension no greater than four in vector fields on a space of an arbitrary (finite) number of variables. Our classification amends and essentially generalizes earlier works on the subject.

Quantum reduction for affine superalgebras

Abstract: We extend the homological method of quantization of generalized Drinfeld–Sokolov reductions to affine superalgebras. This leads, in particular, to a unified representation theory of superconformal algebras.

An Introduction to Lie Groups and the Geometry of Homogeneous Spaces

Abstract: Lie groups Maximal tori and the classification theorem The geometry of a compact Lie group Homogeneous spaces The geometry of a reductive homogeneous space Symmetric spaces Generalized flag manifolds Advanced topics Bibliography Index.

A course in algebra

Abstract: Algebraic structures Elements of linear algebra Elements of polynomial algebra Elements of group theory Vector spaces Linear operators Affine and projective spaces Tensor algebra Commutative algebra Groups Linear representations and associative algebras Lie groups Answers to selected exercises Bibliography Index Algebraic structures Elements of linear algebra Elements of polynomial algebra Elements of group theory Vector spaces Linear operators Affine and projective spaces Tensor algebra Commutative algebra Groups Linear representations and associative algebras Lie groups Answers to selected exercises Bibliography Index

Representations of Cuntz-Pimsner algebras

Abstract: Let X be a Hilbert bimodule over a C*-algebra A. We analyse the structure of the associated Cuntz-Pimsner algebra O X and related algebras using representation-theoretic methods. In particular, we study the ideals L(I) in O x induced by appropriately invariant ideals I in A, and identify the quotients O X /L(I) as relative Cuntz-Pimsner algebras of Muhly and Solel. We also prove a gauge-invariant uniqueness theorem for O X , and investigate the relationship between O X and an alternative model proposed by Doplicher, Pinzari and Zuccante.

The local converse theorem for SO(2n+1) and applications

Abstract: In this paper we characterize irreducible generic representations of SO 2n+1 (k) (where k is a p-adic field) by means of twisted local gamma factors (the Local Converse Theorem). As applications, we prove that two irreducible generic cuspidal automorphic representations of SO 2n+1 (A) (where A is the ring of adeles of a number field) are equivalent if their local components are equivalent at almost all local places (the Rigidity Theorem); and prove the Local Langlands Reciprocity Conjecture for generic supercuspidal representations of SO 2n+1 (k).

The Algebraic Approach to the Discrete Cosine and Sine Transforms and Their Fast Algorithms

Abstract: It is known that the discrete Fourier transform (DFT) used in digital signal processing can be characterized in the framework of the representation theory of algebras, namely, as the decomposition matrix for the regular module ${\mathbb{C}}[Z_n] = {\mathbb{C}}[x]/(x^n - 1)$. This characterization provides deep insight into the DFT and can be used to derive and understand the structure of its fast algorithms. In this paper we present an algebraic characterization of the important class of discrete cosine and sine transforms as decomposition matrices of certain regular modules associated with four series of Chebyshev polynomials. Then we derive most of their known algorithms by pure algebraic means. We identify the mathematical principle behind each algorithm and give insight into its structure. Our results show that the connection between algebra and digital signal processing is stronger than previously understood.

A New Proof of the Mullineux Conjecture

Abstract: Let Sd denote the symmetric group on d letters. In 1979 Mullineux conjectured a combinatorial algorithm for calculating the effect of tensoring with an irreducible Sd-module with the one dimensional sign module when the ground field has positive characteristic. Kleshchev proved the Mullineux conjecture in 1996. In the present article we provide a new proof of the Mullineux conjecture which is entirely independent of Kleshchev's approach. Applying the representation theory of the supergroup GL(m v n) and the supergroup analogue of Schur-Weyl Duality it becomes straightforward to calculate the combinatorial effect of tensoring with the sign representation and, hence, to verify Mullineux's conjecture. Similar techniques also allow us to classify the irreducible polynomial representations of GL(m v n) of degree d for arbitrary m, n, and d.

Twisted K-theory and loop group representations

Abstract: This is the third paper of a series relating the equivariant twisted $K$-theory of a compact Lie group $G$ to the ``Verlinde space'' of isomorphism classes of projective lowest-weight representations of the loop groups. Here, we treat arbitrary compact Lie groups. In addition, we discuss the relation to semi-infinite cohomology, the fusion product of Conformal Field theory, the r\^ole of energy and the topological Peter-Weyl theorem.

Poisson orders, symplectic reflection algebras and representation theory

Abstract: We introduce a new class of algebras called Poisson orders. This class includes the symplectic reflection algebras of Etingof and Ginzburg, many quantum groups at roots of unity, and enveloping algebras of restricted Lie algebras in positive characteristic. Quite generally, we study this class of algebras from the point of view of Poisson geometry, exhibiting connections between their representation theory and some well-known geometric constructions. As an application, we employ our results in the study of symplectic reflection algebras, completing work of Etingof and Ginzburg on when these algebras are finite over their centres, and providing a framework for the study of their representation theory in the latter case.

Catégories dérivées et variétés de Deligne-Lusztig

Abstract: We prove a conjecture of Broue about the Jordan decomposition of blocks of finite reductive groups. We show that a block of a finite connected reductive group, in non-describing characteristic, is Morita-equivalent to a quasi-isolated block of a Levi subgroup. This involves showing that some local system over a Deligne-Lusztig variety has its mod l cohomology concentrated in one degree. We reduce this question to a question about tamely ramified local systems by proving that the category of perfect complexes for the group is generated by the images of the Deligne-Lusztig functors. Then, we describe the ramification at infinity of local systems associated to characters of tori.

Quantum Groups, the loop Grassmannian, and the Springer resolution

Abstract: We establish equivalences of derived categories of the following 3 categories: (1) Principal block of representations of the quantum at a root of 1; (2) G-equivariant coherent sheaves on the Springer resolution; (3) Perverse sheaves on the loop Grassmannian for the Langlands dual group. The equivalence (1)-(2) is an `enhancement' of the known expression for quantum group cohomology in terms of nilpotent variety, due to Ginzburg-Kumar. The equivalence (2)-(3) is a step towards resolving an old mystery surrounding the existense of two completely different realizations of the affine Hecke algebra which have played a key role in the proof of the Deligne-Langlands-Lusztig conjecture. One realization is in terms of locally constant functions on the flag manifold of a p-adic reductive group, while the other is in terms of equivariant K-theory of a complex (Steinberg) variety for the dual group. Our equivalence (2)-(3) may be viewed as a `categorification' of the isomorphism between the corresponding two geometric realizations of the fundamental polynomial representation of the affine Hecke algebra. The composite of the two equivalences above yields an equivalence between abelian categories of quantum group representations and perverse sheaves. A similar equivalence at an even root of unity can be deduced, following Lusztig program, from earlier deep results of Kazhdan-Lusztig and Kashiwara-Tanisaki. Our approach is independent of these results and is totally different (it does not rely on representation theory of Kac-Moody algebras). It also gives way to proving Humphreys' conjectures on tilting U_q(g)-modules, as will be explained in a separate paper.

QUANTUM CHANNELS, WAVELETS, DILATIONS AND REPRESENTATIONS OF $\mathcal{O}_{n}$

Abstract: We show that the representations of the Cuntz C � - algebras On which arise in wavelet analysis and dilation theory can be classified through a simple analysis of completely positive maps on finite-dimensional space Based on this analysis, we find an application in quantum information theory; namely, a structure theorem for the fixed point set of a unital quantum channel We also include some open problems motivated by this work There has been considerable recent interest in the analysis of com- pletely positive maps on finite-dimensional space There are a num- ber of reasons for this including connections with wavelet analysis (3, 5, 15), dilation theory (10, 16), representation theory of the Cuntz C ∗ -algebras On (3, 5, 9, 10), and quantum information theory (1, 17, 21, 22) The results obtained in the current paper have implications for each of these areas In presenting this work, another goal we have is to push further the connections between the various areas mentioned above In the first section we establish a result for completely positive maps While we focus on the finite-dimensional setting, this is not necessary in the proof A structure theorem for the fixed point set of a unital quantum channel is contained in the second section In particular, we prove that the fixed point set is a C ∗ -algebra which is equal to the commutant of the algebra generated by any choice of row contraction which determines the channel We discuss the 2-dimensional channels (17, 21), and use the theorem to classify them by their fixed point sets The representation theory for On is considered in the third section We focus on a subclass of representations arising in dilation theory and wavelet analysis (3, 5, 9, 10, 15) Each of these representations determines a completely positive map on finite-dimensional space We ask if these representations can be classified just by examining the map

A_{n-1} singularities and nKdV hierarchies

Abstract: According to a conjecture of E. Witten proved by M. Kontsevich, a certain generating function for intersection indices on the Deligne -- Mumford moduli spaces of Riemann surfaces coincides with a certain tau-function of the KdV hierarchy. The generating function is naturally generalized under the name the {\em total descendent potential} in the theory of Gromov -- Witten invariants of symplectic manifolds. The papers arXiv: math.AG/0108100 and arXive: math.DG/0108160 contain two equivalent constructions, motivated by some results in Gromov -- Witten theory, which associate a total descendent potential to any semisimple Frobenius structure. In this paper, we prove that in the case of K.Saito's Frobenius structure on the miniversal deformation of the $A_{n-1}$-singularity, the total descendent potential is a tau-function of the $n$KdV hierarchy. We derive this result from a more general construction for solutions of the $n$KdV hierarchy from $n-1$ solutions of the KdV hierarchy.

Lie Algebra and the Mobility of Kinematic Chains

Abstract: This paper deals with the application of Lie Algebra to the mobility analysis of kinematic chains. It develops an algebraic formulation of a group-theoretic mobility criterion developed recently by two of the authors of this publication. The instantaneous form of the mobility criterion presented here is based on the theory of subspaces and subalgebras of the Lie Algebra of the Euclidean group and their possible intersections. It is shown using this theory that certain results on mobility of over-constraint linkages derived previously using screw theory are not complete and accurate. The theory presented provides for a computational approach that would allow efficient automation of the new group-theoretic mobility criterion. The theory is illustrated using several examples. © 2003 Wiley Periodicals, Inc.

Dimensions of triangulated categories

Abstract: We define a dimension for a triangulated category. We prove a representabilityTheorem for a certain class of functors on finite dimensional triangulatedcategories. We study the dimension of the boundedderived category of an algebra or a scheme and we show in particularthat the bounded derived category of coherent sheaves over avariety has a finite dimension. For a self-injective algebra, a lowerbound for Auslander's representation dimension is given by the dimensionof the stable category. We use this to compute the representationdimension of exterior algebras. This provides the first known examplesof representation dimension >3. We deduce that theLoewy length of the group algebra over F_2 of a finite group is strictly bounded below by2-rank of the group (a conjecture of Benson).

Equivariant Degree Theory

Abstract: This book presents a new degree theory for maps which commute with a group of symmetries. This degree is no longer a single integer but an element of the group of equivariant homotopy classes of maps between two spheres and depends on the orbit types of the spaces. The authors develop completely the theory and applications of this degree in a self-contained presentation starting with only elementary facts. The first chapter explains the basic tools of representation theory, homotopy theory and differential equations needed in the text. Then the degree is defined and its main abstract properties are derived. The next part is devoted to the study of equivariant homotopy groups of spheres and to the classification of equivariant maps in the case of abelian actions. These groups are explicitely computed and the effects of symmetry breaking, products and composition are thorougly studied. The last part deals with computations of the equivariant index of an isolated orbit and of an isolated loop of stationary points. Here differential equations in a variety of situations are considered: symmetry breaking, forcing, period doubling, twisted orbits, first integrals, gradients etc. Periodic solutions of Hamiltonian systems, in particular spring-pendulum systems, are studied as well as Hopf bifurcation for all these situations.

Kostka-Foulkes polynomials and Macdonald spherical functions

Abstract: Generalized Hall-Littlewood polynomials (Macdonald spherical functions) and generalized Kostka-Foulkes polynomials ($q$-weight multiplicities) arise in many places in combinatorics, representation theory, geometry, and mathematical physics. This paper attempts to organize the different definitions of these objects and prove the fundamental combinatorial results from ``scratch'', in a presentation which, hopefully, will be accessible and useful for both the nonexpert and researchers currently working in this very active field. The combinatorics of the affine Hecke algebra plays a central role. The final section of this paper can be read independently of the rest of the paper. It presents, with proof, Lascoux and Sch\"utzenberger's positive formula for the Kostka-Foulkes poynomials in the type A case.

Generalized Blob Algebras and Alcove Geometry

Abstract: A sequence of finite-dimensional quotients of affine Hecke algebras is studied. Each element of the sequence is constructed so as to have a weight space labelling scheme for Specht⁄standard modules. As in the weight space formalism of algebraic Lie theory, there is an action of an affine reflection group on this weight space that fixes the set of labelling weights. A linkage principle is proved in each case. Further, it is shown that the simplest non-trivial example may essentially be identified with the blob algebra (a physically motivated quasihereditary algebra whose representation theory is very well understood by Lie-theory-like methods). An extended role is hence proposed for Soergel's tilting algorithm, away from its algebraic Lie theory underpinning, in determining the simple content of standard modules for these algebras. This role is explicitly verified in the blob algebra case. A tensor space representation of the blob algebra is constructed, as a candidate for a full tilting module (subsequently proven to be so in a paper by Martin and Ryom-Hansen), further evidencing the extended utility of Lie-theoretic methods. Possible generalisations of this representation to other elements of the sequence are discussed.

Central value of automorphic $L-$functions

Abstract: We prove a generalization to the totally real field case of the Waldspurger's formula relating the Fourier coefficient of a half integral weight form and the central value of the L-function of an integral weight form. Our proof is based on a new interpretation of Waldspurger's formula in terms of equality between global distributions. As applications we generalize the Kohnen-Zagier formula for holomorphic forms and prove the equivalence of the Ramanujan conjecture for half integral weight forms and a case of the Lindelof hypothesis for integral weight forms. We also study the Kohnen space in the adelic setting.

Classification of graded Hecke algebras for complex reflection groups

Abstract: The graded Hecke algebra for a flnite Weyl group is intimately related to the geome- try of the Springer correspondence. A construction of Drinfeld produces an analogue of a graded Hecke algebra for any flnite subgroup of GL(V ). This paper classifles all the algebras obtained by applying Drinfeld's construction to complex re∞ection groups. By giving explicit (though non- trivial) isomorphisms, we show that the graded Hecke algebras for flnite real re∞ection groups constructed by Lusztig are all isomorphic to algebras obtained by Drinfeld's construction. The classiflcation shows that there exist algebras obtained from Drinfeld's construction which are not graded Hecke algebras as deflned by Lusztig for real as well as complex re∞ection groups.

A Determinantal Formula for Supersymmetric Schur Polynomials

Abstract: We derive a new formula for the supersymmetric Schur polynomial sλ(x/y). The origin of this formula goes back to representation theory of the Lie superalgebra gl(m/n). In particular, we show how a character formula due to Kac and Wakimoto can be applied to covariant representations, leading to a new expression for sλ(x/y). This new expression gives rise to a determinantal formula for sλ(x/y). In particular, the denominator identity for gl(m/n) corresponds to a determinantal identity combining Cauchy's double alternant with Vandermonde's determinant. We provide a second and independent proof of the new determinantal formula by showing that it satisfies the four characteristic properties of supersymmetric Schur polynomials. A third and more direct proof ties up our formula with that of Sergeev-Pragacz.

Existence of lattices in kac–moody groups over finite fields

Abstract: Let be a Kac–Moody Lie algebra We give an interpretation of Tits' associated group functor using representation theory of and we construct a locally compact "Kac–Moody group" G over a finite field k Using (twin) BN-pairs (G,B,N) and (G,B-,N) for G we show that if k is "sufficiently large", then the subgroup B- is a non-uniform lattice in G We have also constructed an uncountably infinite family of both uniform and non-uniform lattices in rank 2 We conjecture that these form uncountably many distinct conjugacy classes in G The basic tool for the construction of non-uniform lattices in rank 2 is a spherical Tits system for G which we also construct