Showing papers on "Equivariant map published in 2000"

Quiver varieties and finite dimensional representations of quantum affine algebras

Abstract: Introduction 145 1. Quantum affine algebra 150 2. Quiver variety 155 3. Stratification of M0 163 4. Fixed point subvariety 167 5. Hecke correspondence and induction of quiver varieties 169 6. Equivariant K-theory 174 7. Freeness 178 8. Convolution 185 9. A homomorphism Uq(Lg)→ KGw×C ∗ (Z(w))⊗Z[q,q−1] Q(q) 192 10. Relations (I) 194 11. Relations (II) 202 12. Integral structure 214 13. Standard modules 218 14. Simple modules 224 15. The Ue(g)-module structure 233 Added in proof 236 References 236

Methods in Equivariant Bifurcations and Dynamical Systems

Abstract: Symmetries in ODEs and PDEs equivariant bifurcations, a first look invariant manifolds and normal forms linear Lie group actions the equivariant structure of bifurcation equations reduction techniques for equivariant systems relative equilibria and relative periodic orbits bifurcations in equivariant systems heteroclinic cycles perturbation of equivariant systems.

Semisimple Frobenius structures at higher genus

Abstract: We describe genus g>1 potentials of semisimple Frobenius structures. Our formula can be considered as a definition in the axiomatic context of Frobenius manifolds. In Gromov-Witten theory, it becomes a conjecture expressing higher genus GW-invariants in terms of genus 0 GW-invariants of symplectic manifolds with generically semisimple quantum cup-product. The conjecture is supported by the corresponding theorem about equivariant GW-invariants of tori actions with isolated fixed points. The parallel theory of gravitational descendents is also presented.

Equivariant E-Theory for C*-Algebras

Abstract: Introduction Asymptotic morphisms The homotopy category of asymptotic morphisms Functors on the homotopy category Tensor products and descent $C^\ast$-algebra extensions $E$-theory Cohomological properties Proper algebras Stabilization Assembly The Green-Julg theorem Induction and compression A generalized Green-Julg theorem Application to the Baum-Connes conjecture Concluding remarks on assembly for proper algebras References.

The quantization conjecture revisited

Abstract: A strong version of the quantization conjecture of Guillemin and Sternberg is proved. For a reductive group action on a smooth, compact, polarized variety (X, L), the cohomologies of L over the GIT quotient X//G equal the invariant part of the cohomologies over X. This generalizes the theorem of [GS] on global sections, and strengthens its subsequent extensions ([JK], [li]) to RiemannRoch numbers. Remarkable by-products are the invariance of cohomology of vector bundles over X//G under a small change in the defining polarization or under shift desingularization, as well as a new proof of Boutot's theorem. Also studied are equivariant holomorphic forms and the equivariant Hodgeto-de Rham spectral sequences for X and its strata, whose collapse is shown. One application is a new proof of the Borel-Weil-Bott theorem of [Ti] for the moduli stack of C-bundles over a curve, and of analogous statements for the moduli stacks and spaces of bundles with parabolic structures. Collapse of the Hodge-to-de Rham sequences for these stacks is also shown.

Riemann-Roch for equivariant Chow groups

Abstract: The purpose of this paper is to prove an equivariant Riemann-Roch theorem for schemes or algebraic spaces with an action of a linear algebraic group $G$. For a $G$-space $X$, this theorem gives an isomorphism between a completion of the equivariant Grothendieck group and a completion of equivariant equivariant Chow groups. The key to proving this isomorphism is a geometric description of completions of the equivariant Grothendieck group. Besides Riemann-Roch, this result has some purely $K$-theoretic applications. In particular, we prove a conjecture of K\"ock (in the case of regular schemes) and extend to arbitrary characteristic a result of Segal on representation rings.

Computer algebra methods for equivariant dynamical systems

Abstract: Grobner bases.- Algorithms for the computation of invariants and equivariants.- Symmetric bifurcation theory.- 4. Orbit space reduction.

Perverse coherent sheaves (after Deligne)

Abstract: This note is mostly an exposition of an unpublished result of Deligne, which introduces an analogue of perverse $t$-structure on the derived category of coherent sheaves on a Noetherian scheme with a dualizing complex Construction extends to the category of coherent sheaves equivariant under an action of an algebraic group; though proof of the general statement in this case does not require new ideas, it provides examples (such as sheaves on the nilpotent cone of a semi-simple group equivariant under the adjoint action) where construction of coherent "intersection cohomology" sheaves works

Equivariant Self-Similar Wave Maps from Minkowski Spacetime into 3-Sphere

Abstract: We prove existence of a countable family of spherically symmetric self-similar wave maps from 3+1 Minkowski spacetime into the 3-sphere. These maps can be viewed as excitations of the ground state solution found previously by Shatah. The first excitation is particularly interesting in the context of the Cauchy problem since it plays the role of a critical solution sitting at the threshold for singularity formation. We analyze the linear stability of our wave maps and show that the number of unstable modes about a given map is equal to its nodal number. Finally, we formulate a condition under which these results can be generalized to higher dimensions.

Combinatorial Intersection Cohomology for Fans

Abstract: We continue the approach toward a purely combinatorial "virtual" intersection cohomology for possibly non-rational fans, based on our investigation of equivariant intersection cohomology for toric varieties (see math.AG/9904159). Fundamental objects of study are "minimal extension sheaves" on "fan spaces". These are flabby sheaves of graded modules over a sheaf of polynomial rings, satisfying three relatively simple axioms that characterize the properties of the equivariant intersection cohomology sheaf on a toric variety, endowed with the finite topology given by open invariant subsets. These sheaves are models for the "pure" objects of a "perverse category"; a "Decomposition Theorem" is shown to hold. -- Formalizing those fans that define "equivariantly formal" toric varieties (where equivariant and non-equivariant intersection cohomology determine each other by Kunneth type formulae), we study "quasi-convex" fans (including fans with convex or with "co-convex" support). For these, there is a meaningful "virtual intersection cohomology". We characterize quasi-convex fans by a topological condition on the support of their boundary fan and prove a generalization of Stanley's "Local-Global" formula realizing the intersection Poincare polynomial of a complete toric variety in terms of local data. Virtual intersection cohomology of quasi-convex fans is shown to satify Poincare duality. To describe the local data in terms of virtual intersection cohomology of lower-dimensional complete polytopal fans, one needs a "Hard Lefschetz" type theorem. It requires a vanishing condition that is known to hold for rational cones, but yet remains to be proven in the general case.

Equivariant Kasparov theory and generalized homomorphisms

Abstract: Let G be a locally compact group. We describe elements of KK^G (A,B) by equivariant homomorphisms, following Cuntz's treatment in the non-equivariant case. This yields another proof for the universal property of KK^G: It is the universal split exact stable homotopy functor. To describe a Kasparov triple (E, phi, F) by an equivariant homomorphism, we have to arrange for the Fredholm operator F to be equivariant. This can be done if A is of the form K(L^2G) otimes A' and more generally if the group action on A is proper in the sense of Rieffel and Exel.

Covering dynamical systems: twofold covers.

Abstract: We study the relation between a dynamical system, which is unchanged ~equivariant! under a discrete symmetry group G and another locally identical dynamical system with no residual symmetry. We also study the converse mapping: lifting a dynamical system without symmetry to a multiple cover, which is equivariant under G. This is done in R 3 for the two element rotation and inversion groups. Comparisons are done for the equations of motion, the strange attractors that they generate, and the branched manifolds that classify these strange attractors. A dynamical system can have many inequivalent multiple covers, all equivariant under the same symmetry group G. These are distinguished by the value of a certain topological index. Many examples are presented. A new global bifurcation, the ‘‘peeling bifurcation,’’ is described.

Dispersion and collapse of wave maps

Abstract: We study numerically the Cauchy problem for equivariant wave maps from 3 + 1 Minkowski spacetime into the 3-sphere. On the basis of numerical evidence combined with stability analysis of self-similar solutions we formulate two conjectures. The first conjecture states that singularities which are produced in the evolution of sufficiently large initial data are approached in a universal manner given by the profile of a stable self-similar solution. The second conjecture states that the codimension-one stable manifold of a self-similar solution with exactly one instability determines the threshold of singularity formation for a large class of initial data. Our results can be considered as a toy-model for some aspects of the critical behaviour in the formation of black holes.

Equivariant Kasparov theory and generalized homomorphisms

Abstract: Let G be a locally compact group. We describe elements of KK^G (A,B) by equivariant homomorphisms, following Cuntz's treatment in the non-equivariant case. This yields another proof for the universal property of KK^G: It is the universal split exact stable homotopy functor. To describe a Kasparov triple (E, phi, F) by an equivariant homomorphism, we have to arrange for the Fredholm operator F to be equivariant. This can be done if A is of the form K(L^2G) otimes A' and more generally if the group action on A is proper in the sense of Rieffel and Exel.

Surfaces convexes fuchsiennes dans les espaces lorentziens à courbure constante

Abstract: We show existence and uniqueness of the equivariant isometric immersions of Riemannian surfaces into Lorentz space-forms under conditions implying convexity, when we impose that the associated representations leave a point invariant.

Intersection cohomology on nonrational polytopes

Abstract: Viewing a fan as a partially ordered set (of cones) we consider a category of sheaves on the fan which corresponds to a category of equivariant sheaves on the corresponding toric variety if the fan is rational. In this category we define an object which corresponds to the equivariant intersection cohomology complex. Our first main result is the ``elementary'' decomposition theorem for the direct image under subdivision of fans We also develop the Borel-Moore- Verdier duality in the derived category of sheaves on the fan.

Deconstructing Monopoles and Instantons

Abstract: We give a unifying description of the Dirac monopole on the 2-sphere S2, of a graded monopole on a (2, 2)-supersphere S2, 2 and of the BPST instanton on the 4-sphere S4, by constructing a suitable global projector p via equivariant maps. This projector determines the projective modules of finite type of sections of the corresponding vector bundle. The canonical connection ∇ = p ◦ d is used to compute the topological charge which is found to be equal to -1 for the three cases. The transposed projector q = pt gives the value +1 for the charges; this showing that transposition of projectors, although an isomorphism in K-theory, is not the identity map. We also study the invariance under the action of suitable Lie groups.

Chern characters for proper equivariant homology theories and applications to K- and L-theory

Abstract: We construct for an equivariant homology theory for proper equivariant CW -complexes an equivariant Chern character under certain conditions. This applies for instance to the sources of the assembly maps in the Farrell-Jones Conjecture with respect to the family F of nite subgroups and in the Baum-Connes Conjecture. Thus we get an explicit calculation of Q Z Kn(RG) and Q Z Ln(RG) for a commutative ring R with Q R and of Q Z K top n (C r (G; F )) for F = R; C in terms of group homology, provided the Farrell-Jones Conjecture with respect to F resp. the Baum-Connes Conjecture is true.

Equivariant Cohomology and Localization of Path Integrals

Abstract: Equivariant Cohomology and the Localization Principle.- Finite-Dimensional Localization Theory for Dynamical Systems.- Quantum Localization Theory for Phase Space Path Integrals.- Equivariant Localization on Simply Connected Phase Spaces: Applications to Quantum Mechanics, Group Theory and Spin Systems.- Equivariant Localization on Multiply Connected Phase Spaces: Applications to Homology and Modular Representations.- Beyond the Semi-Classical Approximation.- Equivariant Localization in Cohomological Field Theory.- Appendix A: BRST Quantization.- Appendix B: Other Models of Equivariant Cohomology.

Submersions and equivariant Quillen metrics

Abstract: Dans cet article, on calcule le comportement de la metrique de Quillen equivariante par snbmersions. On etend ainsi une formule

On family rigidity theorems, I

Abstract: Note that Ind(P ) is a virtual G-representation. Let chg(Ind(P )) with g ∈G be the equivariant Chern character of Ind(P ) evaluated at g. In this paper, we first prove a family fixed-point formula that expresses chg(Ind(P )) in terms of the geometric data on the fixed points X of the fiber of π . Then by applying this formula, we generalize the Witten rigidity theorems and several vanishing theorems proved in [Liu3] for elliptic genera to the family case. Let G = S1. A family elliptic operator P is called rigid on the equivariant Chern character level with respect to this S1-action, if chg(Ind(P )) ∈H ∗(B) is independent of g ∈ S1. When the base B is a point, we recover the classical rigidity and vanishing theorems. When B is a manifold, we get many nontrivial higher-order rigidity and vanishing theorems by taking the coefficients of certain expansion of chg . For the history of the Witten rigidity theorems, we refer the reader to [T], [BT], [K], [L2], [H], [Liu1], and [Liu4]. The family vanishing theorems that generalize those vanishing theorems in [Liu3], which in turn give us many higher-order vanishing theorems in the family case. In a forthcoming paper, we extend our results to general loop group representations and prove much more general family vanishing theorems that generalize the results in [Liu3]. We believe there should be some applications of our results to topology and geometry, which we hope to report on a later occasion. This paper is organized as follows. In Section 1, we prove the equivariant family index theorem. In Section 2, we prove the family rigidity theorem. In the last part of Section 2, motivated by the family rigidity theorem, we state a conjecture. In Section 3, we generalize the family rigidity theorem to the nonzero anomaly case. As corollaries, we derive several vanishing theorems.

Auslander-Buchweitz Approximations of Equivariant Modules

Abstract: This book, first published in 2000, focuses on homological aspects of equivariant modules. It presents a homological approximation theory in the category of equivariant modules, unifying the Cohen-Macaulay approximations in commutative ring theory and Ringel's theory of delta-good approximations for quasi-hereditary algebras and reductive groups. The book provides a detailed introduction to homological algebra, commutative ring theory and homological theory of comodules of co-algebras over an arbitrary base. It aims to overcome the difficulty of generalising known homological results in representation theory. This book will be of interest to researchers and graduate students in algebra, specialising in commutative ring theory and representation theory.

Geometric Methods in Degree Theory for Equivariant Maps

Abstract: Fundamental domains and extension of equivariant maps.- Degree theory for equivariant maps of finite-dimensional manifolds: Topological actions.- Degree theory for equivariant maps of finite-dimensional manifolds: Smooth actions.- A winding number of equivariant vector fields in infinite dimensional banach spaces.- Some applications.

Projectively Equivariant Quantization Map

Abstract: Let M be a manifold endowed with a symmetric affine connection Γ. The aim of this Letter is to describe a quantization map between the space of second-order polynomials on the cotangent bundle T* M and the space of second-order linear differential operators, both viewed as modules over the group of diffeomorphisms and the Lie algebra of vector fields on M. This map is an isomorphism, for almost all values of certain constants, and it depends only on the projective class of the affine connection Γ.

Gerbes on complex reductive Lie groups

Abstract: We construct a gerbe over a complex reductive Lie group G attached to an invariant bilinear form on a maximal diagonalizable subalgebra which is Weyl group invariant and satisfies a parity condition. By restriction to a maximal compact subgroup K, one then gets a gerbe over K. For a simply-connected group, the parity condition is the same used by Pressley and Segal; in general, it was introduced by Deligne and the author. The gerbe is defined by geometric methods, using the so-called Grothendieck manifold. It is equivariant under the conjugation action of G; its restriction to a semisimple orbit is not always trivial. The paper starts with a discussion of gerbe data (in the sense of Chatterjee and Hitchin) and of gerbes as geometric objects (sheaves of groupoids); the relation between the two approaches is presented. There is an Appendix on equivariant gerbes, discussed from both points of view.