CAMBRIDGE STUDIES IN ADVANCED MATHEMATICS 29 Editorial Board D.J.H. Garling, D. Gorenstein, T. torn Dieck, P. Walters Reflection groups and Coxeter groups Downloaded from https://www.cambridge.org/core. University of Sussex Library, on 10 Jul 2018 at 00:06:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9780511623646 Downloaded from https://www.cambridge.org/core. University of Sussex Library, on 10 Jul 2018 at 00:06:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9780511623646 In this graduate textbook Professor Humphreys presents a concrete and up-to- date introduction to the theory of Coxeter groups. He assumes that the reader has a good knowledge of algebra, but otherwise the book is self-contained making it suitable either for courses and seminars or for self-study. The first part is devoted to establishing concrete examples. Chapter 1 develops the most important facts about finite reflection groups and related geometry, leading to the presentation of such groups as Coxeter groups. In Chapter 2 these groups are classified by Coxeter graphs, and actual realizations are described. Chapter 3 discusses in detail the polynomial invariants of finite reflection groups. The first part ends with the construction in Chapter 4 of the affine Weyl groups, a class of Coxeter groups which plays a major role in Lie theory. The second part (which is logically independent of, but motivated by, the first) starts by developing from scratch the properties of Coxeter groups in general, including the Bruhat ordering. In Chapter 6, it is shown how earlier examples and others fit into the general classification of Coxeter graphs. Chapter 7 in- troduces the seminal work of Kazhdan and Lusztig on representations of Hecke algebras associated with Coxeter groups. Finally, Chapter 8 sketches a number of interesting complementary topics as well as connections with Lie theory. The book concludes with an extensive bibliography on Coxeter groups and their applications. Downloaded from https://www.cambridge.org/core. University of Sussex Library, on 10 Jul 2018 at 00:06:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9780511623646 Already published 1 W.M.L. Holcombe Algebraic automata theory 2 K. Petersen Ergodic theory 3 P.T. Johnstone Stone spaces 4 W.H. Schikhof Ultrametric calculus 5 J.-P. Kahane Some random series of functions, Snd edition 6 H. Cohn Introduction to the construction of class fields 7 J. Lambek & P.J. Scott Introduction to higher-order categorical logic 8 H. Matsumura Commutative ring theory 9 C.B. Thomas Characteristic classes and the cohomology of finite groups 10 M. Aschbacher Finite group theory 11 J.L. Alperin Local representation theory 12 P. Koosis The logarithmic integral I 13 A. Pietsch Eigenvalues and s-numbers 14 S.J. Patterson An introduction to the theory of the Riemann zeta-function 15 H.J. Baues Algebraic homotopy 16 V.S. Varadarajan Introduction to harmonic analysis on semisimple Lie groups 17 W. Dicks & M. Dunwoody Groups acting on graphs 18 L.J. Corwin & F.P. Greenleaf Representations of nilpotent Lie groups and their applications 19 R. Fritsch & R. Piccinini Cellular structures in topology 20 H Klingen Introductory lectures on Siegel modular forms 22 M.J. Collins Representations and characters of finite groups 24 H. Kunita Stochastic flows and stochastic differential equations 25 P. Wojtaszczyk Banach spaces for analysts 26 J.E. Gilbert & M.A.M. Murray Clifford algebras and Dirac operators in harmonic analysis 27 A. Frohlich & M.J. Taylor Algebraic number theory 28 K. Goebel & W.A. Kirk Topics in metric fixed point theory 29 J.F. Humphreys Reflection groups and Coxeter groups 30 D.J. Benson Representations and cohomology I 31 D.J. Benson Representations and cohomology II 32 C. AUday & V. Puppe Cohomological methods in transformation groups 33 C. Soule et al Lectures on Arakelov geometry 34 A. Ambrosetti & G. Prodi A primer of nonlinear analysis 35 J. Palis & F. Takens Hyperbolicity, stability and chaos at homoclinic bifurcations 37 Y. Meyer Wavelets and operators I 38 C. Weibel An introduction to homological algebra 39 W. Bruns & J. Herzog Cohen-Macaulay rings 40 V. Snaith Explicit Brauer induction 41 G. Laumon Cohomology of Drinfield modular varieties I 42 E.B. Davies Spectral theory and differential operators 43 J. Diestel, H. Jarchow & A. Tonge Absolutely summing operators 44 P. Mattila Geometry of sets and measures in Euclidean spaces 45 R. Pinsky Positive harmonic functions and diffusion 46 G. Tenenbaum Introduction to analytic and probabilistic number theory 47 C. Peskine An algebraic introduction to complex projective geometry I 50 I. Porteous Clifford algebras and the classical groups 51 M. Audin Spinning Tops 54 J. Le Potier Lectures on Vector bundles Downloaded from https://www.cambridge.org/core. University of Sussex Library, on 10 Jul 2018 at 00:06:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9780511623646 Reflection groups and Coxeter groups JAMES E. HUMPHREYS Professor of Mathematics University of Massachusetts, Amherst I CAMBRIDGE UNIVERSITY PRESS Downloaded from https://www.cambridge.org/core. University of Sussex Library, on 10 Jul 2018 at 00:06:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9780511623646 CAMBRIDGE university press Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi, Dubai, Tokyo, Mexico City Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521436137 © Cambridge University Press 1990 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 1990 First paperback edition (with corrections) 1992, 1994, 1997 A catalogue record for this publication is available from the British Library Library of Congress Cataloguing in Publication Data ISBN 978-0-521-37510-8 Hardback ISBN 978-0-521-43613-7 Paperback Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Information regarding prices, travel timetables, and other factual information given in this work is correct at the time of first printing but Cambridge University Press does not guarantee the accuracy of such information thereafter. Downloaded from https://www.cambridge.org/core. University of Sussex Library, on 10 Jul 2018 at 00:06:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9780511623646 Contents Contents vii Preface xi I Finite and afflne reflection groups 1 1 Finite reflection groups 3 1.1 Reflections 3 1.2 Roots 6 1.3 Positive and simple systems 7 1.4 Conjugacy of positive and simple systems 10 1.5 Generation by simple reflections 10 1.6 The length function 12 1.7 Deletion and Exchange Conditions 13 1.8 Simple transitivity and the longest element 15 1.9 Generators and relations 16 1.10 Parabolic subgroups 18 1.11 Poincare polynomials 20 1.12 Fundamental domains 21 1.13 The lattice of parabolic subgroups 24 1.14 Reflections inW 24 1.15 The Coxeter complex 25 1.16 An alternating sum formula 26 2 Classification of finite reflection groups 29 2.1 Isomorphisms 29 2.2 Irreducible components 30 2.3 Coxeter graphs and associated bilinear forms . 31 2.4 Some positive definite graphs 32 2.5 Some positive semidefinite graphs 33 2.6 Subgraphs 35 2.7 Classification of graphs of positive type 36 vu Downloaded from https://www.cambridge.org/core. Cambridge University Main, on 10 Jul 2018 at 00:05:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9780511623646 viii Contents 2.8 Crystallographic groups 38 2.9 Crystallographic root systems and Weyl groups 39 2.10 Construction of root systems 41 2.11 Computing the order of W 43 2.12 Exceptional Weyl groups 45 2.13 Groups of types H and H 46 3 4 3 Polynomial invariants of finite reflection groups 49 3.1 Polynomial invariants of a finite group 49 3.2 Finite generation 50 3.3 A divisibility criterion 52 3.4 The key lemma 52 3.5 Chevalley's Theorem 54 3.6 The module of covariants 56 3.7 Uniqueness of the degrees 58 3.8 Eigenvalues 60 3.9 Sum and product of the degrees 62 3.10 Jacobian criterion for algebraic independence 63 3.11 Groups with free rings of invariants 65 3.12 Examples 66 3.13 Factorization of the Jacobian 68 3.14 Induction and restriction of class functions 70 3.15 Factorization of the Poincare polynomial 71 3.16 Coxeter elements 74 3.17 Action on a plane 76 3.18 The Coxeter number 79 3.19 Eigenvalues of Coxeter elements 80 3.20 Exponents and degrees of Weyl groups 82 4 Affine reflection groups 87 4.1 Affine reflections 87 4.2 Affine Weyl groups 88 4.3 Alcoves 89 4.4 Counting hyperplanes 91 4.5 Simple transitivity 92 4.6 Exchange Condition 94 4.7 Coxeter graphs and extended Dynkin diagrams 95 4.8 Fundamental domain 96 4.9 A formula for the order of W 97 4.10 Groups generated by affine reflections 99 Downloaded from https://www.cambridge.org/core. Cambridge University Main, on 10 Jul 2018 at 00:05:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9780511623646 IX II General theory of Coxeter groups 103 5 Coxeter groups 105 5.1 Coxeter systems 105 5.2 Length function 107 5.3 Geometric representation of W 108 5.4 Positive and negative roots Ill 5.5 Parabolic subgroups 113 5.6 Geometric interpretation of the length function 114 5.7 Roots and reflections 116 5.8 Strong Exchange Condition 117 5.9 Bruhat ordering 118 5.10 Subexpressions 120 5.11 Intervals in the Bruhat ordering 121 5.12 Poincare series 122 5.13 Fundamental domain for W 124 6 Special cases 129 6.1 Irreducible Coxeter systems 129 6.2 More on the geometric representation 130 6.3 Radical of the bilinear form 131 6.4 Finite Coxeter groups 132 6.5 Affine Coxeter groups 133 6.6 Crystallographic Coxeter groups 135 6.7 Coxeter groups of rank 3 137 6.8 Hyperbolic Coxeter groups 138 6.9 List of hyperbolic Coxeter groups 141 7 Hecke algebras and Kazhdan-Lusztig polynomials 145 7.1 Generic algebras ,. 145 7.2 Commuting operators 147 7.3 Conclusion of the proof 149 7.4 Hecke algebras and inverses 150 7.5 Computing the /^-polynomials 152 7.6 Special case: finite Coxeter groups 154 7.7 An involution on H 155 7.8 Further properties of ^-polynomials 156 7.9 Kazhdan-Lusztig polynomials 157 7.10 Uniqueness 159 7.11 Existence 160 7.12 Examples 162 7.13 Inverse Kazhdan-Lusztig polynomials 164 7.14 Multiplication formulas 166 7.15 Cells and representations of Hecke algebras 167 Downloaded from https://www.cambridge.org/core. Cambridge University Main, on 10 Jul 2018 at 00:05:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9780511623646 x Contents 8 Complements 171 8.1 The Word Problem 171 8.2 Reflection subgroups 172 8.3 Involutions 173 8.4 Coxeter elements and their eigenvalues 174 8.5 Mobius function of the Bruhat ordering 175 8.6 Intervals and Bruhat graphs 176 8.7 Shellability 177 8.8 Automorphisms of the Bruhat ordering 178 8.9 Poincare series of affine Weyl groups 179 8.10 Representations of finite Coxeter groups 180 8.11 Schur multipliers 181 8.12 Coxeter groups and Lie theory 182 References 185 Index 203 Downloaded from https://www.cambridge.org/core. Cambridge University Main, on 10 Jul 2018 at 00:05:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9780511623646