Matrix Representations of Groups
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Subjects include representations of arbitrary groups, representations of finite groups, multiplication of representations, and bounded representations and Weyl's theorem. All of the important elementary results are featured, a number of advanced topics are discussed, and several special representations are worked out in detail. 1968 edition.
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Matrix Representations of Groups - Morris Newman
Bibliographical Note
This Dover edition, first published in 2019, is an unabridged and newly reset republication of the work originally published in 1968 as Volume 60 in the Applied Mathematics Series issued by the National Bureau of Standards, Washington, D.C.
Library of Congress Cataloging-in-Publication Data
Names: Newman, Morris, 1924–2007, author.
Title: Matrix representations of groups / Morris Newman.
Description: Dover edition. | Mineola, New York : Dover Publications, Inc., 2019. | Originally published: Washington, D.C. : National Bureau of Standards, 1968 (Applied mathematics series ; volume 60). | Includes bibliographical references.
Identifiers: LCCN 2018059409 | ISBN 9780486832456 | ISBN 0486832457
Subjects: LCSH: Representations of groups. | Matrices.
Classification: LCC QA176 .N49 2019 | DDC 512/.22—dc23
LC record available at https://lccn.loc.gov/2018059409
Manufactured in the United States by LSC Communications
83245701 2019
www.doverpublications.com
Contents
Foreword
Introduction
I Representations of Arbitrary Groups
1 Reducibility
Examples
2 Schur’s Lemma
3 Representations
Examples
4 Character of a Representation
5 Theorems of Burnside, Frobenius, and Schur
6 An Application to Matrix Groups
7 Further Consequences
II Representations of Finite Groups
8 Theorems of Maschke and Schur
9 Characters of Finite Groups
10 A Divisibility Theorem
III Kinds of Representations
11 Classification of Representations
Examples
IV The Principal Results
12 Further Formulas for Characters
13 The Matrix Associated With a Representation
14 The Regular Representation
15 The Principal Formulas
16 Further Results
Examples
Examples
17 The Symmetric Group
Examples
V Some Theorems of Burnside
18 Conjugacy Classes
Examples and Applications
VI Multiplication of Representations
19 Kronecker Products
20 Powers of a Representation
Examples
21 Direct Products
Examples
VII Bounded Representations and Weyl’s Theorem
22 Hermitian and Bounded Representations
Appendix A. The Elements of the Theory of Algebraic Numbers
1 Fields. Polynomials Over Fields. Irreducibility
2 Algebraic Numbers Over k
3 Algebraic Extensions of k
4 Generators of Fields. Fundamental Systems. Subfields of k ( θ )
5 Algebraic Integers
6 Ideals
Appendix B. The Roots of Unity
References
Index
Foreword
The theory of group representations is of fundamental importance in such disciplines as particle physics and crystallography and has been a major force in group theory. Thus the celebrated theorem of J. Thompson and W. Feit, that every group of odd order is solvable, depends heavily for its proof on this theory; and the new classifications of particles are described by means of certain special continuous groups and their representations.
For some time there has been a need for a simple but complete exposition of this subject and the present volume should meet this need. It should be of value to the worker in the field who has occasion to use the subject or who must understand it. The many special representations worked out in detail should also prove quite useful.
A. V. Astin, Director
National Bureau of Standards
Introduction
In this volume the most important facts about group representations are developed, entirely along the original matrix-theoretic lines set down by Burnside, Frobenius, and Schur in their fundamental memoirs on this subject. In the writer’s opinion the approach through matrices is the one most easily grasped by the beginning student and is also the one which is most easily applied to other parts of mathematics and other disciplines. The more general approach through modules is not discussed.
Very little is presupposed about groups, but the reader should certainly be familiar with classical matrix theory: a good reference source containing all necessary material is the tract by MacDuffee [24].¹ Another is the volume by Marcus [25] in this Series. Any nonstandard results from group theory or matrix theory are proved in the text.
An appendix on the elements of the theory of algebraic numbers has been included, so that the volume is self-contained in this respect. This appendix (which parallels the introductory material in Hecke’s book [19] quite closely) serves by itself as a complete introduction to the classical theory of algebraic numbers, up to and including the unique factorization theorem for ideals. There is also an appendix on the roots of unity, containing an elementary proof of the irreducibility of the cyclotomic polynomial.
The entire development has arbitrarily been limited to the case when the ground field is the field of complex numbers (so that modular representations are not even discussed). The experienced reader will see that many of the proofs are valid for arbitrary fields, or can be modified slightly to be so. Furthermore many important topics, such as projective representations, are not mentioned. On the other hand all of the important elementary results have been included, a number of advanced topics are treated, and a considerable number of special representations have been worked out in detail.
The volume follows in spirit the lectures given by Schur and prepared by Stiefel at Zurich in 1936 [27]. Also the original papers of Burnside, Frobenius, and Schur were consulted frequently, and many of the discussions follow these papers quite closely.
Without doubt the most important modern book on this subject is the book by Curtis and Reiner [9], and this has been consulted as well. The ultimate selection of subjects was of course a matter of taste and reflects the writer’s personal likes and dislikes.
The writer’s principal objective was to make available the results and techniques of the subject of group representations to an audience with at most a standard mathematical background, and to indicate some interesting applications of this subject. The volume should be accessible to a serious reader with some knowledge of matrix theory, who is prepared to make an effort to understand it. The writer has lectured from this volume to mathematically unsophisticated audiences with good results.
The writer thanks R. C. Thompson for his critical reading of the manuscript, which disclosed a number of gaps and inaccuracies. He also thanks Doris Burrell for her painstaking efforts in preparing the typed manuscript.
¹Figures in brackets indicate the literature references at the end of this paper.
Chapter I
Representations of Arbitrary Groups
1 Reducibility
Let A = {A} be a set (finite or infinite) of n × n matrices over the complex field C. Then A is said to be reducible if fixed positive integers, p, q and a fixed nonsingular matrix S exist such that for each A ∈ A,
where A11 is p × p, A21 q × p, and A22 q × q. Otherwise A is said to be irreducible. If the form (1) can be achieved with A21 = 0 as well for all A ∈ A, then A is said to be fully reducible. Thus if T is any n × n nonsingular matrix, then T−1AT = {T−1AT} is reducible if and only if A is reducible, and fully reducible if and only if A is fully reducible.
Examples
(a) The set consisting of a single n × n matrix A alone, n > 1, is reducible. In fact there is a nonsingular matrix S such that S−1 AS is lower triangular.
(c) Let A be a set of n ×n column stochastic matrices (having all column sums 1). Then A is reducible. To see this, choose