Table Of ContentSets and groups
Sets and groups
A first course in algebra
J. A. Green
Professor of Mathematics
University of Warwick
Routledge & Kegan Paul
London and New York
First published in 1965
Second, revised edition 1988 by
Routledge & Kegan Paul Ltd
11 New Fetter Lane, London EC4P 4EE
Published in the USA by
Routledge & Kegan Paul Inc.
in association with Methuen Inc.
29 West 35th Street, New York, NY 10001
Set in 1O/12pt Times
by Cotswold Typesetting Ltd, Gloucester
and printed in Great Britain
by Richard Clay Ltd,
Bungay, Suffolk
© J. A. Green 1965, 1988
Softcover reprint of the hardcover 1st edition 1988
No part of this book may be reproduced in
any form without permission from the publisher
except for the quotation of brief passages
in criticism
Library of Congress Cataloging in Publication Data
Green, J. A. (James Alexander)
Sets and groups.
Includes index.
1. Groups, Theory of 2. Set theory.
I. Title.
QA171.G68 1987 512 86-33887
British Library CIP Data also available
ISBN-13: 978-94-011-6097-1 e-ISBN-13: 978-94-011-6095-7
DOl: 10.1007/978-94-011-6095-7
Contents
Preface to first edition IX
Preface to revised edition Xl
Sets 1
1.1 Sets 1
1.2 Subsets 3
1.3 Intersection 5
1.4 Union 6
1.5 The algebra of sets 7
1.6 Difference and complement 8
1.7 Pairs. Product of sets 10
1.8 Sets of sets 11
Exercises 14
2 Equivalence relations 16
2.1 Relations on a set 16
2.2 Equivalence relations 17
2.3 Partitions 18
2.4 Equivalence classes 19
2.5 Congruence of integers 21
2.6 Algebra of congruences 23
Exercises 25
3 Maps 26
3.1 Maps 26
3.2 Equality of maps 27
3.3 Injective, surjective, bijective maps.
Inverse maps 28
3.4 Product of maps 30
vi CONTENTS
3.5 Identity maps 32
3.6 Products of bijective maps 33
3.7 Permutations 34
3.8 Similar sets 37
Exercises 39
4 Groups 42
4.1 Binary operations on a set 42
4.2 Commutative and associative operations 44
4.3 Units and zeros 47
4.4 Gruppoids, semigroups and groups 49
4.5 Examples of groups 51
4.6 Elementary theorems on groups 53
Exercises 55
5 Subgroups 57
5.1 Subsets closed to an operation 57
5.2 Subgroups 58
5.3 Subgroup generated by a subset 59
5.4 Cyclic groups 61
5.5 Groups acting on sets 62
5.6 Stabilizers 64
Exercises 66
6 Co sets 68
6.1 The quotient sets of a subgroup 68
6.2 Maps of quotient sets 70
6.3 Index. Transversals 71
6.4 Lagrange's theorem 72
6.5 Orbits and stabilizers 73
6.6 Conjugacy classes. Centre of a group 77
6.7 Normal subgroups 81
6.8 Quotient groups 82
Exercises 84
7 Homomorphisms 87
7.1 Homomorphisms 87
7.2 Some lemmas on homomorphisms 88
CONTENTS vii
7.3 Isomorphism 90
7.4 Kernel and image 91
7.5 Lattice diagrams 94
7.6 Homomorphisms and subgroups 96
7.7 The second isomorphism theorem 100
7.8 Direct products and direct sums of groups 102
Exercises 108
8 Rings and fields 110
8.1 Definition of a ring. Examples 110
8.2 Elementary theorems of rings. Subrings 114
8.3 Integral domains 118
8.4 Fields. Division rings 119
8.5 Polynomials 124
8.6 Homomorphisms. Isomorphism of rings 129
8.7 Ideals 132
8.8 Quotient rings 134
8.9 The Homomorphism Theorem for rings 138
8.10 Principal ideals in a commutative ring 139
8.11 The Division Theorem for polynomials 141
8.12 Polynomials over a field 145
8.13 Divisibility in Z and in F[X] 148
8.14 Euclid's algorithm 155
Exercises 161
9 Vector spaces and matrices 165
9.1 Vector spaces over a field 165
9.2 Examples of vector spaces 168
9.3 Two geometric interpretations of vectors 169
9.4 Subs paces 174
9.5 Linear combinations. Spanning sets 176
9.6 Linear dependence. Basis of a vector space 180
9.7 The Basis Theorem. Dimension 184
9.8 Linear maps. Isomorphism of vector spaces 188
9.9 Matrices 192
9.10 Laws of matrix algebra. The ring Mn{F) 197
9.11 Row space of a matrix. Echelon matrices 204
9.12 Systems of linear equations 209
Vlll CONTENTS
9.13 Matrices and linear maps 216
9.14 Invertible matrices. The group GLn(F) 226
Exercises 237
Tables 241
List of notations 247
Answers to exercises 249
Index 255
Preface to first edition
Mathematics is attractive and useful and limited because it
attempts a precise expression of its ideas; people learn it in the hope
of benefiting both from the ideas themselves and from the way they
are expressed. But mathematics is always escaping from its own
limits, and it produces new ideas and new language which
represent a continual challenge to mathematical teaching.
The first three chapters of this book describe sets, relations and
mappings; these are the basic ideas in terms of which most modern
mathematics can be expressed. They are certainly 'abstract' ideas,
but they are not difficult to grasp, because they come from notions
which most people are used to from everyday experience. What is
characteristic of modern mathematics is that it uses these few ideas
over and over again, to build up theories which cover a great
variety of situations. The rest of the book is an introduction to one
such theory, namely the theory of groups.
The book is set out so that each new idea is described in its
general form, and then illustrated by examples. These examples
form a large part of the exposition, and require only a small
amount of traditional mathematics to understand. There are also
numerous exercises for the reader, at the end of each chapter.
It is a pleasure to thank my friends Drs W. Ledermann, L. W.
Morland and S. Swierczkowski, who each read and commented
valuably on the manuscript.
1. A. GREEN
Preface to revised edition
In preparing this revised edition of Sets and Groups I have made
some changes to the original text, and have added a substantial
amount of new material. The main changes to the original text are
that 'mappings' are now called 'maps', and that they are written 'on
the left' instead of 'on the right'. It is hoped in this way to keep up
with current mathematical fashion on these important matters.
The introduction of new material-particularly the two new,
long, chapters 8 and 9 - calls for some explanation. The original
Sets and Groups, like the other books in the 'Library of
Mathematics' series, was short and inexpensive; it was envisaged
as a preparatory text for further study at first-year university or
college level, particularly in algebra. Unfortunately book produc
tion costs have risen so fast in recent years that it is no longer
possible to offer Sets and Groups, in its original form, at a price
attractive to students. It seemed to me that the best solution was to
enlarge the book so that it would cover most ofthe algebra likely to
appear in first-year courses in British universities; to make it in fact
a first course in modern algebra, suitable for specialist as well as
non-specialist mathematics students.
To achieve this, I have added material on groups to chapter 7,
and I have written new chapters on rings and fields (chapter 8) and
on vector spaces and matrices (chapter 9). I have retained, without
essential change, the early chapters of Sets and Groups, which
introduce the reader to modern mathematical language, and to
algebraic structures (sets with binary operations).
This book is written for all who are beginning to study algebra at
university or college level, and it is expected that the reader will
select the parts which he or she needs. To facilitate this, the last two
chapters are largely independent of each other, and of the latter
half (sections 7.5-7.8) of chapter 7.
January 1986 J. A. Green
