438
pages
English
Documents
Le téléchargement nécessite un accès à la bibliothèque YouScribe Tout savoir sur nos offres
438
pages
English
Ebook
Le téléchargement nécessite un accès à la bibliothèque YouScribe Tout savoir sur nos offres
Abstract Algebra
Theory and Applications
Thomas W. Judson
Stephen F. Austin State University
August 10, 2011ii
Copyright 1997 by Thomas W. Judson.
Permission is granted to copy, distribute and/or modify this document under
the terms of the GNU Free Documentation License, Version 1.2 or any later
version published by the Free Software Foundation; with no Invariant Sections,
no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is
included in the appendix entitled \GNU Free Documentation License".
A current version can always be found via abstract.pugetsound.edu.Preface
This text is intended for a one- or two-semester undergraduate course in
abstract algebra. Traditionally, these courses have covered the theoretical
aspects of groups, rings, and elds. However, with the development of
computing in the last several decades, applications that involve abstract
algebra and discrete mathematics have become increasingly important, and
many science, engineering, and computer science students are now electing
to minor in mathematics. Though theory still occupies a central role in the
subject of abstract algebra and no student should go through such a course
without a good notion of what a proof is, the importance of applications
such as coding theory and cryptography has grown signi cantly.
Until recently most abstract algebra texts included few if any applications.
However, one of the major problems in teaching an abstract algebra course
is that for many students it is their rst encounter with an environment that
requires them to do rigorous proofs. Such students often nd it hard to see
the use of learning to prove theorems and propositions; applied examples
help the instructor provide motivation.
This text contains more material than can possibly be covered in a single
semester. Certainly there is adequate material for a two-semester course, and
perhaps more; however, for a one-semester course it would be quite easy to
omit selected chapters and still have a useful text. The order of presentation
of topics is standard: groups, then rings, and nally elds. Emphasis can be
placed either on theory or on applications. A typical one-semester course
might cover groups and rings while briey touching on eld theory, using
Chapters 1 through 6, 9, 10, 11, 13 (the rst part), 16, 17, 18 (the rst
part), 20, and 21. Parts of these chapters could be deleted and applications
substituted according to the interests of the students and the instructor. A
two-semester course emphasizing theory might cover Chapters 1 through 6,
9, 10, 11, 13 through 18, 20, 21, 22 (the rst part), and 23. On the other
iiiiv PREFACE
hand, if applications are to be emphasized, the course might cover Chapters
1 through 14, and 16 through 22. In an applied course, some of the more
theoretical results could be assumed or omitted. A chapter dependency chart
appears below. (A broken line indicates a partial dependency.)
Chapters 1{6
Chapter 8 Chapter 9 Chapter 7
Chapter 10
Chapter 11
Chapter 13 Chapter 16 Chapter 12 Chapter 14
Chapter 17 Chapter 15
Chapter 18 Chapter 20 Chapter 19
Chapter 21
Chapter 22
Chapter 23
Though there are no speci c prerequisites for a course in abstract algebra,
students who have had other higher-level courses in mathematics will generally
be more prepared than those who have not, because they will possess a bit
more mathematical sophistication. Occasionally, we shall assume some basic
linear algebra; that is, we shall take for granted an elementary knowledge
of matrices and determinants. This should present no great problem, since
most students taking a course in abstract algebra have been introduced to
matrices and determinants elsewhere in their career, if they have not already
taken a sophomore- or junior-level course in linear algebra.PREFACE v
Exercise sections are the heart of any mathematics text. An exercise set
appears at the end of each chapter. The nature of the exercises ranges over
several categories; computational, conceptual, and theoretical problems are
included. A section presenting hints and solutions to many of the exercises
appears at the end of the text. Often in the a proof is only sketched,
and it is up to the student to provide the details. The exercises range in
di culty from very easy to very challenging. Many of the more substantial
problems require careful thought, so the student should not be discouraged
if the solution is not forthcoming after a few minutes of work.
There are additional exercises or computer projects at the ends of many
of the chapters. The computer projects usually require a knowledge of
programming. All of these exercises and projects are more substantial in
nature and allow the exploration of new results and theory.
Sage (sagemath.org) is a free, open source, software system for ad-
vanced mathematics, which is ideal for assisting with a study of abstract
algebra. Comprehensive discussion about Sage, and a selection of relevant
exercises, are provided in an electronic format that may be used with the
Sage Notebook in a web browser, either on your own computer, or at a public
server such as sagenb.org. Look for this supplement at the book’s website:
abstract.pugetsound.edu. In printed versions of the book, we include a
brief description of Sage’s capabilities at the end of each chapter, right after
the references.
Acknowledgements
I would like to acknowledge the following reviewers for their helpful comments
and suggestions.
David Anderson, University of Tennessee, Knoxville
Robert Beezer, University of Puget Sound
Myron Hood, California Polytechnic State University
Herbert Kasube, Bradley University
John Kurtzke, University of Portland
Inessa Levi, University of Louisville
Geo rey Mason, University of California, Santa Cruzvi PREFACE
Bruce Mericle, Mankato State University
Kimmo Rosenthal, Union College
Mark Teply, University of Wisconsin
I would also like to thank Steve Quigley, Marnie Pommett, Cathie Gri n,
Kelle Karshick, and the rest of the sta at PWS for their guidance throughout
this project. It has been a pleasure to work with them.
Thomas W. JudsonContents
Preface iii
1 Preliminaries 1
1.1 A Short Note on Proofs . . . . . . . . . . . . . . . . . . . . . 1
1.2 Sets and Equivalence Relations . . . . . . . . . . . . . . . . . 4
2 The Integers 23
2.1 Mathematical Induction . . . . . . . . . . . . . . . . . . . . . 23
2.2 The Division Algorithm . . . . . . . . . . . . . . . . . . . . . 27
3 Groups 37
3.1 Integer Equivalence Classes and Symmetries . . . . . . . . . . 37
3.2 De nitions and Examples . . . . . . . . . . . . . . . . . . . . 42
3.3 Subgroups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4 Cyclic Groups 59
4.1 Cyclic Subgroups . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.2 Multiplicative Group of Complex Numbers . . . . . . . . . . 63
4.3 The Method of Repeated Squares . . . . . . . . . . . . . . . . 68
5 Permutation Groups 76
5.1 De nitions and Notation . . . . . . . . . . . . . . . . . . . . . 77
5.2 Dihedral Groups . . . . . . . . . . . . . . . . . . . . . . . . . 85
6 Cosets and Lagrange’s Theorem 94
6.1 Cosets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
6.2 Lagrange’s Theorem . . . . . . . . . . . . . . . . . . . . . . . 97
6.3 Fermat’s and Euler’s Theorems . . . . . . . . . . . . . . . . . 99
viiviii CONTENTS
7 Introduction to Cryptography 102
7.1 Private Keyy . . . . . . . . . . . . . . . . . . . . 103
7.2 Public Keyy . . . . . . . . . . . . . . . . . . . . 106
8 Algebraic Coding Theory 113
8.1 Error-Detecting and Correcting Codes . . . . . . . . . . . . . 113
8.2 Linear Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
8.3 Parity-Check and Generator Matrices . . . . . . . . . . . . . 126
8.4 E cient Decoding . . . . . . . . . . . . . . . . . . . . . . . . 133
9 Isomorphisms 142
9.1 De nition and Examples . . . . . . . . . . . . . . . . . . . . . 142
9.2 Direct Products . . . . . . . . . . . . . . . . . . . . . . . . . . 147
10 Normal Subgroups and Factor Groups 156
10.1 Factor Groups and Normal Subgroups . . . . . . . . . . . . . 156
10.2 The Simplicity of the Alternating Group . . . . . . . . . . . . 159
11 Homomorphisms 166
11.1 Group Homomorphisms . . . . . . . . . . . . . . . . . . . . . 166
11.2 The Isomorphism Theorems . . . . . . . . . . . . . . . . . . . 169
12 Matrix Groups and Symmetry 176
12.1 Matrix Groups . . . . . . . . . . . . . . . . . . . . . . . . . . 176
12.2 Symmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
13 The Structure of Groups 197
13.1 Finite Abelian . . . . . . . . . . . . . . . . . . . . . . 197
13.2 Solvable Groups . . . . . . . . . . . . . . . . . . . . . . . . . 202
14 Group Actions 210
14.1 Groups Acting on Sets . . . . . . . . . . . . . . . . . . . . . . 210
14.2 The Class Equation . . . . . . . . . . . . . . . . . . . . . . . 214
14.3 Burnside’s Counting Theorem . . . . . . . . . . . . . . . . . . 216
15 The Sylow Theorems 228
15.1 The Sylow . . . . . . . . . . . . . . . . . . . . . . . 228
15.2 Examples and Applications . . . . . . . . . . . . . . . . . . . 232CONTENTS ix
16 Rings 240
16.1 Rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . .