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Galois Theory, 2nd Edition




Galois Theory, 2nd Edition

David A. Cox

ISBN: 978-1-118-21842-6 March 2012 602 Pages

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Praise for the First Edition

". . .will certainly fascinate anyone interested in abstract algebra: a remarkable book!"
—Monatshefte fur Mathematik

Galois theory is one of the most established topics in mathematics, with historical roots that led to the development of many central concepts in modern algebra, including groups and fields. Covering classic applications of the theory, such as solvability by radicals, geometric constructions, and finite fields, Galois Theory, Second Edition delves into novel topics like Abel’s theory of Abelian equations, casus irreducibili, and the Galois theory of origami.

In addition, this book features detailed treatments of several topics not covered in standard texts on Galois theory, including:

  • The contributions of Lagrange, Galois, and Kronecker
  • How to compute Galois groups
  • Galois's results about irreducible polynomials of prime or prime-squared degree
  • Abel's theorem about geometric constructions on the lemniscates
  • Galois groups of quartic polynomials in all characteristics

Throughout the book, intriguing Mathematical Notes and Historical Notes sections clarify the discussed ideas and the historical context; numerous exercises and examples use Maple and Mathematica to showcase the computations related to Galois theory; and extensive references have been added to provide readers with additional resources for further study.

Galois Theory, Second Edition is an excellent book for courses on abstract algebra at the upper-undergraduate and graduate levels. The book also serves as an interesting reference for anyone with a general interest in Galois theory and its contributions to the field of mathematics.

Preface to the First Edition xvii

Preface to the Second Edition xxi

Notation xxiii

1 Basic Notation xxiii

2 Chapter-by-Chapter Notation xxv


1 Cubic Equations 3

1.1 Cardan's Formulas 4

1.2 Permutations of the Roots 10

1.3 Cubic Equations over the Real Numbers 15

2 Symmetric Polynomials 25

2.1 Polynomials of Several Variables 25

2.2 Symmetric Polynomials 30

2.3 Computing with Symmetric Polynomials (Optional) 42

2.4 The Discriminant 46

3 Roots of Polynomials 55

3.1 The Existence of Roots 55

3.2 The Fundamental Theorem of Algebra 62


4 Extension Fields 73

4.1 Elements of Extension Fields 73

4.2 Irreducible Polynomials 81

4.3 The Degree of an Extension 89

4.4 Algebraic Extensions 95

5 Normal and Separable Extensions 101

5.1 Splitting Fields 101

5.2 Normal Extensions 107

5.3 Separable Extensions 109

5.4 Theorem of the Primitive Element 119

6 The Galois Group 125

6.1 Definition of the Galois Group 125

6.2 Galois Groups of Splitting Fields 130

6.3 Permutations of the Roots 132

6.4 Examples of Galois Groups 136

6.5 Abelian Equations (Optional) 143

7 The Galois Correspondence 147

7.1 Galois Extensions 147

7.2 Normal Subgroups and Normal Extensions 154

7.3 The Fundamental Theorem of Galois Theory 161

7.4 First Applications 167

7.5 Automorphisms and Geometry (Optional) 173


8 Solvability by Radicals 191

8.1 Solvable Groups 191

8.2 Radical and Solvable Extensions 196

8.3 Solvable Extensions and Solvable Groups 201

8.4 Simple Groups 210

8.5 Solving Polynomials by Radicals 215

8.6 The Casus Irreducbilis (Optional) 220

9 Cyclotomic Extensions 229

9.1 Cyclotomic Polynomials 229

9.2 Gauss and Roots of Unity (Optional) 238

10 Geometric Constructions 255

10.1 Constructible Numbers 255

10.2 Regular Polygons and Roots of Unity 270

10.3 Origami (Optional) 274

11 Finite Fields 291

11.1 The Structure of Finite Fields 291

11.2 Irreducible Polynomials over Finite Fields (Optional) 301


12 Lagrange, Galois, and Kronecker 315

12.1 Lagrange 315

12.2 Galois 334

12.3 Kronecker 347

13 Computing Galois Groups 357

13.1 Quartic Polynomials 357

13.2 Quintic Polynomials 368

13.3 Resolvents 386

13.4 Other Methods 400

14 Solvable Permutation Groups 413

14.1 Polynomials of Prime Degree 413

14.2 Imprimitive Polynomials of Prime-Squared Degree 419

14.3 Primitive Permutation Groups 429

14.4 Primitive Polynomials of Prime-Squared Degree 444

15 The Lemniscate 463

15.1 Division Points and Arc Length 464

15.2 The Lemniscatic Function 470

15.3 The Complex Lemniscatic Function 482

15.4 Complex Multiplication 489

15.5 Abel's Theorem 504

A Abstract Algebra 515

A.1 Basic Algebra 515

A.2 Complex Numbers 524

A.3 Polynomials with Rational Coefficients 528

A.4 Group Actions 530

A.5 More Algebra 532

Index 557

“There is barely a better introduction to the subject, in all its theoretical and practical aspects, than the book under review.”  (Zentralblatt MATH, 1 December 2012)

"the great merit of this book (one of many expositions of the subject) is that everything is taken at a slow pace, with many examples to illustrate every idea. You get the (probably true) impression that the author loves this material, has taught it to undergraduates at Amherst College many times, has learned by experience the ideas which students find difficult, and has then taken great trouble to dissect these ideas and to pick out exactly the right examples and exercises to make them part of the reader’s mental equipment." (The Mathematical Gazette 2016)