Linear Algebra As an Introduction to Abstract Mathematics - UC Davis
Linear Algebra
As an Introduction to Abstract Mathematics
Lecture Notes for MAT67 University of California, Davis written Fall 2007, last updated November 15, 2016
Isaiah Lankham Bruno Nachtergaele
Anne Schilling
Copyright c 2007 by the authors. These lecture notes may be reproduced in their entirety for non-commercial purposes.
Contents
1 What is Linear Algebra?
1
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 What is Linear Algebra? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Systems of linear equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.1 Linear equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.2 Non-linear equations . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3.3 Linear transformations . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3.4 Applications of linear equations . . . . . . . . . . . . . . . . . . . . . 7
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 Introduction to Complex Numbers
11
2.1 Definition of complex numbers . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Operations on complex numbers . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1 Addition and subtraction of complex numbers . . . . . . . . . . . . . 12
2.2.2 Multiplication and division of complex numbers . . . . . . . . . . . . 13
2.2.3 Complex conjugation . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2.4 The modulus (a.k.a. norm, length, or magnitude) . . . . . . . . . . . 16
2.2.5 Complex numbers as vectors in R2 . . . . . . . . . . . . . . . . . . . 18
2.3 Polar form and geometric interpretation for C . . . . . . . . . . . . . . . . . 19
2.3.1 Polar form for complex numbers . . . . . . . . . . . . . . . . . . . . . 19
2.3.2 Geometric multiplication for complex numbers . . . . . . . . . . . . . 20
2.3.3 Exponentiation and root extraction . . . . . . . . . . . . . . . . . . . 21
2.3.4 Some complex elementary functions . . . . . . . . . . . . . . . . . . . 22
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
ii
3 The Fundamental Theorem of Algebra and Factoring Polynomials
26
3.1 The Fundamental Theorem of Algebra . . . . . . . . . . . . . . . . . . . . . 26
3.2 Factoring polynomials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4 Vector Spaces
36
4.1 Definition of vector spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.2 Elementary properties of vector spaces . . . . . . . . . . . . . . . . . . . . . 39
4.3 Subspaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.4 Sums and direct sums . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
5 Span and Bases
48
5.1 Linear span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.2 Linear independence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.3 Bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.4 Dimension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
6 Linear Maps
64
6.1 Definition and elementary properties . . . . . . . . . . . . . . . . . . . . . . 64
6.2 Null spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
6.3 Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
6.4 Homomorphisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.5 The dimension formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.6 The matrix of a linear map . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.7 Invertibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
7 Eigenvalues and Eigenvectors
85
7.1 Invariant subspaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
7.2 Eigenvalues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
7.3 Diagonal matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
7.4 Existence of eigenvalues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
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7.5 Upper triangular matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 7.6 Diagonalization of 2 ? 2 matrices and applications . . . . . . . . . . . . . . . 96 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
8 Permutations and the Determinant of a Square Matrix
102
8.1 Permutations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
8.1.1 Definition of permutations . . . . . . . . . . . . . . . . . . . . . . . . 102
8.1.2 Composition of permutations . . . . . . . . . . . . . . . . . . . . . . 105
8.1.3 Inversions and the sign of a permutation . . . . . . . . . . . . . . . . 107
8.2 Determinants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
8.2.1 Summations indexed by the set of all permutations . . . . . . . . . . 110
8.2.2 Properties of the determinant . . . . . . . . . . . . . . . . . . . . . . 112
8.2.3 Further properties and applications . . . . . . . . . . . . . . . . . . . 115
8.2.4 Computing determinants with cofactor expansions . . . . . . . . . . . 116
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
9 Inner Product Spaces
120
9.1 Inner product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
9.2 Norms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
9.3 Orthogonality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
9.4 Orthonormal bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
9.5 The Gram-Schmidt orthogonalization procedure . . . . . . . . . . . . . . . . 129
9.6 Orthogonal projections and minimization problems . . . . . . . . . . . . . . 132
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
10 Change of Bases
139
10.1 Coordinate vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
10.2 Change of basis transformation . . . . . . . . . . . . . . . . . . . . . . . . . 141
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
11 The Spectral Theorem for Normal Linear Maps
147
11.1 Self-adjoint or hermitian operators . . . . . . . . . . . . . . . . . . . . . . . 147
11.2 Normal operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
11.3 Normal operators and the spectral decomposition . . . . . . . . . . . . . . . 151
iv
11.4 Applications of the Spectral Theorem: diagonalization . . . . . . . . . . . . 153 11.5 Positive operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 11.6 Polar decomposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 11.7 Singular-value decomposition . . . . . . . . . . . . . . . . . . . . . . . . . . 159 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
List of Appendices
A Supplementary Notes on Matrices and Linear Systems
164
A.1 From linear systems to matrix equations . . . . . . . . . . . . . . . . . . . . 164
A.1.1 Definition of and notation for matrices . . . . . . . . . . . . . . . . . 165
A.1.2 Using matrices to encode linear systems . . . . . . . . . . . . . . . . 168
A.2 Matrix arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
A.2.1 Addition and scalar multiplication . . . . . . . . . . . . . . . . . . . . 171
A.2.2 Multiplication of matrices . . . . . . . . . . . . . . . . . . . . . . . . 175
A.2.3 Invertibility of square matrices . . . . . . . . . . . . . . . . . . . . . . 179
A.3 Solving linear systems by factoring the coefficient matrix . . . . . . . . . . . 181
A.3.1 Factorizing matrices using Gaussian elimination . . . . . . . . . . . . 182
A.3.2 Solving homogeneous linear systems . . . . . . . . . . . . . . . . . . . 192
A.3.3 Solving inhomogeneous linear systems . . . . . . . . . . . . . . . . . . 195
A.3.4 Solving linear systems with LU-factorization . . . . . . . . . . . . . . 199
A.4 Matrices and linear maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
A.4.1 The canonical matrix of a linear map . . . . . . . . . . . . . . . . . . 204
A.4.2 Using linear maps to solve linear systems . . . . . . . . . . . . . . . . 205
A.5 Special operations on matrices . . . . . . . . . . . . . . . . . . . . . . . . . . 211
A.5.1 Transpose and conjugate transpose . . . . . . . . . . . . . . . . . . . 211
A.5.2 The trace of a square matrix . . . . . . . . . . . . . . . . . . . . . . . 212
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
B The Language of Sets and Functions
218
B.1 Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
B.2 Subset, union, intersection, and Cartesian product . . . . . . . . . . . . . . . 220
B.3 Relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
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