Math 55a Lecture Notes - Evan Chen
Math 55a Lecture Notes
Evan Chen Fall 2014
This is Harvard College's famous Math 55a, instructed by Dennis Gaitsgory. The formal name for this class is "Honors Abstract and Linear Algebra" but it generally goes by simply "Math 55a".
The permanent URL is . html, along with all my other course notes.
Contents
1 September 2, 2014
5
1.1 Boring stuff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Equivalence relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 September 4, 2014
7
2.1 Review of equivalence relations go here . . . . . . . . . . . . . . . . . . . 7
2.2 Universal property of a quotient . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Homomorphisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3 September 9, 2014
9
3.1 Direct products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Commutative diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3 Sub-things . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.4 Let's play Guess the BS! . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.5 Kernels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.6 Normality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.7 Examples of normal groups . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4 September 11, 2014
13
4.1 Rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2 Ring homomorphisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3 Modules, and examples of modules . . . . . . . . . . . . . . . . . . . . . . 14
4.4 Abelian groups are Z-modules . . . . . . . . . . . . . . . . . . . . . . . . 15
4.5 Homomorphisms of R-modules . . . . . . . . . . . . . . . . . . . . . . . . 15
4.6 Matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.7 Sub-modules and Ideals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5 September 16, 2015
17
5.1 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2 Direct Sums of Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.3 Direct Products of Modules . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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Evan Chen (Fall 2014)
Contents
5.4 Sub-Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 5.5 Free Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.6 Return to the Finite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6 September 18, 2014
23
6.1 Linearly independent, basis, span . . . . . . . . . . . . . . . . . . . . . . . 23
6.2 Dimensions and bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.3 Corollary Party . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.4 Proof of Theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7 September 23, 2014
28
7.1 Midterm Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.2 Endomorphisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.3 Given a map we can split into invertible and nilpotent parts . . . . . . . . 29
7.4 Eigen-blah . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
7.5 Diagonalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
8 September 25, 2014
33
8.1 Eigenspaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
8.2 Generalized eigenspaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
8.3 Spectral Theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
8.4 Lemmata in building our proof . . . . . . . . . . . . . . . . . . . . . . . . 35
8.5 Proof of spectral theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
8.6 Recap of Proof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
9 September 30, 2014
38
9.1 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
9.2 Taking polynomials of an endomorphism . . . . . . . . . . . . . . . . . . . 38
9.3 Minimal Polynomials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
9.4 Spectral Projector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
9.5 Polynomials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
10 October 2, 2014
42
10.1 Jordan Canonical Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
10.2 A big proposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
10.3 Young Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
10.4 Proof of Existence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
11 October 7, 2014
46
11.1 Order of a Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
11.2 Groups of prime powers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
11.3 Abelian groups and vector spaces are similar . . . . . . . . . . . . . . . . 47
11.4 Chinese Remainder Theorem . . . . . . . . . . . . . . . . . . . . . . . . . 48
11.5 Not algebraically closed . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
12 October 9, 2014
50
12.1 Group Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
12.2 How do G-sets talk to each other? . . . . . . . . . . . . . . . . . . . . . . 50
12.3 Common group actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
12.4 More group actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
12.5 Transitive actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
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Contents
12.6 Orbits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 12.7 Corollaries of Sylow's Theorem . . . . . . . . . . . . . . . . . . . . . . . . 54 12.8 Proof of (b) of Sylow's Theorem assuming (a) . . . . . . . . . . . . . . . . 55
13 October 14, 2014
56
13.1 Proof of the first part of Sylow's Theorem . . . . . . . . . . . . . . . . . . 56
13.2 Abelian group structure on set of modules . . . . . . . . . . . . . . . . . . 56
13.3 Dual Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
13.4 Double dual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
13.5 Real and Complex Vector Spaces . . . . . . . . . . . . . . . . . . . . . . . 57
13.6 Obvious Theorems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
13.7 Inner form induces a map . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
14 October 16, 2014
60
14.1 Artificial Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
14.2 Orthogonal Subspace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
14.3 Orthogonal Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
14.4 Adjoint operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
14.5 Spectral theory returns . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
14.6 Things not mentioned in class that any sensible person should know . . . 64
14.7 Useful definitions from the homework . . . . . . . . . . . . . . . . . . . . 64
15 October 21, 2014
66
15.1 Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
15.2 Basic Properties of Tensor Products . . . . . . . . . . . . . . . . . . . . . 66
15.3 Computing tensor products . . . . . . . . . . . . . . . . . . . . . . . . . . 67
15.4 Complexification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
16 October 23, 2014
69
16.1 Tensor products gain module structure . . . . . . . . . . . . . . . . . . . . 69
16.2 Universal Property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
16.3 Tensor products of vector spaces . . . . . . . . . . . . . . . . . . . . . . . 70
16.4 More tensor stuff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
16.5 Q & A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
17 October 28, 2014
73
17.1 Midterm Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
17.1.1 Problem 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
17.1.2 Problem 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
17.1.3 Problem 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
17.2 The space nsub(V ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 17.3 The space nquot(V ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 17.4 The Wedge Product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
17.5 Constructing the Isomorphism . . . . . . . . . . . . . . . . . . . . . . . . 78
17.6 Why do we care? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
18 October 30, 2014
81
18.1 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
18.2 Completing the proof that nsub(V ) = nquot(V ) . . . . . . . . . . . . . . . 82 18.3 Wedging Wedges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
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Contents
19 November 4, 2014
85
19.1 Representations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
19.2 Group Actions, and Sub-Representations . . . . . . . . . . . . . . . . . . 85
19.3 Invariant Subspaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
19.4 Covariant subspace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
19.5 Quotient spaces and their representations . . . . . . . . . . . . . . . . . . 87
19.6 Tensor product of representations . . . . . . . . . . . . . . . . . . . . . . 87
20 November 6, 2014
89
20.1 Representations become modules . . . . . . . . . . . . . . . . . . . . . . . 89
20.2 Subrepresentations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
20.3 Schur's Lemma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
20.4 Splittings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
20.5 Table of Representations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
20.6 Induced and Restricted Representations . . . . . . . . . . . . . . . . . . . 93
21 November 11, 2014
94
21.1 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
21.2 Homework Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
21.3 A Theorem on Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
21.4 The Sum of the Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 96
21.5 Re-Writing the Sum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
21.6 Some things we were asked to read about . . . . . . . . . . . . . . . . . . 98
22 November 13, 2014
100
22.1 Irreducibles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
22.2 Products of irreducibles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
22.3 Regular representation decomposes . . . . . . . . . . . . . . . . . . . . . . 101
22.4 Function invariants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
22.5 A Concrete Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
23 November 18, 2014
104
23.1 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
23.2 The symmetric group on five elements . . . . . . . . . . . . . . . . . . . . 104
23.3 Representations of S5/(S3 ? S2) ? finding the irreducible . . . . . . . . . 106
23.4 Secret of the Young Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . 107
23.5 The General Theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
24 November 20, 2014
109
24.1 Reducing to some Theorem with Hom's . . . . . . . . . . . . . . . . . . . 109
24.2 Reducing to a Combinatorial Theorem . . . . . . . . . . . . . . . . . . . . 110
24.3 Doing Combinatorics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
25 December 2, 2014
113
26 December 4, 2014
114
26.1 Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
26.2 Functors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
26.3 Natural Transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
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Evan Chen (Fall 2014)
1 September 2, 2014
?1 September 2, 2014
?1.1 Boring stuff
Sets include R, Z, et cetera. A subset Y X is exactly what you think it is. Q, {0}, {1}, , R R. Yay.
X1 X2, X1 X2. . . . Gaitsgory what are you doing For a fixed universe X, we write Y , X \ Y , X - Y for {x X | x / Y }.
Lemma 1.1 For Y X,
Y = Y.
Proof. Trivial. darn this is being written out? x Y x / Y x Y.
Hence Y = Y .
Lemma 1.2 (X1 X2) = X1 X2.
Proof. Compute x X1 X2 x / X1X2 x / X1x / X2 x X1x X2 x X1X2.
Lemma 1.3 X1 X2 = X1 X2.
Proof. HW. But this is trivial and follows either from calculation or from applying the previous two lemmas.
Given a set X we can consider its power set P(X). It has 2n elements.
?1.2 Functions
Given two sets X and Y a map (or function) X -f Y is an assignment x X to an element f x Y .
Examples: X = {55 students}, Y = Z. Then f (x) = $ in cents (which can be negative).
Definition 1.4. A function f is injective (or a monomorphism) if x = y = f x = f y.
Definition 1.5. A function f is surjective (or an epimorphism) if y Y x X : f x = y.
Composition;
X -f Y -g Z.
5
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