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¹Åµä´úÊý¼¸ºÎ¸ÅÂÛ(Topics in Classical Algebraic Geometry)

Contents
1 Polarity 1
1.1 Polar hypersurfaces . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 The polar pairing . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 First polars . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.3 Second polars . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.4 The Hessian . . . . . . . . . . . . . . . . . . . . . . . . 7
1.1.5 Parabolic points . . . . . . . . . . . . . . . . . . . . . . . 11
1.1.6 The Steinerian . . . . . . . . . . . . . . . . . . . . . . . 12
1.2 Dual hypersurface . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2.1 The polar map . . . . . . . . . . . . . . . . . . . . . . . 14
1.2.2 Dual varieties . . . . . . . . . . . . . . . . . . . . . . . . 15
1.2.3 Pl¡§ucker equations . . . . . . . . . . . . . . . . . . . . . . 18
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2 Conics 23
2.1 Polar triangles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.1.1 Conjugate triangles . . . . . . . . . . . . . . . . . . . . . 26
2.1.2 Poncelet related conics . . . . . . . . . . . . . . . . . . . 28
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3 Plane cubics 37
3.1 Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.1.1 Weierstrass equation . . . . . . . . . . . . . . . . . . . . 37
3.1.2 Hesse equation . . . . . . . . . . . . . . . . . . . . . . . 38
3.1.3 Hesse pencil . . . . . . . . . . . . . . . . . . . . . . . . 40
3.2 Polars of a plane cubic . . . . . . . . . . . . . . . . . . . . . . . 42
3.2.1 The Hessian of a cubic hypersurface . . . . . . . . . . . . 42
3.2.2 The Hessian of a plane cubic . . . . . . . . . . . . . . . . 43
3.2.3 The dual cubic . . . . . . . . . . . . . . . . . . . . . . . 45
iii
iv CONTENTS
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4 Determinantal equations 49
4.1 Plane curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.1.1 The problem . . . . . . . . . . . . . . . . . . . . . . . . 49
4.1.2 Plane curves . . . . . . . . . . . . . . . . . . . . . . . . 50
4.1.3 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.1.4 Quadratic Cremona transformations . . . . . . . . . . . . 57
4.1.5 Moduli space . . . . . . . . . . . . . . . . . . . . . . . . 61
4.2 Determinantal equations for hypersurfaces . . . . . . . . . . . . . 63
4.2.1 Cohen-Macauley sheaves . . . . . . . . . . . . . . . . . . 63
4.2.2 Determinants with linear entries . . . . . . . . . . . . . . 65
4.2.3 The case of curves . . . . . . . . . . . . . . . . . . . . . 67
4.2.4 The case of surfaces . . . . . . . . . . . . . . . . . . . . 68
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5 Theta characteristics 73
5.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.1.1 Symmetric determinants . . . . . . . . . . . . . . . . . . 73
5.1.2 Quadratic forms over a field of characteristic 2 . . . . . . 75
5.1.3 Hyperelliptic curves . . . . . . . . . . . . . . . . . . . . 78
5.1.4 2-torsion points on a hyperelliptic curve . . . . . . . . . . 79
5.1.5 Theta characteristics on a hyperelliptic curve . . . . . . . 80
5.1.6 Jacobian variety . . . . . . . . . . . . . . . . . . . . . . 82
5.1.7 Riemann¡¯s theta function . . . . . . . . . . . . . . . . . . 83
5.1.8 Theta functions with characteristics . . . . . . . . . . . . 84
5.1.9 Hyperelliptic curves again . . . . . . . . . . . . . . . . . 87
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6 Plane Quartics 91
6.1 Odd theta characteristics . . . . . . . . . . . . . . . . . . . . . . 91
6.1.1 28 bitangents . . . . . . . . . . . . . . . . . . . . . . . . 91
6.1.2 Steiner complexes . . . . . . . . . . . . . . . . . . . . . 94
6.1.3 Aronhold sets . . . . . . . . . . . . . . . . . . . . . . . . 97
6.2 Quadratic determinant equations . . . . . . . . . . . . . . . . . . 99
6.2.1 Coble¡¯s construction . . . . . . . . . . . . . . . . . . . . 99
6.2.2 Symmetric quadratic determinants . . . . . . . . . . . . . 102
6.3 Even theta characteristics . . . . . . . . . . . . . . . . . . . . . . 107
6.3.1 Contact cubics . . . . . . . . . . . . . . . . . . . . . . . 107
6.3.2 Cayley octads . . . . . . . . . . . . . . . . . . . . . . . . 108
CONTENTS v
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
7 Planar Cremona transformations 111
7.1 Homaloidal linear systems . . . . . . . . . . . . . . . . . . . . . 111
7.1.1 Cremona transformations . . . . . . . . . . . . . . . . . . 111
7.1.2 Exceptional configurations . . . . . . . . . . . . . . . . . 112
7.1.3 The bubble space . . . . . . . . . . . . . . . . . . . . . . 115
7.2 De Jonqui`eres transformations . . . . . . . . . . . . . . . . . . . 121
7.2.1 Hyperelliptic curves . . . . . . . . . . . . . . . . . . . . 123
7.3 Elementary transformations . . . . . . . . . . . . . . . . . . . . . 125
7.3.1 Segre-Hirzebruch minimal ruled surfaces . . . . . . . . . 125
7.3.2 Elementary transformations . . . . . . . . . . . . . . . . 129
7.3.3 Birational automorphisms of P1
 P1 . . . . . . . . . . . 131
7.3.4 De Jonqui`eres transformations again . . . . . . . . . . . . 134
7.4 Characteristic matrices . . . . . . . . . . . . . . . . . . . . . . . 135
7.4.1 Composition of characteristic matrices . . . . . . . . . . . 140
7.4.2 Weyl groups . . . . . . . . . . . . . . . . . . . . . . . . 143
7.4.3 Noether¡¯s inequality . . . . . . . . . . . . . . . . . . . . 144
7.5 Cremona group . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
7.5.1 Noether¡¯s factorization theorem . . . . . . . . . . . . . . 145
7.5.2 Noether-Fano inequality . . . . . . . . . . . . . . . . . . 146
7.5.3 Noether¡¯s Theorem . . . . . . . . . . . . . . . . . . . . . 147
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
8 Del Pezzo surfaces 155
8.1 First properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
8.1.1 Varieties of minimal degree . . . . . . . . . . . . . . . . 155
8.1.2 A blow-up model . . . . . . . . . . . . . . . . . . . . . . 156
8.1.3 (􀀀2)-curves . . . . . . . . . . . . . . . . . . . . . . . . . 159
8.2 Anti-canonical models . . . . . . . . . . . . . . . . . . . . . . . 161
8.2.1 Surfaces of degree d in Pd . . . . . . . . . . . . . . . . . 166
8.3 Lines on Del Pezzo surfaces . . . . . . . . . . . . . . . . . . . . 168
8.3.1 Cremona isometries . . . . . . . . . . . . . . . . . . . . 168
8.3.2 Lines on Del Pezzo surfaces . . . . . . . . . . . . . . . . 173
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
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