Algebraic Curves over a Finite Field

J.W.P. Hirschfeld is professor emeritus of mathematics at the University of Sussex. His books include "Projective Geometries over Finite Fields". G. Korchmaros is professor of mathematics at the University of Basilicata in Italy. F. Torres is professor of mathematics at the University of Campinas in Brazil.

"This book is well-written and I greatly enjoyed reading it. The wealth of information and examples in this book give the reader a firm foundation and develop an intuition for the subject. The authors have used it as a textbook for a two-year course, and it would be a fine introduction to any advanced undergraduate or graduate student wanting to learn this subject."--Thomas Hagedorn, MAA Reviews

• Preface xi PART 1. GENERAL THEORY OF CURVES 1 Chapter 1. Fundamental ideas 3 1.1 Basic definitions 3 1.2 Polynomials 6 1.3 Affine plane curves 6 1.4 Projective plane curves 9 1.5 The Hessian curve 13 1.6 Projective varieties in higher-dimensional spaces 18 1.7 Exercises 18 1.8 Notes 19 Chapter 2. Elimination theory 21 2.1 Elimination of one unknown 21 2.2 The discriminant 30 2.3 Elimination in a system in two unknowns 31 2.4 Exercises 35 2.5 Notes 36 Chapter 3. Singular points and intersections 37 3.1 The intersection number of two curves 37 3.2 B'ezout's Theorem 45 3.3 Rational and birational transformations 49 3.4 Quadratic transformations 51 3.5 Resolution of singularities 55 3.6 Exercises 61 3.7 Notes 62 Chapter 4. Branches and parametrisation 63 4.1 Formal power series 63 4.2 Branch representations 75 4.3 Branches of plane algebraic curves 81 4.4 Local quadratic transformations 84 4.5 Noether's Theorem 92 4.6 Analytic branches 99 4.7 Exercises 107 4.8 Notes 109 Chapter 5. The function field of a curve 110 5.1 Generic points 110 5.2 Rational transformations 112 5.3 Places 119 5.4 Zeros and poles 120 5.5 Separability and inseparability 122 5.6 Frobenius rational transformations 123 5.7 Derivations and differentials 125 5.8 The genus of a curve 130 5.9 Residues of differential forms 138 5.10 Higher derivatives in positive characteristic 144 5.11 The dual and bidual of a curve 155 5.12 Exercises 159 5.13 Notes 160 Chapter 6. Linear series and the Riemann-Roch Theorem 161 6.1 Divisors and linear series 161 6.2 Linear systems of curves 170 6.3 Special and non-special linear series 177 6.4 Reformulation of the Riemann-Roch Theorem 180 6.5 Some consequences of the Riemann-Roch Theorem 182 6.6 The Weierstrass Gap Theorem 184 6.7 The structure of the divisor class group 190 6.8 Exercises 196 6.9 Notes 198 Chapter 7. Algebraic curves in higher-dimensional spaces 199 7.1 Basic definitions and properties 199 7.2 Rational transformations 203 7.3 Hurwitz's Theorem 208 7.4 Linear series composed of an involution 211 7.5 The canonical curve 216 7.6 Osculating hyperplanes and ramification divisors 217 7.7 Non-classical curves and linear systems of lines 228 7.8 Non-classical curves and linear systems of conics 230 7.9 Dual curves of space curves 238 7.10 Complete linear series of small order 241 7.11 Examples of curves 254 7.12 The Linear General Position Principle 257 7.13 Castelnuovo's Bound 257 7.14 A generalisation of Clifford's Theorem 260 7.15 The Uniform Position Principle 261 7.16 Valuation rings 262 7.17 Curves as algebraic varieties of dimension one 268 7.18 Exercises 270 7.19 Notes 271 PART 2. CURVES OVER A FINITE FIELD 275 Chapter 8. Rational points and places over a finite field 277 8.1 Plane curves defined over a finite field 277 8.2 Fq-rational branches of a curve 278 8.3 Fq-rational places, divisors and linear series 281 8.4 Space curves over Fq 287 8.5 The Stohr-Voloch Theorem 292 8.6 Frobenius classicality with respect to lines 305 8.7 Frobenius classicality with respect to conics 314 8.8 The dual of a Frobenius non-classical curve 326 8.9 Exercises 327 8.10 Notes 329 Chapter 9. Zeta functions and curves with many rational points 332 9.1 The zeta function of a curve over a finite field 332 9.2 The Hasse-Weil Theorem 343 9.3 Refinements of the Hasse-Weil Theorem 348 9.4 Asymptotic bounds 353 9.5 Other estimates 356 9.6 Counting points on a plane curve 358 9.7 Further applications of the zeta function 369 9.8 The Fundamental Equation 373 9.9 Elliptic curves over Fq 378 9.10 Classification of non-singular cubics over Fq 381 9.11 Exercises 385 9.12 Notes 388 PART 3. FURTHER DEVELOPMENTS 393 Chapter 10. Maximal and optimal curves 395 10.1 Background on maximal curves 396 10.2 The Frobenius linear series of a maximal curve 399 10.3 Embedding in a Hermitian variety 407 10.4 Maximal curves lying on a quadric surface 421 10.5 Maximal curves with high genus 428 10.6 Castelnuovo's number 431 10.7 Plane maximal curves 439 10.8 Maximal curves of Hurwitz type 442 10.9 Non-isomorphic maximal curves 446 10.10 Optimal curves 447 10.11 Exercises 453 10.12 Notes 454 Chapter 11. Automorphisms of an algebraic curve 458 11.1 The action of K-automorphisms on places 459 11.2 Linear series and automorphisms 464 11.3 Automorphism groups of plane curves 468 11.4 A bound on the order of a K-automorphism 470 11.5 Automorphism groups and their fixed fields 473 11.6 The stabiliser of a place 476 11.7 Finiteness of the K-automorphism group 480 11.8 Tame automorphism groups 483 11.9 Non-tame automorphism groups 486 11.10 K-automorphism groups of particular curves 501 11.11 Fixed places of automorphisms 509 11.12 Large automorphism groups of function fields 513 11.13 K-automorphism groups fixing a place 532 11.14 Large p-subgroups fixing a place 539 11.15 Notes 542 Chapter 12. Some families of algebraic curves 546 12.1 Plane curves given by separated polynomials 546 12.2 Curves with Suzuki automorphism group 564 12.3 Curves with unitary automorphism group 572 12.4 Curves with Ree automorphism group 575 12.5 A curve attaining the Serre Bound 585 12.6 Notes 587 Chapter 13. Applications: codes and arcs 590 13.1 Algebraic-geometry codes 590 13.2 Maximum distance separable codes 594 13.3 Arcs and ovals 599 13.4 Segre's generalisation of Menelaus' Theorem 603 13.5 The connection between arcs and curves 607 13.6 Arcs in ovals in planes of even order 611 13.7 Arcs in ovals in planes of odd order 612 13.8 The second largest complete arc 615 13.9 The third largest complete arc 623 13.10 Exercises 625 13.11 Notes 625 Appendix A. Background on field theory and group theory 627 A.1 Field theory 627 A.2 Galois theory 633 A.3 Norms and traces 635 A.4 Finite fields 636 A.5 Group theory 638 A.6 Notes 649 Appendix B. Notation 650 Bibliography 655 Index 689