Drinfeld Modular Curves

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1231 Ernst-Ulrich Gekeler

Drinfeld Modular Curves

Springer-Verlag Berlin Heidelberg New York London Paris Tokyo

Author

Ernst-Ulrich Gekeler Max-Planck Institut fur Mathematik Gottfried-Claren-Str. 26, 5300 Bonn 3, Federal Republic of Germany

Mathematics Subject Classification (1980): 12A90, 10D 12, 10D07, 14H 25 ISBN 3-540-17201-7 Springer-Verlag Berlin Heidelberg New York ISBN 0-387-17201-7 Springer-Verlag New York Berlin Heidelberg

This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under § 54 of the German Copyright Law where copies are made for other than private use, a fee is payable to "Verwertungsgesellschaft Wort", Munich.

© Springer-Verlag Berlin Heidelberg 1986 Printed in Germany Printing and binding: Druckhaus Beltz, Hemsbach/Bergstr. 2146/3140-543210

Table of Contents Introduction

o

Notations

I

Drinfeld Modules 1. Algebraic Theory

2

2. Analytic Theory

5

3. The Operation of

7

GL(r,Afl

4. The Modular Schemes for

r

and

2

9

II

III

1. Adelic Description of Lattices

10

2. Lattice Invariants

13

3. Morphisms of Lattices

15

Partial Zeta Functions 1. Relations with Lattice Sums

17

2. The Rational Function

20

3. Evaluation at IV

V

s

=

0

Z (Sl a,n and s

22

Drinfeld Modules of Rank 1 1. The Case of a Rational Function Field

25

2. Normalization

26

3. Some Lemmata

30

4. Computation of Lattice Invariants

33

5. Distinguished 1-D-Modules

38

Modular Curves over

C

1. The "Upper Half-Plane"

40

2. Group Actions

43

3. Modular Forms

47

4. Elliptic Points

50

5. Modular Forms and Differentials

51

Appendix: The First Betti Number of

r

54

IV

VI

VII

Expansions around Cusps 1. Preparations

58

2. Formulae

60

3. Computation of the Factors

61

4. The 6-Functions

65

5. Some Consequences

71

Modular Forms and Functions

1- The Field of Modular Functions 2. The Field of Definition of the Elliptic Points 3. Behavior of E (q-1) at Elliptic Points 4. The Graded Algebra of Modu La r' Forms

VIII

78 82 83 85

5. Higher Modular Curves

86

6. Modular Forms for Congruence Subgroups

92

Complements 1. Hecke Operators

94

2. Connections with the Classification of Elliptic Curves

96

3. Some Open Questions

99

Index

101

List of Symbols

102

Bibliography

104

Introduction The analogy of the arithmetic of number fields with that of "function fields"

(i.e. function fields in one variable over a finite field of

constants) has been known for a long time. This analogy starts with elementary things (structure of rings of integers, ramification theory, product formula ..• ), but reaches into such deep fields like for example -

(abelian and non-abelian) class field theory;

- Iwasawa theory; - special values of L-functions (conjectures of Birch and SwinnertonDyer and of Stark, relations with K-theory); - diophantine geometry (conjecture