Toroidal Compactification of Siegel Spaces

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812 IIIIIII

Yukihiko Namikawa

Toroidal Compactificatic)n of Siegel Spaces I

Springer-Verlag Berlin Heidelberg New York 1980

!

Author Yukihiko Namikawa Department of Mathematics, Nagoya University Furocho, Chikusa-Ku Nagoya, 464/Japan

AMS Subject Classifications (1980): 14 L1 ?, 20 G 20, 32 J 05, 32 M 15, 32N15 ISBN 3-540-10021-0 Springer-Verlag Berlin Heidelberg New York ISBN 0-387-10021-0 Springer-Verlag NewYork Heidelberg Berlin 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 the publisher, the amount of the fee to be determined by agreement with the publisher. © by Springer-Vertag Berlin Heidelberg 1980 Printed in Germany Printing and binding: Beltz Offsetdruck, Hemsbach/Bergstr. 2141/3t40-543210

To M y P a r e n t s

~aL t 6 o a L

TaC ~ p a y ~ S a ~

aOTO5.

Introduction

One of the simplest but the richest object to study in

mathematics

is a unit disc

in the complex plane.

A m o n g several g e n e r a l i z a t i o n s

of it the n o t i o n

of h e r m i t i a n b o u n d e d s y m m e t r i c - d o m a i n w o u l d be the most m e a n i n g f u l

one, w h i c h is a g e n e r a l i z a t i o n in the field of d i f f e r e n t i a l g e o m e t r y (for full e x p o s i t i o n see [13] for example). As a g e n e r a l i z a t i o n of

of an a r i t h m e t i c group

G = Aut(D)

subgroup

symmetric domain

SL(2, ~ )

r

of b i h o l o m o r p h i c

D.

acting on

w h i c h is a d i s c r e t e

The q u o t i e n t

automorphisms

space

F\V

with a structure of a normal complex analytic

D

we have a notion

subgroup of the Lie

of a h e r m i t i a n b o u n d e d

is n a t u r a l l y endowed

space.

Two facts stand in the way of s t u d y i n g the geometric

of

r\~.

The first is that

rise to s i n g u l a r i t i e s

on

r

may have fixed points in

r\D.

overcome by taking a suitable

w h i c h acts on

~

without

The second is that

problem to compactify

r\~

This d i f f i c u l t y

subgroup

r'

fixed points. r\~

F

of finite index Here arises

the

The first answer to this

[27 ] in the case of the Siegel

u p p e r h a l f plane and finally by B a i l y - B o r e l others

which give

can be, however,

may not be compact.

suitably.

p r o b l e m was given first by Satake form.

of

D

structure

[4] in the most general

The second answer was quite r e c e n t l y given by Mumford and [2], s u g g e s t e d by the early work by Siegel

[30] and Igusa

[15].

The aim of this lecture note is to exhibit these theories of

c o m p a c t i f i c a t i o n of

r\D

in the case of the Siegel u p p e r h a l f plane.

Thanks to this r e s t r i c t i o n one can see the whole theory e l e m e n t a r i l y and e x p l i c i t l y in thi