Foundations of the Theory of Klein Surfaces

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219 Norman L. Alling

The University of Rochester, Rochester, NYIUSA

Newcomb Greenleaf

The University of Texas, Austin, Tx/USA

Foundations of the Theory of Klein Surfaces

Springer-Verlag Berlin· Heidelberg· NewYork 1971

Lecture Notes in Mathematics A collection of informal reports and seminars Edited by A. Dold, Heidelberg and B. Eckmann, Zurich

219 Norman L. Alling

The University of Rochester, Rochester, NYIUSA

Newcomb Greenleaf

The University of Texas, Austin, Tx/USA

Foundations of the Theory of Klein Surfaces

Springer-Verlag Berlin· Heidelberg· NewYork 1971

AMS Subject Classifications (1970): 14H05, 14)25, 30A46

ISBN 3-540-05577-0 Springer-Verlag Berlin· Heidelberg· New York ISBN 0-387-05577-0 Springer-Verlag New York· 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-Verlag Berlin' Heidelberg 1971. Library of Congress Catalog Card Number 73-172693. Printed in Germany. Offsetdruck: Julius Beltz, Hemsbach

INTRODUCTION

It has long been known that the category

of compact Rie-

mann surfaces and non-constant analytic maps, and the category of complex-algebraic function fields and complex isomorphisms are, via two contravariant functions, coequivalent; thus an analytic theory and an algebraic theory are tied together. While investigating several Banach algebras on compact Riemann surfaces

I

with non-empty boundary

oX

([A Z]' [A [A the 4]), 3],

first author posed the following question for himself:

what is the

simplest algebraic object which can be associated with

I,

which let that

can be recovered?

I

E(I)

The answer seems to be the following:

be the field of all functions

f(oX)

R U [ro}.

surface

I

f

I

meromorphic on

such

This field is an algebraic function field

in one variable over the reals. verse question:

from

It is natural then to ask the con-

given such a field

E,

is there a compact Riemann

(possibly with boundary) such that

answer to this question, interestingly, is no.

E

E(1)?

The

The following field,

long known to algebraic geometers, supplies a counter-example to such a conjecture:

1 et

E = R( x,y ),

where

X

z+

y2

=

-1.

The

present collaboration began at this juncture. Let

be the category of all real-algebraic function fields

and all real-linear isomorphisms.

Given such a field E,

braic geometers have long known how to associate a curve E;

for example let

X;: {o:

0

a valuation ring of

E

the algeX with over

R}.

The usual topology put on such a curve is the Zariski topology in

IV which the proper closed sets are the finite sets. does not utilize the topology on fold.

Such a to