Algebraic Spaces

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203 Donald Knutson Columbia University in the City of New York, New York, NY/USA

Algebraic Spaces

Springer-Verlag Berlin· Heidelberg· NewYork 1971

AMS Subject Classifications (1970): 14-02, 14A 15, 14A20, 14F20, 18F 10

ISBN 3-540-05496-0 Springer-Verlag Berlin . Heidelberg . New York ISBN 0-387-05496-0 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 with the publisher.

© by Springer-Verlag Berlin' Heidelberg 1971. Library of Congress Catalog Card Nurnber73-164957.Printed in Germany. Offsetdruck: Julius Beltz, Hernsbach

PREFACE The core of this book is the author's thesis, Algebraic Spaces, written under Michael Artin at the Massachusetts Institute of Technology. The object there as here was to work out the foundations

a

la EGA for the theory of algebraic spaces, and hence give the

necessary background for Artin's fundamental papers Algebraization of Formal Moduli I, II. While working on this book, I was supported by M.I.T., Boston College, Columbia University, and the Advanced Science Summer Seminar at Bowdoin College, sponsored by the National Science Foundation. To all these institutions, I extend my gratitude. My special thanks goes to Professor Michael Artin both for many helpful discussions and for his initial suggestion that I undertake this project. Donald Knutson

CONTENTS Introduction Chapter One: The Etale Topology of Schemes 1. 2. 3. 4. 5.

29

Grothendieck Topologies and Descent Theory The Zariski Topology of Schemes The Flat Topology of Schemes The Etale Topology of Schemes Etale Equivalence Relations

29 38 52

59 72

Chapter Two: Algebraic Spaces

91

1. The Category of Algebraic Spaces ••.• 91 2. The Etale Topology of Algebraic Spaces • • • • • 101 3. Descent Theory for Algebraic Spaces • • • • . • • • 106 4. Quasicoherent Sheaves and Cohomology • 113 • • • 120 5. Local Constructions . 6. Points and the Zariski Topology • • 129 7. Proper and Projective Morphisms • 139 8. Integral Algebraic Spaces • • 144 Chapter Three: Quasicoherent Sheaves on Noetherian Locally Separated Algebraic Spaces . . • 1. 2. 3. 4. 5.

The Completeness / Extension Lemma The Serre Criterion Schemehood and Nilpotents Chevalley's Theorem. Devissage • • • • •

Chapter Four: The Finiteness Theorem 1. 2. 3. 4.

Actions of a Finite Group •• Symmetric powers of Projective Spaces Chow's Lemma The Finiteness Theorem

• • 153 153 • • 159 • 165 • • 169

• 173 • • • • • 176 . . . • • • . 177

• 185 • • • 192 • 202

VI Chapter Five: Formal Algebraic Spaces

• 204

1. Affine Formal Schemes . . . . •

2. Formal Algebraic Spaces • • . • •

• 204 • • 215

3. The Theorem of Holomorphic Func