Buildings of Spherical Type and Finite BN-Pairs

These notes are a slightly revised and extended version of mim- graphed notes written on the occasion of a seminar on buildings and BN-pairs held at Oberwolfach in April 1968. Their main purpose is to present the solution of the following two problems: (A

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386

Jacques Tits

Buildings of Spherical Type and Finite BN-Pairs

Springer-Verlag Berlin Heidelberg New York Tokyo

Author

Jacques Tits College de France, 11, pI. Marcelin Berthelot 75231 Paris Cedex 05, France

1st Edition 1974 2nd Corrected Printing 1986

Mathematics Subject Classification (1970): Primary: 20G 15, 20G40, Secondary: 05B25, 05B30, 14M 15, 15A63, 17C40, 17005, 20005, 20H 15,50-00, 50A 15, 50A20, 50025,50030,50045, 50050,50099

ISBN 3-540-06757-4 Springer-Verlag Berlin Heidelberg New York Tokyo ISBN 0-387-06757-4 Springer-Verlag New York Heidelberg Berlin Tokyo

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 "VerwertungsgesellschaftWort", Munich. © by Springer-Verlag Berlin Heidelberg 1974 Printed in Germany Printing and binding: Beltz Offsetdruck, Hemsbach/Bergstr. 2146/3140-543210

Table of contents

Introduction 1.

Complexes

2.

Coxeter complexes

3.

Buildings

4.

Reduction

5.

The building of a semi-simple algebraic group

1

13

60 75

6.

Buildings of type

An

7.

Buildings of type

C • I. Polar spaces n

102

8.

Buildings of type

C • II. Projective embeddings of polar spaces n

117

9.

Buildings of type

C • III. Non-embeddable polar spaces n

177

10.

Buildings of type

F

200

11.

Finite BN-pairs of irreducible type and rank

12.

Appendix 1.

Shadows

232

Appendix 2.

Generators and relations

249

D

n

,E

92

n

4

>3

218

Addenda

274

Bibliography

277

Index of notations

287

Index

288

Introduction

These notes are a slightly revised and extended version of mimeographed notes written on the occasion of a seminar on buildings and BN-pairs held at Oberwolfach in April 1968.

Their main purpose is to present the solution of the

following two problems:

(A)

Determination of the buildings of rank

spherical type, other than ~

and H

group", about the excluded types

("of spherical type" means "with finite Weyl

4

H,

>; and irreducible,

cf. the addenda on p. 274).

Roughly speaking,

those buildings all turn out to be associated to simple algebraic or classical groups (cf. 6.;, 6.1;, 8.4.;, 8.22, 9.1, 10.2).

An easy application provides

of all finite groups with BN-pairs of irreducible type and rank

the enumeration

>;, up to normal

subgroups contained in B (cf. 11.7).

(B)

Determination of all isomorphisms between buildings of rank

>

2

and spherical type associated to algebraic or classical simple groups and, in particular, description of the full automorphism groups of such buildings

(cf. 5.8, 5.9,

5.10, 6.6, 6.1;, 8.6, 9.;, 10.4).

Except for the appendices, the notes are rather strictly oriented toward these goals.

In particular, the general theory of buildings and BN-pairs is de-

veloped only inasmuch as it