The Monodromy Groups of Isolated Singularities of Complete Intersections

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1293 Wolfgang Ebeling

The Monodromy Groups of Isolated Singularities of Complete Intersections

Spri nger-Verlag Berlin Heidelberg New York London Paris Tokyo

Lecture Notes in Mathematics Edited by A. Dold and B. Eckmann Subseries: Mathematisches Institut der Universitat und Max-Planck-Institut fUr Mathematik, Bonn - vol. 12 F. Hirzebruch Adviser:

1293 Wolfgang Ebeling

The Monodromy Groups of Isolated Singularities of Complete Intersections

Spri nger-Verlag Berlin Heidelberg New York London Paris Tokyo

Author

Wolfgang Ebeling Mathematisches Institut der Universitat Wegelerstr. 10,5300 Bonn 1, Federal Republic of Germany

Mathematics Subject Classification (1980): 14B05, 14D05, 14M 10, 32B30, 57R45; 10C30, 10E45, 20H 15, 51 F 15 ISBN 3-540-18686-7 Springer-Verlag Berlin Heidelberg New York ISBN 0-387-18686-7 Springer-Verlag New York Berlin Heidelberg

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© Springer-Verlag Berlin Heidelberg 1987 Printed in Germany Printing and binding: Druckhaus Beltz, Hemsbach/Bergstr. 2146/3140-543210

For Bettina, Bastian, and Mirja

INTRODUCTION For the analysis of hyper surface singularities and their deformations the Milnor lattice and the invariants associated with it play an 1\

important role. The middle homology group of a Milnor fibre of the singularity is a free abelian group, which is endowed with a bilinear form, the intersection form. This bilinear form is symmetric or skewsymmetric, if the dimension is even or odd respectively. This group with this additional structure is called the Milnor lattice monodromy group

r

H. The

of the singularity is a subgroup of the automorp.hism

group of this lattice. It is generated by reflections, respectively symplectic transvections, corresponding to certain elements of the Milnor lattice, the vanishing cyCles, which are defined in a geometric way. These form the set

c H

of vanishing cycles. The monodromy group is al­

ready generated by the respective automorphisms corresponding to the elements of certain geometrically distinguished bases of vanishing cycles. The set of Dynkin diagrams (or intersection diagrams) corresponding to such bases yields another invariant. A survey of the relations between these invariants and their importance for the deformation theory of the singularities in the hypersurface case is given by E. Brieskorn in his expository article [Brieskorn

3]. By stabilizing we can restrict ourselves to the symmetric case in

the hypersurface situation. For the simple hypersurface singularitie