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Polarizations on Abelian Varieties and Self-dual [ell ]-adic Representations of Inertia Groups

Published online by Cambridge University Press:  04 December 2007

A. Silverberg
Affiliation:
Department of Mathematics, Ohio State University, Columbus, Ohio 43210, U.S.A. E-mail: [email protected]
Yu. G. Zarhin
Affiliation:
Department of Mathematics, Pennsylvania State University, University Park, PA 16802, U.S.A. E-mail: [email protected]
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Abstract

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It is well known that every finite subgroup of GLd(Q[ell ]) is conjugate to a subgroup of GLd(Z[ell ]). However, this does not remain true if we replace general linear groups by symplectic groups. We say that G is a group of inertia type of G is a finite group which has a normal Sylow-p-subgroup with cyclic quotient. We show that if [ell ]>d+1, and G is a subgroup of Sp2d(Q[ell ]) of inertia type, then G is conjugate in GL2d(Q[ell ]) to a subgroup of Sp2d(Z[ell ]). We give examples which show that the bound is sharp. We apply these results to construct, for every odd prime [ell ], isogeny classes of Abelian varieties all of whose polarizations have degree divisible by [ell ]2. We prove similar results for Euler characteristic of invertible sheaves on Abelian varieties over fields of positive characteristic.

Type
Research Article
Copyright
© 2001 Kluwer Academic Publishers