Polarizations on abelian varieties and self-dual ell-adic representations of inertia groups

Abstract

It is well-known that every finite subgroup of GL_d(Q_{\ell}) is conjugate to a subgroup of GL_d(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 if 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 Sp_{2d}(Q_{\ell}) of inertia type, then G is conjugate in GL_{2d}(Q_{\ell}) to a subgroup of \Sp_{2d}(Z_{\ell}). Despite the fact that G can fail to be conjugate in \GL_{2d}(Q_\ell) to a subgroup of \Sp_{2d}(Z_\ell), we prove that it can nevertheless be embedded in \Sp_{2d}(F_\ell) in such a way that the characteristic polynomials are preserved (mod \ell), as long as \ell>3. The latter result holds for arbitrary finite groups, not necessarily of inertia type, and holds also for symmetric forms, not just alternating forms. We give examples which show that the bounds are 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. This paper is a revised version of ANT-0151, titled `Self-dual ell-adic representations of finite groups'.