Essential p-dimension and Chern numbers

Abstract

Let $X$ be a smooth, projective, geometrically connected variety over a field $k$ containing a root of unity of order $p$. If $X$ has a Chern number prime to $p$, we show that every action of a finite $p$-group on $X$ factors through a subgroup of ${\mathrm{GL}}_n(k)$, where $n=\dim X$. This allows one to transfer properties of representations of finite $p$-groups to their actions on $X$. We deduce a fixed-point theorem which, unlike previously known results of this kind, is sensitive to the arithmetic of the base field. We also obtain a bound on the orders of cyclic $p$-subgroups of the Cremona groups: for instance ${\mathrm{Cr}}_n(\mathbb{Q})$ contains no element of order $p^2$ when $p\ge n+2$. The method is based on the following observation, of independent interest. For an affine algebraic group $G$ over a field of characteristic zero, ${\mathrm{ed}}_p(G)+\dim G$ is the least dimension of a smooth projective variety $Y$ with a generically free $G$-action such that the degree map ${\mathrm{CH}}_G(Y)\to {\mathbb{F}}_p$ is nonzero. A key input for our result is Karpenko and Merkurjev's computation of the essential $p$-dimension of $p$-groups.