On automorphisms of $p$-torsion ${\mathbf{G}}_m$-gerbes

Abstract

Rouquier proved that for a smooth projective variety $X$, the group scheme ${\mathrm{Pic}}_X^0⋊{\mathrm{Aut}}_X^0$ is an invariant of the derived category of $X$. This was generalized to the twisted case by Olsson, who associated a group algebraic space ${\mathrm{Aut}}_{\mathcal{X}}^0$ to a ${\mathbf{G}}_m$-gerbe $\mathcal{X}\to X$ and proved it to be a twisted derived invariant. In characteristic zero, Olsson showed that ${\mathrm{Aut}}_{\mathcal{X}}^0$ is an extension of ${\mathrm{Aut}}_X^0$ by a subgroup scheme of ${\mathrm{Pic}}_X$. It is important in applications to compute ${\mathrm{Aut}}_{\mathcal{X}}^0$ in positive characteristic as well, which is the problem we consider here. We use deformation theory and representability results of Bragg and Olsson to prove that in many cases of interest, Olsson's description of ${\mathrm{Aut}}_{\mathcal{X}}^0$ as an extension also holds in characteristic $p$. As a corollary, we show that twisted derived equivalent abelian varieties are isogenous, simultaneously generalizing results of Honigs and Lane. We also provide an example showing that Olsson's extension description does not hold in general. This uses a result of Illusie on crystalline and flat cohomology and a result of Yang on the Brauer group of an ordinary variety, which we generalize to cohomology in arbitrary degree.

arXiv comment15 pages, comments welcome, improvements made to exposition in v2 and Corollary 3.4 added