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

Abstract

Olsson showed in [Ols25] that if $\mathcal{X} \to X$ is a $\mathbf{G}_m$-gerbe over a smooth projective variety over an algebraically closed field $k$ such that the Brauer class of $\mathcal{X}$ has order prime to the characteristic of $k$, then the homomorphism of $k$-group algebraic spaces $\operatorname{Aut}^0_{\mathcal{X}} \to \operatorname{Aut}^0_X$ is surjective. We provide an example to show that this need not be the case when the Brauer class of $\mathcal{X}$ has order equal to the characteristic. Our main tools are deformation theory of the fppf sheafified Artin--Mazur formal groups and nice properties of the flat cohomology of ordinary varieties in positive characteristic which are presumably well-known, but which we collect and give an exposition of here. We additionally prove some sufficient conditions for surjectivity of $\operatorname{Aut}^0_{\mathcal{X}} \to \operatorname{Aut}^0_X$ using representability results of Bragg and Olsson.