Torsors and gerbes

Abstract

Let $A$ be an abelian sheaf on a site $X_{\tau}$ on which we have an action of a finite group $G$. Given an $A$-torsor (respectively a gerbe banded by $A$), we would like to know under what conditions it is induced from an $A^G$-torsor (respectively a gerbe banded by $A^G$). It is necessary for the torsor or the gerbe to admit a lift of the group action. Given such a $G$-action, we define an obstruction class which is a cohomology class in $H^2(G,A(X))$ for torsors and in $H^3(G,A(X))$ for gerbes. We prove that if a torsor or a gerbe is induced from the invariants, then the obstruction class is zero. Moreover, the vanishing of the obstruction class is sufficient for the reverse conclusion assuming the group cohomology classes locally vanish. The first two sheaf cohomology groups $H^i(X,A)$ can be realized concretely as the group of $A$-torsors and the group of abelian gerbes banded by $A$. Using this realization and the notion of obstruction classes, we describe morphisms of cohomologies in low degrees which come from a certain spectral sequence. A particular example of interest is the Hoschild-Serre spectral sequence that arises in the case of a finite etale Galois cover of a scheme $X$ with Galois group $G$.