Between the genus and the $Γ$-genus of an integral quadratic $Γ$-form

Abstract

Let Γbe a finite group and (V,q) be a regular quadratic Γ-form defined over an integral domain $\mathcal{O}_S$ of a global function field (of odd characteristic). We use flat cohomology to classify the quadratic Γ-forms defined over $\mathcal{O}_S$ that are locally Γ-isomorphic for the flat topology to (V,q) and compare between the genus c(q) and the Γ-genus c_Γ(q) of q. We show that c_Γ(q) should not inject in c(q). The suggested obstruction arises from the failure of the Witt cancellation theorem for $\mathcal{O}_S$.