On composition of torsors

Abstract

Let $K$ be a field, let $X$ be a connected smooth $K$-scheme and let $G,H$ be two smooth connected $K$-group schemes. Given $Y\to X$ a $G$-torsor and $Z\to Y$ an $H$-torsor, we study whether one can find an extension $E$ of $G$ by $H$ so that the composite $Z\to X$ is an $E$-torsor. We give both positive and negative results, depending on the nature of the groups $G$ and $H$.

20 pages. Final accepted version