The Other Group of as Galois Extension

Abstract

Let $k\subseteq K$ be a finite Galois extension of fields with Galois group $G$. Let $\mathscr{G}$ be the automorphism $k$-group scheme of $K$. We construct a canonical $k$-subgroup scheme $\underline{G}\subset\mathscr{G}$ with the property that $Spec_k(K)$ is a $k$-torsor for $\underline{G}$. $\underline{G}$ is a constant $k$-group if and only if $G$ is abelian, in which case $G=\underline{G}$.