The behavior of differential, quadratic and bilinear forms under purely inseparable field extensions

Abstract

Let $F$ be a field of characteristic $p$ and let $E/F$ be a purely inseparable field extension. We study the group $H_p^{n+1}(F)$ of classes of differential forms under the restriction map $H_p^{n+1}(F)\to H_p^{n+1}(E)$ and give a system of generators of the kernel $H_p^{n+1}(E/F)$. In the case $p=2$, we use this to determine the kernel $W_q(E/F)$ of the restriction map $W_q(F) \to W_q(E)$ between the group of nonsingular quadratic forms over $F$ and over $E$. We also deduce the corresponding result for the bilinear Witt kernel $W(E'/F)$ of the restriction map $W(F) \to W(E')$, where $E'/F$ denotes a modular purely inseparable field extension.