On some semidirect products of skew braces arising in Hopf-Galois theory

Abstract

We classify skew braces that are the semidirect product of an ideal and a left ideal. As a consequence, given a Galois extension of fields $L/K$ whose Galois group is the semidirect product of a normal subgroup $A$ and a subgroup $B$, we classify the Hopf-Galois structures on $L/K$ that realize $L^A$ via a normal Hopf subalgebra and $L^B$ via a Hopf subalgebra. We show that the Hopf algebra giving such a Hopf-Galois structure is the smash product of these Hopf subalgebras, and use this description to study generalized normal basis generators and questions of integral module structure in extensions of local fields.

20 pages