  By work of C. Greither and B. Pareigis as well as N. P. Byott, the
enumeration of Hopf-Galois structures on a Galois extension of fields with
Galois group $G$ may be reduced to that of regular subgroups of $\mbox{Hol}(N)$
isomorphic to $G$ as $N$ ranges over all groups of order $|G|$, where
$\mbox{Hol}(-)$ denotes the holomorph. In this paper, we shall give a
description of such subgroups of $\mbox{Hol}(N)$ in terms of bijective crossed
homomorphisms $G\longrightarrow N$, and then use it to study two questions
related to non-existence of Hopf-Galois structures.
