Decompositions of the wreath product of certain directed graphs into directed hamiltonian cycles

Abstract: We affirm several special cases of a conjecture that first appears in Alspach et al.~(1987) which stipulates that the wreath (lexicographic) product of two hamiltonian decomposable directed graphs is also hamiltonian decomposable. Specifically, we show that the wreath product of hamiltonian decomposable directed graph $G$, such that $|V(G)|$ is even and $|V(G)|\geqslant 3$, with a directed $m$-cycle such that $m \geqslant 4$ or the complete symmetric directed graph on $m$ vertices such that $m\geqslant 3$, is hamiltonian decomposable. We also show the wreath product of a directed $n$-cycle, where $n$ is even, with a directed $m$-cycle, where $m \in {2,3}$, is not hamiltonian decomposable.