Hamiltonian decompositions of the wreath product of hamiltonian decomposable digraphs

Abstract: We affirm most open cases of a conjecture that first appeared 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 a hamiltonian decomposable directed graph $G$, such that $|V(G)|$ is even and $|V(G)|\geqslant 2$, with a hamiltonian decomposable directed graph $H$, such that $|V(H)| \geqslant 4$, is also hamiltonian decomposable except possibly when $G$ is a directed cycle and $H$ is a directed graph of odd order that admits a decomposition into $c$ directed hamiltonian cycle where $c$ is odd and $3\leqslant c \leqslant |V(H)|-2$.