Semi-perfect 1-Factorizations of the Hypercube

Abstract

A 1-factorization $\mathcal{M}=\{M_1,M_2,\dots ,M_n\}$ of a graph $G$ is called perfect if the union of any pair of 1-factors $M_i,M_j$ with $i\ne j$ is a Hamilton cycle. It is called $k$-semi-perfect if the union of any pair of 1-factors $M_i,M_j$ with $1\le i\le k$ and $k+1\le j\le n$ is a Hamilton cycle. We consider 1-factorizations of the discrete cube $Q_d$. There is no perfect 1-factorization of $Q_d$, but it was previously shown that there is a 1-semi-perfect 1-factorization of $Q_d$ for all $d$. Our main result is to prove that there is a $k$-semi-perfect 1-factorization of $Q_d$ for all $k$ and all $d$, except for one possible exception when $k=3$ and $d=6$. This is, in some sense, best possible. We conclude with some questions concerning other generalisations of perfect 1-factorizations.