Symmetric Hamilton Cycle Decompositions of Complete Multigraphs

Let $n ≥ 3$ and $⋋ ≥ 1$ be integers. Let $⋋K_n$ denote the complete multigraph with edge-multiplicity $⋋$. In this paper, we show that there exists a symmetric Hamilton cycle decomposition of $⋋K_{2m}$ for all even $⋋ ≥ 2$ and $m ≥ 2$. Also we show that there exists a symmetric Hamilton cycle decomposition of $⋋K_{2m} − F$ for all odd $⋋ ≥ 3$ and $m ≥ 2$. In fact, our results together with the earlier results (by Walecki and Brualdi and Schroeder) completely settle the existence of symmetric Hamilton cycle decomposition of $⋋K_n$ (respectively, $⋋K_n − F$, where $F$ is a 1-factor of $⋋K_n$) which exist if and only if $⋋(n − 1)$ is even (respectively, $⋋(n − 1)$ is odd), except the non-existence cases n ≡ 0 or 6 (mod 8) when ⋋ = 1