On the existence of unparalleled even cycle systems

Abstract

A $2t$-cycle system of order $v$ is a set $\mathcal{C}$ of cycles whose edges partition the edge-set of $K_v-I$ (i.e., the complete graph minus the $1$-factor $I$). If $v\equiv 0 \pmod{2t}$, a set of $v/2t$ vertex-disjoint cycles of $\mathcal{C}$ is a parallel class. If $\mathcal{C}$ has no parallel classes, we call such a system unparalleled. We show that there exists an unparalleled $2t$-cycle system of order $v \equiv 0 \pmod{2t}$ if and only if $v>2t>2$.