Uniquely 2-colourable 4-cycle decompositions

Abstract

A cycle system of order $n$ is a decomposition of the edges of the complete graph $K_n$ into cycles of a fixed length. A cycle system is said to be $k$-colourable if we can assign $k$ colours to its vertices so that no cycle is monochromatic. A $k$-colourable cycle system is uniquely $k$-colourable if its colouring is unique up to the permutation of colour classes. In this paper, we construct uniquely $2$-colourable $4$-cycle systems of order $n$ for all admissible $n\ge 49$, and also uniquely $2$-colourable $4$-cycle decompositions of $K_n-I$, for all admissible $n\ge 50$. These constructions contribute to the broader study of uniquely colourable cycle systems and open new directions for future research.