Equitable chromatic threshold of Kronecker products of complete graphs

Abstract

A proper vertex coloring of a graph is equitable if the sizes of color classes differ by at most 1. The equitable chromatic threshold of a graph $G$, denoted by $\chi_=^*(G)$, is the minimum $k$ such that $G$ is equitably $k^\prime$-colorable for all $k^\prime \ge k$. Let $G\times H$ denote the direct product of graphs $G$ and $H$. For $n\ge m\ge 2$ we prove that $\chi_=^*(K_{m} \times K_n)$ equals $\lceil\frac{mn}{m+1}\rceil$ if $n\equiv 2,...,m (\textup{mod} m+1)$, and equals $m\lceil\frac{n}{s^\star}\rceil$ if $n\equiv 0,1 (\textup{mod} m+1)$, where $s^\star$ is the minimum positive integer such that $s^\star \nmid n$ and $s^\star\ge m+2.$