A note on zero-sum Ramsey numbers of complete graphs

Abstract

For a graph $H$ with $3\mid e(H)$, the zero-sum Ramsey number $R(H,{\mathbb{Z}}_3)$ is the least integer $N$ such that every labeling of the edges of $K_N$ by elements of ${\mathbb{Z}}_3$ contains a copy of $H$ whose edge labels sum to zero. We determine the last previously unresolved infinite family in the complete-graph case modulo $3$. More precisely, we prove that \(R(K_n,\Z_3)=n+3\) for every $n\ge 10$ satisfying $n\equiv 1(\mathrm{mod}3)$. Consequently, for $k\ge 1$, \(R(K_{9k+7},\Z_3)=9k+10\), resolving a problem of Caro and Mifsud.