Bipartite Rainbow Numbers of Matchings

Abstract

Given two graphs $G$ and $H$, let $f(G,H)$ denote the maximum number $c$ for which there is a way to color the edges of $G$ with $c$ colors such that every subgraph $H$ of $G$ has at least two edges of the same color. Equivalently, any edge-coloring of $G$ with at least $rb(G,H)=f(G,H)+1$ colors contains a rainbow copy of $H$, where a rainbow subgraph of an edge-colored graph is such that no two edges of it have the same color. The number $rb(G,H)$ is called the {\it rainbow number of $H$ with respect to $G$}, and simply called the {\it bipartite rainbow number of $H$} if $G$ is the complete bipartite graph $K_{m,n}$. Erdős, Simonovits and Sós showed that $rb(K_n,K_3)=n$. In 2004, Schiermeyer determined the rainbow numbers $rb(K_n,K_k)$ for all $n\ge k\ge 4$, and the rainbow numbers $rb(K_n,k{K}_2)$ for all $k\ge 2$ and $n\ge 3k+3$. In this paper we will determine the rainbow numbers $rb(K_{m,n},k{K}_2)$ for all $k\ge 1$.

8 pages