Linear bounds for rich-neighbor edge-colorings

Abstract: An edge in a proper edge-coloring is rich if all edges adjacent to it receive pairwise distinct colors. A rich-neighbor edge-coloring is a proper edge-coloring in which every non-isolated edge is adjacent to a rich edge. The rich-neighbor chromatic index $\rn(G)$ is the minimum number of colors in such a coloring. We prove that every loopless multigraph $G$ satisfies $\rn(G)\le3χ'(G)$, where $χ'(G)$ is the chromatic index. Exact values are also determined for complete bipartite graphs and complete graphs. For all positive integers $m,n$, $\rn(K_{m,n})=m+n-1$, except that $\rn(K_{2,2})=4$. For $n\ge3$, $\rn(K_n)=2n-3$, except that $\rn(K_4)=6$ and $\rn(K_6)=10$. Moreover, for $n\ge7$, the rich edges in every optimal rich-neighbor edge-coloring of $K_n$ form a star forest.