On $(1^2,2^2)$-packing edge-coloring of sparse subcubic graphs

Abstract

For positive integers $\ell$ and $k$, a $(1^\ell, 2^k)$-packing edge-coloring of a graph $G$ is a partition of $E(G)$ into $\ell$ matchings and $k$ induced matchings. A graph is $d$-irregular if it has no adjacent vertices of degree $d$. Yang and Wu proved that every $3$-irregular subcubic graph admits a $(1,2^4)$-packing edge-coloring, which answered an open question of Hocquad, Lajou, and Lu\v zar in the affirmative. In this paper, we prove an analogue result that every $3$-irregular subcubic multigraph is $(1^2,2^2)$-packing edge-colorable. Our result is sharp since there are $3$-irregular subcubic graphs that are not $(1,2^3)$-packing edge-colorable and $(1^2,2)$-packing edge-colorable, respectively. Hocquad, Lajou, and Lu\v zar conjectured that every subcubic planar graph is $(1^2,2^3)$-packing edge-colorable. Furthermore, they found a subcubic planar graph with girth $3$ that is not $(1^2,2^2)$-packing edge-colorable. For every fixed integer $k \ge 3$, we found graphs with girth $k$ that are not $(1^2,2)$- and not $(1,2^3)$-packing edge-colorable. It is natural to consider the question "what is the minimum positive integer $g$ such that every subcubic planar graph with girth at least $g$ is $(1^2,2^2)$-packing edge-colorable?". We prove $g$ is finite and in fact $g \le 20$. We also provide an example showing $g \ge 6$.