Extending partial edge-colorings of bounded size in Cartesian products of graphs

Abstract

This paper studies edge-precoloring extensions in Cartesian products of graphs, motivated by a conjecture of Casselgren, Petros, and Fufa. We formulate a general hypothesis stating that if every edge-precoloring of $G$ and $H$ of sizes $k<χ'(G)$ and $l<χ'(H)$, respectively, is extendable, then any edge-precoloring of $G \square H$ of size $k+l+1$ can be extended to a proper $(χ'(G)+χ'(H))$-coloring. We provide partial progress toward this conjecture by establishing the result in cases where $k<Δ(G)$, $G$ is a triangle-free $r$-regular graph and $H$ is a star, an even cycle, a path or, more generally, an arbitrary tree $F$. Furthermore, we prove the conjecture in the case where $G$ is a subcubic graph and $H = K_2$.

Figures (3)

• Figure 1: Scenarios of three vertical edges
• Figure 2: The extension of the graph $G$
• Figure 3: Neighborhood of $U$ in the case of identical pair of colors on the precolored edges

Theorems & Definitions (8)

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