Coloring squares of planar graphs with small maximum degree (2105.11235v1)

Abstract: For a graph $G$, by $\chi_2(G)$ we denote the minimum integer $k$, such that there is a $k$-coloring of the vertices of $G$ in which vertices at distance at most 2 receive distinct colors. Equivalently, $\chi_2(G)$ is the chromatic number of the square of $G$. In 1977 Wegner conjectured that if $G$ is planar and has maximum degree $\Delta$, then $\chi_2(G) \leq 7$ if $\Delta \leq 3$, $\chi_2(G) \leq \Delta+5$ if $4 \leq \Delta \leq 7$, and $\lfloor 3\Delta/2 \rfloor +1$ if $\Delta \geq 8$. Despite extensive work, the known upper bounds are quite far from the conjectured ones, especially for small values of $\Delta$. In this work we show that for every planar graph $G$ with maximum degree $\Delta$ it holds that $\chi_2(G) \leq 3\Delta+4$. This result provides the best known upper bound for $6 \leq \Delta \leq 14$.