
A proper incidence $k$-coloring of a graph $G$ is a coloring of the incidences using $k$ colors in such a way that every two adjacent incidences have distinct colors. The minimum integer $k$ such that $G$ has a proper incidence $k$-coloring is the incidence chromatic number of $G$, denoted by $\chi_{i}(G)$. An incidence $(k,l)$-coloring of $G$ is a proper incidence $k$-coloring such that $|A_v|\leq l$ for each $v\in V(G)$. The authors provide the following conjecture. Conjecture 1. $\chi_{i}(G) \leq \Delta(G)+2 $ holds for every planar graph $G$. The authors confirm the conjecture for outer-1-planar graphs $G$ with $\Delta(G) \geq 8$ or $g(G) \geq 4$. Specifically, they prove the following results. Theorem 1. Every outer-1-planar graph $G$ has an incidence $(\Delta(G) + 3, 2)$-coloring. Theorem 2. Every outer-1-planar graph $G$ with $\Delta(G)\ge 8$ has an incidence $(\Delta(G) + 2, 2)$- coloring. Theorem 3. Every outer-1-planar graph $G$ with $g(G)\ge 4$ has an incidence $(\Delta(G) + 2, 2)$- coloring.
Coloring of graphs and hypergraphs, outer-1-planar graph, planar graph, incidence coloring, Planar graphs; geometric and topological aspects of graph theory
Coloring of graphs and hypergraphs, outer-1-planar graph, planar graph, incidence coloring, Planar graphs; geometric and topological aspects of graph theory
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