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On the strong chromatic number of graphs

The strong chromatic number, $χ_S(G)$, of an $n$-vertex graph $G$ is the smallest number $k$ such that after adding $k\lceil n/k\rceil-n$ isolated vertices to $G$ and considering {\bf any} partition of the vertices of the resulting graph into disjoint subsets $V_1, \ldots, V_{\lceil n/k\rceil}$ of size $k$ each, one can find a proper $k$-vertex-coloring of the graph such that each part $V_i$, $i=1, \ldots, \lceil n/k\rceil$, contains exactly one vertex of each color. For any graph $G$ with maximum degree $Δ$, it is easy to see that $χ_S(G)\geqΔ+1$. Recently, Haxell proved that $χ_S(G) \leq 3Δ-1$. In this paper, we improve this bound for graphs with large maximum degree. We show that $χ_S(G)\leq 2Δ$ if $Δ\geq n/6$ and prove that this bound is sharp.