Star coloring

In graph-theoretic mathematics, a star coloring of a graph G is a (proper) vertex coloring in which every path on four vertices uses at least three distinct colors. Equivalently, in a star coloring, the induced subgraphs formed by the vertices of any two colors has connected components that are star graphs. Star coloring has been introduced by Grünbaum (1973).The star chromatic number χ s ( G ) {displaystyle chi _{s}(G)} of G is the least number of colors needed to star color G. In graph-theoretic mathematics, a star coloring of a graph G is a (proper) vertex coloring in which every path on four vertices uses at least three distinct colors. Equivalently, in a star coloring, the induced subgraphs formed by the vertices of any two colors has connected components that are star graphs. Star coloring has been introduced by Grünbaum (1973).The star chromatic number χ s ( G ) {displaystyle chi _{s}(G)} of G is the least number of colors needed to star color G. One generalization of star coloring is the closely related concept of acyclic coloring, where it is required that every cycle uses at least three colors, so the two-color induced subgraphs are forests. If we denote the acyclic chromatic number of a graph G by χ a ( G ) {displaystyle chi _{a}(G)} , we have that χ a ( G ) ≤ χ s ( G ) {displaystyle chi _{a}(G)leq chi _{s}(G)} , and in fact every star coloring of G is an acyclic coloring. The star chromatic number has been proved to be bounded on every proper minor closed class by Nešetřil & Ossona de Mendez (2003). This results was further generalized by Nešetřil & Ossona de Mendez (2006) to all low-tree-depth colorings (standard coloring and star coloring being low-tree-depth colorings with respective parameter 1 and 2). It was demonstrated by Albertson et al. (2004) that it is NP-complete to determine whether χ s ( G ) ≤ 3 {displaystyle chi _{s}(G)leq 3} , even when G is a graph that is both planar and bipartite.Coleman & Moré (1984) showed that finding an optimal star coloring is NP-hard even when G is a bipartite graph.