Equitable coloring of corona products of cubic graphs is harder than ordinary coloring
DOI:
https://doi.org/10.26493/1855-3974.687.99bKeywords:
Corona graph, cubic graph, equitable chromatic number, equitable graph coloring, NP-hardness, polynomial algorithmAbstract
A graph is equitably k-colorable if its vertices can be partitioned into k independent sets in such a way that the number of vertices in any two sets differ by at most one. The smallest k for which such a coloring exists is known as the equitable chromatic number of G and it is denoted by χ = (G). In this paper the problem of determinig χ = for coronas of cubic graphs is studied. Although the problem of ordinary coloring of coronas of cubic graphs is solvable in polynomial time, the problem of equitable coloring becomes NP-hard for these graphs. We provide polynomially solvable cases of coronas of cubic graphs and prove the NP-hardness in a general case. As a by-product we obtain a simple linear time algorithm for equitable coloring of such graphs which uses χ = (G) or χ = (G) + 1 colors. Our algorithm is best possible, unless P=NP. Consequently, cubical coronas seem to be the only known class of graphs for which equitable coloring is harder than ordinary coloring.
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