Vertex Partitions of Graphs into Cographs and Stars

A cograph is a graph that contains no path on four vertices as an induced subgraph. A cographk-partition of a graph G = (V,E) is a vertex partition of G into k sets V-1, ..., V-k < subset of> V so that the graph induced by V-i is a cograph for 1 i k. Gimbel and Neetil [5] studied the complexity aspects of the cograph k-partitions and raised the following questions: Does there exist a triangle-free planar graph that is not cograph 2-partitionable? If the answer is yes, what is the complexity of the associated decision problem? In this article, we prove that such an example exists and that deciding whether a triangle-free planar graph admits a cograph 2-partition is NP-complete. We also show that every graph with maximum average degree at most ??? admits a cograph 2-partition such that each component is a star on at most three vertices. (C) 2013 Wiley Periodicals, Inc.