Characterizing paths graphs on bounded degree trees by minimal forbidden induced subgraphs

An undirected graph GG is called a VPT graph if it is the vertex intersection graph of a family of paths in a tree. The class of graphs which admit a VPT representation in a host tree with maximum degree at most hh is denoted by [h,2,1][h,2,1]. The classes [h,2,1][h,2,1] are closed under taking induced subgraphs, therefore each one can be characterized by a family of minimal forbidden induced subgraphs. In this paper we associate the minimal forbidden induced subgraphs for [h,2,1][h,2,1] which are VPT with (color) hh-critical graphs. We describe how to obtain minimal forbidden induced subgraphs from critical graphs, even more, we show that the family of graphs obtained using our procedure is exactly the family of VPT minimal forbidden induced subgraphs for [h,2,1][h,2,1]. The members of this family together with the minimal forbidden induced subgraphs for VPT (Lévêque et al., 2009; Tondato, 2009), are the minimal forbidden induced subgraphs for [h,2,1][h,2,1], with h≥3h≥3. By taking h=3h=3 we obtain a characterization by minimal forbidden induced subgraphs of the class V PT∩EPT=EPT∩Chordal=[3,2,2]=[3,2,1] (see Golumbic and Jamison, 1985).