Characterizing [h,2,1] graphs by minimal forbidden induced subgraphs

TL;DR

This paper characterizes the minimal forbidden induced subgraphs for the class [h,2,1] of graphs, which are VPT graphs with a host tree of maximum degree h, using h-critical graphs and their properties.

Contribution

It introduces a method to derive minimal forbidden induced subgraphs for [h,2,1] from h-critical graphs, providing a complete characterization for these graph classes.

Findings

The family of forbidden subgraphs for [h,2,1] is exactly those obtained from h-critical graphs.

For h=3, the characterization aligns with the class VPT ∩ EPT.

The approach unifies the understanding of minimal forbidden subgraphs across different h values.

Abstract

An undirected graph G 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 h is denoted by [h,2,1]. The classes [h,2,1] are closed by 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] which are VPT with (color) h-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]. The members of this family together with the minimal forbidden induced subgraphs for VPT, are the minimal forbidden induced subgraphs for [h,2,1], with $h\geq 3$. Notice that by taking h=3 we obtain a characterization by minimal forbidden induced subgraphs of the class VPT $\cap$ EPT=EPT $\cap$ Chordal=[3,2,2]=[3,2,1].