Induced subgraphs and tree-decompositions VII. Basic obstructions in $H$-free graphs

TL;DR

This paper characterizes when classes of H-free graphs are 'clean', meaning large treewidth guarantees certain induced subgraphs, and extends results to include forbidden connected graphs containing H.

Contribution

It provides a complete characterization of H-free graph classes that are clean and strengthens results to include forbidden connected subgraphs containing H.

Findings

H-free classes are clean iff H is a forest of subdivided stars

Forbidden connected graphs containing H also lead to clean classes

Provides a description of unavoidable induced subgraphs in large graphs

Abstract

We say a class $\mathcal{C}$ of graphs is clean if for every positive integer $t$ there exists a positive integer $w(t)$ such that every graph in $\mathcal{C}$ with treewidth more than $w(t)$ contains an induced subgraph isomorphic to one of the following: the complete graph $K_t$, the complete bipartite graph $K_{t,t}$, a subdivision of the $(t\times t)$-wall or the line graph of a subdivision of the $(t \times t)$-wall. In this paper, we adapt a method due to Lozin and Razgon (building on earlier ideas of Wei{\ss}auer) to prove that the class of all $H$-free graphs (that is, graphs with no induced subgraph isomorphic to a fixed graph $H$) is clean if and only if $H$ is a forest whose components are subdivided stars. Their method is readily applied to yield the above characterization. However, our main result is much stronger: for every forest $H$ as above, we show that forbidding certain connected graphs containing $H$ as an induced subgraph (rather than $H$ itself) is enough to obtain a clean class of graphs. Along the proof of the latter strengthening, we build on a result of Davies and produce, for every positive integer $\eta$, a complete description of unavoidable connected induced subgraphs of a connected graph $G$ containing $\eta$ vertices from a suitably large given set of vertices in $G$. This is of independent interest, and will be used in subsequent papers in this series.