Induced subgraphs and tree decompositions XII. Grid theorem for pinched graphs

TL;DR

This paper characterizes the structure of c-pinched graphs with large treewidth, showing they must contain specific large induced subgraphs, thus extending the grid theorem to this class.

Contribution

It provides a complete grid-type theorem for c-pinched graphs, identifying the unavoidable large induced subgraphs when the treewidth is large.

Findings

Large treewidth in c-pinched graphs implies presence of specific large induced subgraphs.

Characterization includes complete graphs, bipartite graphs, walls, line-graphs of walls, or Pohoata-Davies graphs.

Main theorem generalizes to graphs with cycles of bounded length.

Abstract

Given an integer $c\in \mathbb{N}$, we say a graph $G$ is $c$-pinched if $G$ does not contain an induced subgraph consisting of $c$ cycles, all going through a single common vertex and otherwise pairwise disjoint and with no edges between them. What can be said about the structure of $c$-pinched graphs? For instance, $1$-pinched graphs are exactly graphs of treewidth $1$. However, bounded treewidth for $c>1$ is immediately seen to be a false hope because complete graphs, complete bipartite graphs, subdivided walls and line graphs of subdivided walls are all examples of $2$-pinched graphs with arbitrarily large treewidth. There is even a fifth obstruction for larger values of $c$, discovered by Pohoata and later independently by Davies, consisting of $3$-pinched graphs with unbounded treewidth and no large induced subgraph isomorphic to any of the first four obstructions. We fuse the above five examples into a grid-type theorem fully describing the unavoidable induced subgraphs of pinched graphs with large treewidth. More precisely, we prove that for every integer $c\in \mathbb{N}$, a $c$-pinched graph $G$ has large treewidth if and only if $G$ contains one of the following as an induced subgraph: a large complete graph, a large complete bipartite graph, a subdivision of a large wall, the line-graph of a subdivision of a large wall, or a large graph from the Pohoata-Davies construction. Our main result also generalizes to an extension of pinched graphs where the lengths of excluded cycles are lower-bounded.