Burling graphs as intersection graphs

Abstract

For a subset $ S $ of $ \mathbb R^d$, $ S$-graphs are the intersection graphs of specific transformations of $ S $. The class of Burling graphs is a class of triangle-free graphs with arbitrarily large chromatic number that has attracted much attention in the last years. In 2012, Pawlik, Kozik, Krawczyk, Laso\'n, Micek, Trotter, and Walczak showed that for every compact and path-connected set $ S \subseteq \mathbb R^2$ that is different from an axis-parallel rectangle, the class of $ S $-graphs contains all Burling graphs. There is, however, a gap between the two classes. In recent years, there have been improvements in understanding the subclasses of $ S $-graphs that are closer or equal to Burling graphs. In this article, we close this gap for every set $ S $ with the mentioned properties: we introduce the class of constrained $ S $-graphs, a subclass of $ S$-graphs, and prove that it is equal to the class of Burling graphs. We also introduce the class of constrained graphs, a subclass of intersection graphs of subsets of $ \mathbb R^2$, and prove that it is equal to the class of Burling graphs.