A tamed family of triangle-free graphs with unbounded chromatic number

Abstract

We construct a hereditary class of triangle-free graphs with unbounded chromatic number, in which every non-trivial graph either contains a pair of non-adjacent twins or has an edgeless vertex cutset of size at most two. This answers in the negative a question of Chudnovsky, Penev, Scott, and Trotignon. The class is the hereditary closure of a family of (triangle-free) twincut graphs $G_1,G_2,\dots$ such that $G_k$ has chromatic number $k$. We also show that every twincut graph is edge-critical.