On the Intersection of Tolerance and Cocomparability Graphs

Abstract

It has been conjectured by Golumbic and Monma in 1984 that the intersection of tolerance and cocomparability graphs coincides with bounded tolerance graphs. The conjecture has been proved under some - rather strong - \emph{structural} assumptions on the input graph; in particular, it has been proved for complements of trees, and later extended to complements of bipartite graphs, and these are the only known results so far. Our main result in this article is that the above conjecture is true for every graph $G$ that admits a tolerance representation with exactly one unbounded vertex; note here that this assumption concerns only the given tolerance \emph{representation} $R$ of $G$, rather than any structural property of $G$. Moreover, our results imply as a corollary that the conjecture of Golumbic, Monma, and Trotter is true for every graph $G=(V,E)$ that has no three independent vertices $a,b,c\in V$ such that $N(a) \subset N(b) \subset N(c)$; this is satisfied in particular when $G$ is the complement of a triangle-free graph (which also implies the above-mentioned correctness for complements of bipartite graphs). Our proofs are constructive, in the sense that, given a tolerance representation $R$ of a graph $G$, we transform $R$ into a bounded tolerance representation $R^{\ast}$ of $G$. Furthermore, we conjecture that any \emph{minimal} tolerance graph $G$ that is not a bounded tolerance graph, has a tolerance representation with exactly one unbounded vertex. Our results imply the non-trivial result that, in order to prove the conjecture of Golumbic, Monma, and Trotter, it suffices to prove our conjecture.