Minimal non-comparability graphs and semi-transitivity

TL;DR

The paper addresses the problem of identifying the intersection between minimal non-comparability graphs and minimal non-word-representable graphs. It classifies minimal non-comparability graphs into those that are semi-transitive and those that are not, and shows the intersection consists of two infinite families, $G_n^9$ and $G_n^4$, plus a single graph $H_1$. It further derives a complete list of minimal non-word-representable graphs containing an all-adjacent vertex by augmenting semi-transitive minimal non-comparability graphs with an all-adjacent vertex, revealing several infinite families of minimal non-word-representable graphs. These results deepen the forbidden-subgraph understanding of word-representable graphs and provide constructive avenues to generate new obstructions.

Abstract

The concept of word-representable graphs has been widely explored in the literature. The class of word-representable graphs is characterized by the existence of a semi-transitive orientation. Specifically, a graph is word-representable if and only if it admits such an orientation. Comparability graphs form a subclass of word-representable graphs. Both word-representable and comparability graphs belong to hereditary graph classes. Every hereditary class can be characterized in terms of their forbidden induced subgraphs. The minimal forbidden induced subgraphs of comparability graphs and word-representable graphs are referred to as minimal non-comparability graphs and minimal non-word-representable graphs, respectively. While the complete set of minimal non-comparability graphs is known, identifying the set of all minimal non-word-representable graphs remains an open problem. In this paper, we precisely determine the set of all minimal non-comparability graphs that are minimal non-word-representable graphs as well. To achieve this, we categorize all minimal non-comparability graphs into those that are semi-transitive and those that are not. Furthermore, as a byproduct of our classification, we establish a characterization and a complete list of minimal non-word-representable graphs that contain an all-adjacent vertex. This is accomplished by introducing an all-adjacent vertex to each minimal non-comparability graph that is semi-transitive. As a result of our study, we identify several infinite families of minimal non-word-representable graphs, expanding the understanding of their structural properties.