Minimal toughness in subclasses of weakly chordal graphs

TL;DR

This paper explores the properties of minimally tough graphs within subclasses of weakly chordal graphs, providing classifications and simplifying existing results on these graphs' toughness characteristics.

Contribution

It offers complete classifications of minimally tough graphs in several subclasses of weakly chordal graphs, extending understanding of toughness in these graph classes.

Findings

Classified minimally tough graphs in co-chordal graphs with large complement diameter

Identified minimally tough net-free co-chordal graphs

Characterized minimally tough complements of forests and $P_4$-free graphs

Abstract

The toughness of a graph $G$ is defined as the largest real number $t$ such that for any set $S\subseteq V(G)$ such that $G-S$ is disconnected, $S$ has at least $t$ times more elements than $G-S$ has components (unless $G$ is complete, in which case the toughness is defined to be infinite). A graph is said to be minimally tough if deleting any edge decreases the toughness. It is an open question whether there exists a minimally tough non-complete chordal graph with toughness exceeding $1$. We initiate the study of minimally tough graphs in the larger class of weakly chordal graphs. We obtain complete classifications of minimally tough graphs in the following subclasses of weakly chordal graphs: co-chordal graphs whose complement has diameter at least $3$, net-free co-chordal graphs, complements of forests, $P_4$-free graphs, and complete multipartite graphs. Our approach leads to simple proofs of two results on minimally tough graphs due to Dallard, Fern\'andez, Katona, Milani\v{c}, and Varga.