Monophonic number of Kneser graphs and strongly 2-monophonic graphs

Abstract

Given a graph $G$ a set $S\subset V(G)$ is called monophonic if every vertex in $G$ lies on some induced path between two vertices in $S$. The monophonic number, $m(G)$, of $G$, which is the smallest cardinality of a monophonic set in $G$, has been studied from various perspectives. In this paper, we establish $m(K(n,r))$ for all Kneser graphs $K(n,r)$, where $n\ge 2r$. In addition, when $r\ge 3$, we prove an even stronger property, notably that every pair of non-adjacent vertices in $K(n,r)$ forms a monophonic set. We call the graphs satisfying this property strongly $2$-monophonic graphs. We present several (sufficient and necessary) conditions for a graph to be strongly $2$-monophonic, and prove that the Cartesian product of any two strongly $2$-monophonic graphs is also such. Besides non-complete Hamming graphs, we also prove that every Johnson graph is strongly $2$-monophonic, whereas chordal graphs, with the exception of the graphs $K_n-e$, do not enjoy this property.