On 1-Konig-Egervary Graphs

Abstract

Let $\alpha(G)$ denote the cardinality of a maximum independent set, while $\mu(G)$ be the size of a maximum matching in $G=\left( V,E\right) $. Let $\xi(G)$ denote the size of the intersection of all maximum independent sets. It is known that if $\alpha(G)+\mu(G)=n(G)=\left\vert V\right\vert $, then $G$ is a K\"{o}nig-Egerv\'{a}ry graph. If $\alpha(G)+\mu(G)=n(G) -1$, then $G$ is a $1$-K\"{o}nig-Egerv\'{a}ry graph. If $G$ is not a K\"{o}nig-Egerv\'{a}ry graph, and there exists a vertex $v\in V$ (an edge $e\in E$) such that $G-v$ ($G-e$) is K\"{o}nig-Egerv\'{a}ry, then $G$ is called a vertex (an edge) almost K\"{o}nig-Egerv\'{a}ry graph (respectively). The critical difference $d(G)$ is $\max\{d(I):I\in\mathrm{Ind}(G)\}$, where $\mathrm{Ind}(G)$ denotes the family of all independent sets of $G$. If $A\in\mathrm{Ind}(G)$ with $d\left( X\right) =d(G)$, then $A$ is a critical independent set. Let $diadem (G)=\bigcup\{S:S$ is a critical independent set in $G\}$, and $\varrho_{v}\left( G\right) $ denote the number of vertices $v\in V\left( G\right) $, such that $G-v$ is a K\"{o}nig-Egerv\'{a}ry graph. In this paper, we characterize all types of almost K\"{o}nig-Egerv\'{a}ry graphs and present interrelationships between them. We also show that if $G$ is a $1$-K\"{o}nig-Egerv\'{a}ry graph, then $\varrho_{v}\left( G\right) \leq n\left( G\right) +d\left( G\right) -\xi\left( G\right) -\beta(G)$, where $\beta(G)=\left\vert diadem(G)\right\vert $. As an application, we characterize the $1$-K\"{o}nig-Egerv\'{a}ry graphs that become K\"{o}nig-Egerv\'{a}ry after deleting any vertex.