A Set and Collection Lemma

A set S is independent if no two vertices from S are adjacent. In this paper we prove that if F is a collection of maximum independent sets of a graph, then there is a matching from S-{intersection of all members of F} into {union of all members of F}-S, for every independent set S. Based on this finding we give alternative proofs for a number of well-known lemmata, as the "Maximum Stable Set Lemma" due to Claude Berge and the "Clique Collection Lemma" due to Andr\'as Hajnal.Comment: 7 pages, 3 figure

On Konig-Egervary Collections of Maximum Critical Independent Sets

Let G be a simple graph with vertex set V(G). A set S is independent if no two vertices from S are adjacent. The graph G is known to be a Konig-Egervary if alpha(G)+mu(G)= |V(G)|, where alpha(G) denotes the size of a maximum independent set and mu(G) is the cardinality of a maximum matching. The number d(X)= |X|-|N(X)| is the difference of X, and an independent set A is critical if d(A) = max{d(I):I is an independent set in G} (Zhang; 1990). Let Omega(G) denote the family of all maximum independent sets. Let us say that a family Gamma of independent sets is a Konig-Egervary collection if |Union of Gamma| + |Intersection of Gamma| = 2alpha(G) (Jarden, Levit, Mandrescu; 2015). In this paper, we show that if the family of all maximum critical independent sets of a graph G is a Konig-Egervary collection, then G is a Konig-Egervary graph. It generalizes one of our conjectures recently validated in (Short; 2015).Comment: 9 pages, 3 figure

On some conjectures concerning critical independent sets of a graph

Let $G$ be a simple graph with vertex set $V(G)$. A set $S\subseteq V(G)$ is independent if no two vertices from $S$ are adjacent. For $X\subseteq V(G)$, the difference of $X$ is $d(X) = |X|-|N(X)|$ and an independent set $A$ is critical if $d(A) = \max \{d(X): X\subseteq V(G) \text{ is an independent set}\}$ (possibly $A=\emptyset$). Let $\text{nucleus}(G)$ and $\text{diadem}(G)$ be the intersection and union, respectively, of all maximum size critical independent sets in $G$. In this paper, we will give two new characterizations of K\"{o}nig-Egerv\'{a}ry graphs involving $\text{nucleus}(G)$ and $\text{diadem}(G)$. We also prove a related lower bound for the independence number of a graph. This work answers several conjectures posed by Jarden, Levit, and Mandrescu.Comment: 10 pages, 3 figure

Critical and Maximum Independent Sets of a Graph

Let G be a simple graph with vertex set V(G). A subset S of V(G) is independent if no two vertices from S are adjacent. By Ind(G) we mean the family of all independent sets of G while core(G) and corona(G) denote the intersection and the union of all maximum independent sets, respectively. The number d(X)= |X|-|N(X)| is the difference of the set of vertices X, and an independent set A is critical if d(A)=max{d(I):I belongs to Ind(G)} (Zhang, 1990). Let ker(G) and diadem(G) be the intersection and union, respectively, of all critical independent sets of G (Levit and Mandrescu, 2012). In this paper, we present various connections between critical unions and intersections of maximum independent sets of a graph. These relations give birth to new characterizations of Koenig-Egervary graphs, some of them involving ker(G), core(G), corona(G), and diadem(G).Comment: 12 pages, 9 figures. arXiv admin note: substantial text overlap with arXiv:1407.736

On the Critical Difference of Almost Bipartite Graphs

A set $S\subseteq V$ is \textit{independent} in a graph $G=\left( V,E\right)$ if no two vertices from $S$ are adjacent. The \textit{independence number} $\alpha(G)$ is the cardinality of a maximum independent set, while $\mu(G)$ is the size of a maximum matching in $G$. If $\alpha(G)+\mu(G)$ equals the order of $G$, then $G$ is called a \textit{K\"{o}nig-Egerv\'{a}ry graph }\cite{dem,ster}. The number $d\left( G\right) =\max\{\left\vert A\right\vert -\left\vert N\left( A\right) \right\vert :A\subseteq V\}$ is called the \textit{critical difference} of $G$ \cite{Zhang} (where $N\left( A\right) =\left\{ v:v\in V,N\left( v\right) \cap A\neq\emptyset\right\}$). It is known that $\alpha(G)-\mu(G)\leq d\left( G\right)$ holds for every graph \cite{Levman2011a,Lorentzen1966,Schrijver2003}. In \cite{LevMan5} it was shown that $d(G)=\alpha(G)-\mu(G)$ is true for every K\"{o}nig-Egerv\'{a}ry graph. A graph $G$ is \textit{(i)} \textit{unicyclic} if it has a unique cycle, \textit{(ii)} \textit{almost bipartite} if it has only one odd cycle. It was conjectured in \cite{LevMan2012a,LevMan2013a} and validated in \cite{Bhattacharya2018} that $d(G)=\alpha(G)-\mu(G)$ holds for every unicyclic non-K\"{o}nig-Egerv\'{a}ry graph $G$. In this paper we prove that if $G$ is an almost bipartite graph of order $n\left( G\right)$, then $\alpha(G)+\mu(G)\in\left\{ n\left( G\right) -1,n\left( G\right) \right\}$. Moreover, for each of these two values, we characterize the corresponding graphs. Further, using these findings, we show that the critical difference of an almost bipartite graph $G$ satisfies $$d(G)=\alpha(G)-\mu(G)=\left\vert \mathrm{core}(G)\right\vert -\left\vert N(\mathrm{core}(G))\right\vert ,$$ where by \textrm{core}$\left( G\right)$ we mean the intersection of all maximum independent sets.Comment: 12 pages, 5 figures. arXiv admin note: text overlap with arXiv:1102.472

Critical Independent Sets of a Graph

Let $G$ be a simple graph with vertex set $V\left( G\right)$. A set $S\subseteq V\left( G\right)$ is independent if no two vertices from $S$ are adjacent, and by $\mathrm{Ind}(G)$ we mean the family of all independent sets of $G$. The number $d\left( X\right) =$ $\left\vert X\right\vert -\left\vert N(X)\right\vert$ is the difference of $X\subseteq V\left( G\right)$, and a set $A\in\mathrm{Ind}(G)$ is critical if $d(A)=\max \{d\left( I\right) :I\in\mathrm{Ind}(G)\}$ (Zhang, 1990). Let us recall the following definitions: $\mathrm{core}\left( G\right)$ = $\bigcap$ {S : S is a maximum independent set}. $\mathrm{corona}\left( G\right)$ = $\bigcup$ {S :S is a maximum independent set}. $\mathrm{\ker}(G)$ = $\bigcap$ {S : S is a critical independent set}. $\mathrm{diadem}(G)$ = $\bigcup$ {S : S is a critical independent set}. In this paper we present various structural properties of $\mathrm{\ker}(G)$, in relation with $\mathrm{core}\left( G\right)$, $\mathrm{corona}\left( G\right)$, and $\mathrm{diadem}(G)$.Comment: 15 pages; 12 figures. arXiv admin note: substantial text overlap with arXiv:1102.113

Monotonic Properties of Collections of Maximum Independent Sets of a Graph

Let G be a simple graph with vertex set V(G). A subset S of V(G) is independent if no two vertices from S are adjacent. The graph G is known to be a Konig-Egervary if alpha(G) + mu(G)= |V(G)|, where alpha(G) denotes the size of a maximum independent set and mu(G) is the cardinality of a maximum matching. Let Omega(G) denote the family of all maximum independent sets, and f be the function from the set of subcollections Gamma of Omega(G) such that f(Gamma) = (the cardinality of the union of elements of Gamma) + (the cardinality of the intersection of elements of Gamma). Our main finding claims that f is "<<"-increasing, where the preorder {Gamma1} << {Gamma2} means that the union of all elements of {Gamma1} is a subset of the union of all elements of {Gamma2}, while the intersection of all elements of {Gamma2} is a subset of the intersection of all elements of {Gamma1}. Let us say that a family {Gamma} is a Konig-Egervary collection if f(Gamma) = 2*alpha(G). We conclude with the observation that for every graph G each subcollection of a Konig-Egervary collection is Konig-Egervary as well.Comment: 15 pages, 7 figure

On an Annihilation Number Conjecture

Let $\alpha(G)$ denote the cardinality of a maximum independent set, while $\mu(G)$ be the size of a maximum matching in the graph $G=\left(V,E\right)$. If $\alpha(G)+\mu(G)=\left\vert V\right\vert$, then $G$ is a K\"onig-Egerv\'ary graph. If $d_{1}\leq d_{2}\leq\cdots\leq d_{n}$ is the degree sequence of $G$, then the annihilation number $h\left(G\right)$ of $G$ is the largest integer $k$ such that $\sum\limits_{i=1}^{k}d_{i}\leq\left\vert E\right\vert$ (Pepper 2004, Pepper 2009). A set $A\subseteq V$ satisfying $\sum \limits_{a\in A} deg(a)\leq\left\vert E\right\vert$ is an annihilation set, if, in addition, $deg\left(v\right) +\sum\limits_{a\in A} deg(a)>\left\vert E\right\vert$, for every vertex $v\in V(G)-A$, then $A$ is a maximal annihilation set in $G$. In (Larson & Pepper 2011) it was conjectured that the following assertions are equivalent: (i) $\alpha\left(G\right) =h\left(G\right)$; (ii) $G$ is a K\"onig-Egerv\'ary graph and every maximum independent set is a maximal annihilating set. In this paper, we prove that the implication "(i) $\Longrightarrow$ (ii)" is correct, while for the opposite direction we provide a series of generic counterexamples. Keywords: maximum independent set, matching, tree, bipartite graph, K\"onig-Egerv\'ary graph, annihilation set, annihilation number.Comment: 17 pages, 11 figure

Critical Sets in Bipartite Graphs

Let G=(V,E) be a graph. A set S is independent if no two vertices from S are adjacent, alpha(G) is the size of a maximum independent set, and core(G) is the intersection of all maximum independent sets. The number d(X)=|X|-|N(X)| is the difference of the set X, and d_{c}(G)=max{d(I):I is an independent set} is called the critical difference of G. A set X is critical if d(X)=d_{c}(G). For a graph G we define ker(G) as the intersection of all critical independent sets, while diadem(G) is the union of all critical independent sets. For a bipartite graph G=(A,B,E), with bipartition {A,B}, Ore defined delta(X)=d(X) for every subset X of A, while delta_0(A)=max{delta(X):X is a subset of A}. Similarly is defined delta_0(B). In this paper we prove that for every bipartite graph G=(A,B,E) the following assertions hold: d_{c}(G)=delta_0(A)+delta_0(B); ker(G)=core(G); |ker(G)|+|diadem(G)|=2*alpha(G).Comment: 13 pages, 8 figure

Critical sets, crowns, and local maximum independent sets

A set $S\subseteq V(G)$ is independent (or stable) if no two vertices from $S$ are adjacent, and by $\mathrm{Ind}(G)$ we mean the set of all independent sets of $G$. A set $A\in\mathrm{Ind}(G)$ is critical (and we write $A\in CritIndep(G)$) if $\left\vert A\right\vert -\left\vert N(A)\right\vert =\max\{\left\vert I\right\vert -\left\vert N(I)\right\vert :I\in \mathrm{Ind}(G)\}$, where $N(I)$ denotes the neighborhood of $I$. If $S\in\mathrm{Ind}(G)$ and there is a matching from $N(S)$ into $S$, then $S$ is a crown, and we write $S\in Crown(G)$. Let $\Psi(G)$ be the family of all local maximum independent sets of graph $G$, i.e., $S\in\Psi(G)$ if $S$ is a maximum independent set in the subgraph induced by $S\cup N(S)$. In this paper we show that $CritIndep(G)\subseteq Crown(G)$ $\subseteq\Psi(G)$ are true for every graph. In addition, we present some classes of graphs where these families coincide and form greedoids or even more general set systems that we call augmentoids.Comment: 19 pages, 11 figure