TY - JOUR

T1 - On the intersection of all critical sets of a unicyclic graph

AU - Levit, Vadim E.

AU - Mandrescu, Eugen

PY - 2014/1/10

Y1 - 2014/1/10

N2 - A set S ⊆ V is independent in a graph G=(V,E) if no two vertices from S are adjacent. The independence number α(G) is the cardinality of a maximum independent set, while μ(G) is the cardinality of a maximum matching in G. If α(G)+μ(G)=|V|, then G is a König-Egerváry graph. The number d(G)=max{|A|-|N(A)|:A⊆V} is the critical difference of G (Zhang, 1990) [22], where N(A)={v:v∈V,N(v)∩A ≠ Ø}. By core(G) (corona(G)) we denote the intersection (union, respectively) of all maximum independent sets, and by ker(G) we mean the intersection of all critical sets. A connected graph having only one cycle is called unicyclic. It is known that the relation ker(G)⊆ core (G) holds for every graph G (Levit, 2012) [14], while the equality is true for bipartite graphs (Levit, 2013) [15]. For König-Egerváry unicyclic graphs, the difference |core(G)|-|ker(G)| may equal any non-negative integer. In this paper we prove that if G is a non-König-Egerváry unicyclic graph, then: (i) ker(G)=core(G) and (ii) |corona(G)|+|core(G)|=2α(G)+1. Pay attention that |corona(G)|+|core(G)|=2α(G) holds for every König-Egerváry graph (Levit, 2011) [11].

AB - A set S ⊆ V is independent in a graph G=(V,E) if no two vertices from S are adjacent. The independence number α(G) is the cardinality of a maximum independent set, while μ(G) is the cardinality of a maximum matching in G. If α(G)+μ(G)=|V|, then G is a König-Egerváry graph. The number d(G)=max{|A|-|N(A)|:A⊆V} is the critical difference of G (Zhang, 1990) [22], where N(A)={v:v∈V,N(v)∩A ≠ Ø}. By core(G) (corona(G)) we denote the intersection (union, respectively) of all maximum independent sets, and by ker(G) we mean the intersection of all critical sets. A connected graph having only one cycle is called unicyclic. It is known that the relation ker(G)⊆ core (G) holds for every graph G (Levit, 2012) [14], while the equality is true for bipartite graphs (Levit, 2013) [15]. For König-Egerváry unicyclic graphs, the difference |core(G)|-|ker(G)| may equal any non-negative integer. In this paper we prove that if G is a non-König-Egerváry unicyclic graph, then: (i) ker(G)=core(G) and (ii) |corona(G)|+|core(G)|=2α(G)+1. Pay attention that |corona(G)|+|core(G)|=2α(G) holds for every König-Egerváry graph (Levit, 2011) [11].

KW - Core

KW - Corona

KW - Critical set

KW - Ker

KW - König-Egerváry graph

KW - Maximum independent set

KW - Unicyclic graph

UR - http://www.scopus.com/inward/record.url?scp=84887988570&partnerID=8YFLogxK

U2 - 10.1016/j.dam.2013.09.006

DO - 10.1016/j.dam.2013.09.006

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AN - SCOPUS:84887988570

SN - 0166-218X

VL - 162

SP - 409

EP - 414

JO - Discrete Applied Mathematics

JF - Discrete Applied Mathematics

ER -