Graphs with equal domination and covering numbers

Abstract

A dominating set of a graph G is a set Dsubseteq V_G such that every vertex in V_G-D is adjacent to at least one vertex in D, and the domination number gamma (G) of G is the minimum cardinality of a dominating set of G. A set Csubseteq V_G is a covering set of G if every edge of G has at least one vertex in C. The covering number beta (G) of G is the minimum cardinality of a covering set of G. The set of connected graphs G for which gamma (G)=beta (G) is denoted by {mathcal {C}}_{gamma =beta }, whereas {mathcal {B}} denotes the set of all connected bipartite graphs in which the domination number is equal to the cardinality of the smaller partite set. In this paper, we provide alternative characterizations of graphs belonging to {mathcal {C}}_{gamma =beta } and {mathcal {B}}. Next, we present a quadratic time algorithm for recognizing bipartite graphs belonging to {mathcal {B}}, and, as a side result, we conclude that the algorithm of Arumugam et al. (Discrete Appl Math 161:1859–1867, 2013) allows to recognize all the graphs belonging to the set {mathcal {C}}_{gamma =beta } in quadratic time either. Finally, we consider the related problem of patrolling grids with mobile guards, and show that it can be solved in O(n log n + m) time, where n is the number of line segments of the input grid and m is the number of its intersection points.