Fair Domination Number in Cactus Graphs

For k ≥ 1, a k-fair dominating set (or just kFD-set) in a graph G is a dominating set S such that |N(v) ∩ S| = k for every vertex v ∈ V \ S. The k-fair domination number of G, denoted by fd_k(G), is the minimum cardinality of a kFD-set. A fair dominating set, abbreviated FD-set, is a kFD-set for some integer k ≥ 1. The fair domination number, denoted by fd(G), of G that is not the empty graph, is the minimum cardinality of an FD-set in G. In this paper, aiming to provide a particular answer to a problem posed in [Y. Caro, A. Hansberg and M.A. Henning, Fair domination in graphs, Discrete Math. 312 (2012) 2905–2914], we present a new upper bound for the fair domination number of a cactus graph, and characterize all cactus graphs G achieving equality in the upper bound of fd₁(G).