Bounds on the domination number of a digraph and its reverse

Abstract

Let D be a digraph. A dominating set of D is the subset S of V(D) such that each vertex in V(D)?S is an out-neighbor of a vertex in S. The minimum cardinality of a dominating set of G is denoted by ?(D). We let D?denote the reverse of D. In [Discrete Math. 197/198 (1999) 179-183], Chartrand, Harary and Yue proved that every connected digraph D of order n ? 2 satisfies ?(D)+ ?(D?) ? 4n 3 and characterized the digraphs D attaining the equality. In this paper, we pose a reduction of the determining problem for (D)+(D?) using the total domination concept. As a corollary of such a reduction and known results, we give new bounds for (D)+(D?) and an alternative proof of Chartrand-Harary-Yue theorem.