Roman Reinforcement Numbers of Digraphs

Let \( D = (V,A) \) be a finite and simple digraph. A Roman dominating function (RDF) on \( D \) is a labeling \( f: V(D) \to \{0,1,2\} \) such that every vertex \( v \) with label \( 0 \) has a vertex \( w \) with label \( 2 \) such that \( wv \) is an arc in \( D \). The weight of an RDF \( f \) is the value \( \omega(f) = \sum_{v \in V} f(v) \). The Roman domination number of a digraph \( D \), denoted by \( \gamma_R(D) \), equals the minimum weight of an RDF on \( D \). The Roman reinforcement number \( r_R(D) \) of a digraph \( D \) is the minimum number of arcs that must be added to \( D \) in order to decrease the Roman domination number. In this paper, we initiate the study of Roman reinforcement number in digraphs and we present some sharp bounds for \( r_R(D) \). In particular, we determine the Roman reinforcement number of some classes of digraphs.