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Let G = (V, E) be a graph of order n and let B(S) be the set of vertices in V \ S that have a neighbor in the vertex set S. The differential of a vertex set S is defined as ∂(S) = |B(S)| − |S| and the maximum value of ∂(S) for any subset S of V is the differential of G. A Roman dominating function of G is a function f : V → {0, 1, 2} such that every vertex u with f (u) = 0 is adjacent to a vertex v with f (v) = 2. The weight of a Roman dominating function is the value f (V ) = u∈V f (u). The minimum weight of a Roman dominating function of a graph G is the Roman domination number of G, written γR(G). We prove that γR(G) = n − ∂(G) and present several combinatorial, algorithmic and complexity-theoretic consequences thereof.

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