On the Outer Independent Double Roman Domination Number

An outer independent (double) Roman dominating function is a (double) Roman dominating function f for which the set of vertices assigned 0 under f is independent. The outer independent (double) Roman domination number (γoid R(G)) γoi R(G) is the minimum weight taken over all outer independent (double) Roman dominating functions of G. A vertex cover number β(G) is the minimum size of any vertex cover sets of a graph G. In this work, we present some contributions to the study of outer independent double Roman domination in graphs. Characterizations of the families of all connected graphs with small outer independent double Roman domination numbers, and tight lower and upper bounds on this parameter are given. We also prove that the decision problem associated with γoid R(G) is NP-complete even when restricted to planar graphs with maximum degree at most four. We moreover prove that 2β(T )+1 ≤ γoid R(T ) ≤ 3β(T ) for any tree T , and show that each integer between the lower and upper bounds is realizable. Finally, we give an exact formula for this parameter concerning the corona graphs.