Complexity Aspects of Variants of Independent Roman Domination in Graphs

For a simple, undirected graph G = (V, E), an independent Roman dominating function (IRDF) f : V → {0, 1, 2} has the property that every vertex v ∈ V with f (v) = 0 is adjacent to at least one vertex u, with f (u) = 2, and no two vertices assigned positive values are adjacent.An independent Roman {2}-dominating function (IR2DF) is an IRDF f with an additional property that, for every vertex v ∈ V with f (v) = 0, there exists at least two vertices x, y ∈ NG(v) with f (x) = f (y) = 1. An independent double Roman dominating function (IDRDF) on G is a function f : V → {0, 1, 2, 3}, such that for every vertex v ∈ V, if f (v) = 0, then v has at least two neighbors x, y with f (x) = f (y) = 2 or one neighbor w with f (w) = 3, and if f (v) = 1, then v must have at least one neighbor w with f (w) ≥ 2, and no two vertices assigned positive values are adjacent. The weight of an IRDF (IR2DF, IDRDF) f is the sum f (V) = v∈V f (v). Given a graph G and a positive integer k, the independent Roman domination problem (IRDP), independent Roman {2}- domination problem (IR2DP) and independent double Roman domination problem (IDRDP), respectively, is to check whether G has an IRDF, IR2DF, and IDRDF of weight at most k. The IRDP, IR2DP and IDRDP are known to be NP-complete for bipartite graphs. We investigate the complexity of these problems for dually chordal graphs and some subclasses of bipartite graphs, namely, star convex bipartite graphs and comb convex bipartite graphs. We also investigate the complexity of IR2DP and IDRDP for chordal graphs. The minimum independent Roman domination problem (MIRDP), minimum independent Roman {2}-domination problem (MIR2DP), and minimum independent double Roman domination problem (MIDRDP), respectively, are to find an IRDF, IR2DF, and IDRDF of minimum weight in the input graph. We show that MIRDP, MIR2DP, and MIDRDP are linear time solvable for bounded treewidth graphs, chain graphs, and threshold graphs, a subclass of split graphs. We also show that theMIRDP,MIR2DP, and MIDRDP are APX-hard for graphs with bounded degree 4. Finally, we show that the domination problem and IRDP (IR2DP, IDRDP) are not equivalent in computational complexity aspects.