Given a graph \(G=(V,E)\), a function \(f:V\rightarrow \{0,1,2\}\) is said to be a Roman Dominating function (RDF) if for every \(v\in V\) with \(f(v)=0\), there exists a vertex \(u\in N(v)\) such that \(f(u)=2\). A Roman Dominating function f is said to be an Independent Roman Dominating function (IRDF), if \(V_1\cup V_2\) forms an independent set, where \(V_i=\{v\in V~\vert ~f(v)=i\}\), for \(i\in \{0,1,2\}\). The total weight of f is equal to \(\sum _{v\in V} f(v)\), and is denoted as w(f). The Roman Domination Number (resp. Independent Roman Domination Number) of G, denoted by \(\gamma _R(G)\) (resp. \(i_R(G)\)), is defined as min\(\{w(f)~\vert ~f\) is an RDF (resp. IRDF) of \(G\}\). For a given graph G, the problem of computing \(\gamma _R(G)\) (resp. \(i_R(G)\)) is defined as the Roman Domination problem (resp. Independent Roman Domination problem).
In this paper, we examine structural parameterizations of the (Independent) Roman Domination problem. We propose fixed-parameter tractable (FPT) algorithms for the (Independent) Roman Domination problem in graphs that are k vertices away from a cluster graph. These graphs have a set of k vertices whose removal results in a cluster graph. We refer to k as the distance to the cluster graph. Specifically, we prove the following results when parameterized by the deletion distance k to cluster graphs: we can find the Roman Domination Number (and Independent Roman Domination Number) in time \(4^kn^{O(1)}\). In terms of lower bounds, we show that the Roman Domination number can not be computed in time \(2^{\epsilon k}n^{O(1)}\), for any \(0<\epsilon <1\) unless a well-known conjecture, SETH fails. In addition, we also show that the Roman Domination problem, parameterized by distance to cluster, does not admit a polynomial kernel unless NP \(\subseteq \) coNP/poly.