Restrained condition on double Roman dominating functions

We continue the study of restrained double Roman domination in graphs. For a graph $G=\big{(}V(G),E(G)\big{)}$, a double Roman dominating function $f$ is called a restrained double Roman dominating function (RDRD function) if the subgraph induced by $\{v\in V(G)\mid f(v)=0\}$ has no isolated vertices. The restrained double Roman domination number (RDRD number) $γ_{rdR}(G)$ is the minimum weight $\sum_{v\in V(G)}f(v)$ taken over all RDRD functions of $G$. We first prove that the problem of computing $γ_{rdR}$ is NP-hard even for planar graphs, but it is solvable in linear time when restricted to bounded clique-width graphs such as trees, cographs and distance-hereditary graphs. Relationships between $γ_{rdR}$ and some well-known parameters such as restrained domination number $γ_{r}$, domination number $γ$ and restrained Roman domination number $γ_{rR}$ are investigated in this paper by bounding $γ_{rdR}$ from below and above involving $γ_{r}$, $γ$ and $γ_{rR}$ for general graphs, respectively. We prove that $γ_{rdR}(T)\geq n+2$ for any tree $T\neq K_{1,n-1}$ of order $n\geq2$ and characterize the family of all trees attaining the lower bound. The characterization of graphs with small RDRD numbers is given in this paper.