On the upper Bound of double Roman dominating function

A double Roman Dominating function on a graph $G$ is a function $f:V\to \{0,1,2,3\}$ such that the following conditions hold. If $f(v)=0$, then vertex $v$ must have at least two neighbors in $V_2$ or one neighbor in $V_3$ and if $f(v)=1$, then vertex $v$ must have at least one neighbor in $V_2\bigcup V_3$. The weight of a double Roman dominating function is the sum $w_f={\sum }_{v\in V(G)}f(v)$. In this paper, we improve the upper bounds of ${\gamma }_{dR}(G)$ that has already obtained and we show that ${\gamma }_{dR}(G)\le {\displaystyle \frac{12n}{11}}$, for any graph with $\delta (G)\ge 2$. This bound improve the bounds that have already been presented in \cite{chen} and \cite{kkcs}. Finally we prove the conjecture posed in \cite{kkcs}.