Quasi total double Roman domination in trees

A quasi total double Roman dominating function (QTDRD-function) on a graph $G=(V(G),E(G))$ is a function $f:V(G)\longrightarrow\{0,1,2,3\}$ having the property that \textrm{(i)} if $f(v)=0$, then vertex $v$ must have at least twoneighbors assigned 2 under $f$ or one neighbor $w$ with $f(w)=3$; \textrm{(ii)} if $f(v)=1$, then vertex $v$ has at least one neighbor $w$ with $f(w)\geq2$, and \textrm{(iii)} if $x$ is an isolated vertex in the subgraph induced by the set of vertices assigned non-zero values, then $f(x)=2$. The weight of a QTDRD-function $f$ is the sum of its function values over the whole vertices, and the quasi total double Roman domination number $\gamma_{qtdR}(G)$ equals the minimum weight of a QTDRD-function on $G$. In this paper, we show that for any tree $T$ of order $n\ge 4$, $\gamma_{qtdR}(T)\le n+\frac{s(T)}{2}$, where $s(T)$ is the number of support vertices of $T$,  that improves a known bound.