Some upper bounds for the $3$-proper index of graphs

A tree $T$ in an edge-colored graph is a {\it proper tree} if no two adjacent edges of $T$ receive the same color. Let $G$ be a connected graph of order $n$ and $k$ be a fixed integer with $2\le k\le n$. For a vertex subset $S \subseteq V(G)$ with $\left|S\right| \ge 2$, a tree containing all the vertices of $S$ in $G$ is called an $S$-tree. An edge-coloring of $G$ is called a \emph{$k$-proper coloring} if for every $k$-subset $S$ of $V(G)$, there exists a proper $S$-tree in $G$. For a connected graph $G$, the \emph{$k$-proper index} of $G$, denoted by $px_k(G)$, is the smallest number of colors that are needed in a $k$-proper coloring of $G$. In this paper, we show that for every connected graph $G$ of order $n$ and minimum degree $δ\geq 3$, $px_{3}(G)\le n\frac{\ln(δ+1)}{δ+1}(1+o_δ(1))+2$. We also prove that for every connected graph $G$ with minimum degree at least $3$, $px_{3}(G) \le px_{3}(G[D])+3$ when $D$ is a connected $3$-way dominating set of $G$ and $px_{3}(G) \le px_{3}(G[D])+1$ when $D$ is a connected $3$-dominating set of $G$. In addition, we obtain tight upper bounds of the 3-proper index for two special graph classes: threshold graphs and chain graphs. Finally, we prove that $px_3(G) \le \lfloor\frac{n}{2}\rfloor$ for any 2-connected graphs with at least four vertices.

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