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The vertex-rainbow index of a graph

The $k$-rainbow index $rx_k(G)$ of a connected graph $G$ was introduced by Chartrand, Okamoto and Zhang in 2010. As a natural counterpart of the $k$-rainbow index, we introduced the concept of $k$-vertex-rainbow index $rvx_k(G)$ in this paper. For a graph $G=(V,E)$ and a set $S\subseteq V$ of at least two vertices, \emph{an $S$-Steiner tree} or \emph{a Steiner tree connecting $S$} (or simply, \emph{an $S$-tree}) is a such subgraph $T=(V',E')$ of $G$ that is a tree with $S\subseteq V'$. For $S\subseteq V(G)$ and $|S|\geq 2$, an $S$-Steiner tree $T$ is said to be a \emph{vertex-rainbow $S$-tree} if the vertices of $V(T)\setminus S$ have distinct colors. For a fixed integer $k$ with $2\leq k\leq n$, the vertex-coloring $c$ of $G$ is called a \emph{$k$-vertex-rainbow coloring} if for every $k$-subset $S$ of $V(G)$ there exists a vertex-rainbow $S$-tree. In this case, $G$ is called \emph{vertex-rainbow $k$-tree-connected}. The minimum number of colors that are needed in a $k$-vertex-rainbow coloring of $G$ is called the \emph{$k$-vertex-rainbow index} of $G$, denoted by $rvx_k(G)$. When $k=2$, $rvx_2(G)$ is nothing new but the vertex-rainbow connection number $rvc(G)$ of $G$. In this paper, sharp upper and lower bounds of $srvx_k(G)$ are given for a connected graph $G$ of order $n$,\ that is, $0\leq srvx_k(G)\leq n-2$. We obtain the Nordhaus-Guddum results for $3$-vertex-rainbow index, and show that $rvx_3(G)+rvx_3(\overline{G})=4$ for $n=4$ and $2\leq rvx_3(G)+rvx_3(\overline{G})\leq n-1$ for $n\geq 5$. Let $t(n,k,\ell)$ denote the minimal size of a connected graph $G$ of order $n$ with $rvx_k(G)\leq \ell$, where $2\leq \ell\leq n-2$ and $2\leq k\leq n$. The upper and lower bounds for $t(n,k,\ell)$ are also obtained.