The minimum size of graphs with given rainbow index

The concept of $k$-rainbow index $rx_k(G)$ of a connected graph $G$, introduced by Chartrand, Okamoto and Zhang, is a natural generalization of the rainbow connection number. Let $t(n,k,\ell)$ denote the minimum size of a connected graph $G$ of order $n$ with $rx_k(G)\leq \ell$, where $2\leq \ell\leq n-1$ and $2\leq k\leq n$. In this paper, we obtain some exact values and some upper bounds for $t(n,k,\ell)$.