New results on the Wiener index of trees with a given diameter

We study the Wiener index of a class of trees with fixed diameter and order. A double broom is a tree such that there exist two vertices $u$ and $v$, such that each leaf of $T$ is adjacent to $u$ or $v$. We prove that for a tree $T$ of diameter $d$ and (sufficiently large) order $n$ such that $n\leq d-2+4 \left\lfloor \sqrt{ \frac{d-1}{2}} \right\rfloor$, $T$ has maximum Wiener index (in the class of trees of diameter $d$ and order $n$) if and only if $T$ is a balanced double broom. Our results are sharp up to a small constant.