Optimal dispersive estimates for the wave equation with C^((n-3)/2) potentials in dimensions 4le nle 7

We prove optimal dispersive estimates for the wave group $e^{it\sqrt{-\Delta+V}}$ for a class of real-valued potentials $V\in C^{(n-3)/2}({\bf R}^n)$, $4\le n\le 7$, such that $\partial_x^\alpha V(x)=O(|x|^{-\delta})$ for $|x|>1$, where $\delta>(n+1)/2$.