Formula_1103

\[\begin{gathered}\tilde{u}(x, \hat{k})=\frac{2 \pi i}{k r}\left(e^{-i k r} \delta(\omega+x / r)-e^{i k r}(\delta(\omega-x / r)+\right. \\\left.+\frac{i k}{2 \pi} f(k, x / r)\right)+o\left(r^{-1}\right) .\end{gathered}\]