Formula_9436

\[\left[-\frac{\hbar^{2}}{2 m}(i \nabla+\boldsymbol{k})^{2}+U(\boldsymbol{r})\right] u_{\boldsymbol{k}}(\boldsymbol{r})=\mathscr{E}_{\boldsymbol{k}} U(\boldsymbol{r}),\]