If the scattering potential has the translation invariance property $V(\mathbf{r}+\mathbf{R})=V(\mathbf{r})$, where $\mathbf{R}$ is a constant vector, show that in the first Born approximation, as in the exact formulation (Exercise 13.7), scattering occurs only when the momentum transfer $\mathbf{q}$ (in units of $\hbar$ ) equals a reciprocal lattice vector $\mathbf{G}$.