Ch 26.1 In a periodic potential $V(\mathbf{r} + \mathbf{R}) = V(\mathbf{r})$, the energy eigenstates can be written as $\psi_{n\mathbf{k}}(\mathbf{r}) = e^{i\mathbf{k}\cdot\mathbf{r}}u_{n\mathbf{k}}(\mathbf{r})$, where $u_{n\mathbf{k}}$ has the same periodicity as the lattice. The quantum number $\m