Problem: Square Lattice

Consider two-dimensional electrons subjected to a weak periodic potential coming from a square lattice of spacing a=5 Å. For vectors far away from the Brillouin zone boundary, the wavefunction can be well described by plane waves. Assume we want to write the wavefunction in the Bloch form, . We consider a state of energy E and wavevector

What will the three lowest energies be at this wavenumber? What are the corresponding functions? (Note: eVÅ ).