1. The problem statement, all variables and given/known data Using geometrical arguments or otherwise, derive how the energy of an electron in the second Brillouin zone may be less than the energy of an electron in the first zone.  2. Relevant equations 3. The attempt at a solution I'm thinking this has something to do with overlapping bands, I think divalent metals show these characteristics. I'm not really sure where to start off from with a derivation though.