Schrodinger in a weak periodic potential

[Waxterzz]

Hi,

I have problems figuring out what both c-coefficients means.

Can someone explain this in a conceptual way?

Also, Vg and V-g are the coefficients of the Fourier series if you expand V, but what does it mean physically?

Thanks in advance.

 

[Waxterzz]

Vg is just the amplitude of your periodic potential, isn't it? :)

(my god)

And V-g, the amplitude of periodic potential in the next Brillouin Zone?