# Wave eqtion of electron in a periodic potential

1. Feb 8, 2010

### JayKo

quoted from Kittel text 8th edition page 170,171
the potential energy is U(x)=$$\Sigma$$$$U_{G}e^{iGx}$$

but why it is also equal to-> U(x)=$$\Sigma$$$$U_{G}e^{iGx}+e^{-iGx}$$

from text explanation, i couldn't get it why there is an extra term of $$e^{iGx}$$

any 1 care to enlight me on this? thanks.

2. Feb 14, 2010

### torehan

There is an extra term e-iGx to make potential energy U(x) a real function.