# Homework Help: Kittel page 112

1. Mar 21, 2008

### ehrenfest

1. The problem statement, all variables and given/known data
This question refers to Kittel's solid-state physics book.

Above equation (23), Kittel says that a cutoff frequency $\omega_D$ is determined by (19) as" and then he gives equation (23). I have absolutely no idea where equation (23) comes from.

2. Relevant equations

3. The attempt at a solution

2. Mar 24, 2008

### genneth

Could you reproduce the equations for those of us who don't have the book handy?