# Kittel page 112

1. Homework Statement
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. Homework Equations

3. The Attempt at a Solution

## Answers and Replies

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