(adsbygoogle = window.adsbygoogle || []).push({}); 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 [itex]\omega_D[/itex] 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

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# Homework Help: Kittel page 112

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