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**1. Homework Statement**

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. Homework Equations**

**3. The Attempt at a Solution**