# Kittel page 174

## Homework Statement

This question refers to Kittel's solid-state physics book.

On this page, in the section about the "Kronig-Penney Model in Reciprocal Space", I do not understand why the sum over s is from 0 to 1/a. What if a is 2?

Also, how do they know that "the boundary conditions are periodic over a ring of unit length"?

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