# Kittel page 174

1. Feb 29, 2008

### ehrenfest

1. The problem statement, all variables and given/known data
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"?

2. Relevant equations

3. The attempt at a solution

Last edited: Feb 29, 2008
2. Mar 1, 2008

### malawi_glenn

Then you only sum over zero.