# Homework Help: Putting a discrete sum of cosines in closed form

1. Apr 12, 2010

### H_man

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

I've just found what I think is the Green's function for a source between two ideal conducting planes at x = 0 and x = l:

2. Relevant equations

$$G(x,x') = \Sigma \frac{icos(\pi n x/l)}{(\pi n /l)}$$

3. The attempt at a solution

The question then wants me to put this in a closed form but I can't really see any useful identities to do this. Any hints/thoughts?? (perhaps my Green's function is wrong :yuck:

ohh..... the summation is from - infinity to infinity... and I must say that it was troubling me that I'll have a singularity in my summation at n=0.

Last edited: Apr 12, 2010