# How to evaluate this summation

1. Jan 17, 2009

### atchoh1983

hello guys,

I have tried to evaluate $$\Sigma$$ e-an2 so many times, but I didn't get it.

where a is just a constant and summation begin from n=1 to infinity.

I know that $$\Sigma$$en is just geometric series which is equal 1/(1-e)

But when n changes to be n2, I have no ideas how to do that.

If anyone know, please give me a suggestion.

with much thanks and appreciate!!

2. Jan 17, 2009

### element4

Im not sure, but I really dont think you can express

$$\sum_{n=1}^{\infty}e^{-an^2}$$

in closed form. But if you meant

$$\sum_{n=1}^{\infty}(e^{-an})^2$$

you can just do the following steps

$$\sum_{n=1}^{\infty}(e^{-an})^2$$ = $$\sum_{n=1}^{\infty}e^{-2an}$$ = $$\sum_{n=1}^{\infty}(e^{2a})^{-n}$$ = $$\frac 1{e^{2n}-1}$$

3. Jan 17, 2009

### peterungar

This is an elliptic theta function, see Handboook of Mathematical Functions by Milton Abramowitz and Irene Stegun, p. 576, 16.27.3 (Put z=1 and q=exp(-a).)