sum(e^(-(n^2)pi*x))

[Wizlem]

I was reading a book on the zeta function and came across this attributed to Jacobi. I have no idea where to find a source about this so maybe someone can give me some direction. Let

\psi(x) = {\sum}^{\infty}}_{n=1}e^{-n^2 \pi x}.

How do you show that

\frac{1+2\psi(x)}{1+2\psi(1/x)} = \frac{1}{\sqrt{x}}

[jackmell]

Try framing it in terms of the theta function:

\theta(t)=\sum_{n=-\infty}^{\infty}e^{-\pi n^2 t}

which has the functional equation:

\theta(t)=t^{-1/2} \theta(1/t)

Now, express your \psi function in terms of this functional equation.

Also, see "Complex Analysis" by Stein and Shakarchi.