I am interested in finding the exact value of the following sum.
$\sum_{l=1}^{\infty}\sum_{m=1}^{\infty}\frac{1}{\left(l^{2}+m^{2}\right)^{3/2}}.$
This looks similar to the Riemann zeta function (https://en.wikipedia.org/wiki/Riemann_zeta_function). I was wondering whether it is possible to evaluate the sum exactly.