This is LaTeX code:

[latex]
S:=\sum_{(m,n) \in \mathbb{N}\times \mathbb{N}}\frac{mn^2}{2^n(m2^n+n2^m)}=\sum_{m=1}^{\infty}\sum_{n=1}^\infty \frac{1}{a_n(a_m+a_n)}=
[/latex]