This is LaTeX code:
[latex]
\sum_{i=1}^{\infty}\sum_{j=1}^{\infty}|a_{i,j}|=\sum_{n=1}^{\infty}|a_{\pi(n)}|= \sum_{j=1}^{\infty}\sum_{i=1}^{\infty}|a_{i,j}|
[/latex]