Limits under multiple \sum
Something like:
\mathop{\sum_{j=1}^{\infty}\sum_{k=1}^{\infty}}_{j>k}
maybe?
Looks like this:
Or if you want it on the same line as j=1 and k=1, then maybe:
\mathop{\sum^{\infty}\sum^{\infty}}_{j=1\ j>k\ k=1}
There is an equivalent alternative.
\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage{mathtools}
\begin{document}
\[
\underset{j>k}{\sum_{j=1}^{\infty}\sum_{k=1}^{\infty}}
\]
\end{document}