Double sum over i and j with i \neq j

Question

I would like to represent a double sum with i\neq j. So I want two capital sigmas with text i\neq j underneath them. How can I achieve this?

Answer 1

\mathop allows you to perform operator-related things (like sub-/superscripting underneath/over top in display style) on non-operator symbols:

\documentclass{article}
\begin{document}
\[ \mathop{\sum\sum}_{i\neq j} \]
\end{document}

Using amsmath, it is also possible to declare a new operator, say \sumsum, if you're using this notation on a more frequent basis:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\DeclareMathOperator*{\sumsum}{\sum\sum}
\begin{document}
\[ \sumsum_{i\neq j} x_{ij} \]
\end{document}