How to put a subscript in the center of two letters

Question

I have the following code \sum_{i<j}^{}\sum Cov(X_i, X_j) which renders

How can I move the i<j subscript to the center underneath the two sigmas (which will result in a "shift")? Preference will be given to the simplest solution.

Answer 1

\documentclass{article}
\usepackage{amsmath}

\DeclareMathOperator{\Cov}{Cov}
\DeclareMathOperator*{\doublesum}{\sum\sum}

\begin{document}

\[\doublesum_{i<j} \Cov(X_i, X_j)\]

\end{document} 

Answer 2

You can use \mathop to stack symbols under (as a subscript) or over (as a superscript) math operators:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}

\[
  \mathop{\sum\sum}_{i < j} \text{Cov}(X_i, X_j)
\]

\end{document}