I would like to place a subscript under two summation signs in a fraction, in an equation. The code showed works when it is not in a fraction, but as soon as I use it in a fraction the subscript is placed after the two summation signs, instead of under the two summation signs.
\begin{equation} \frac{{\sum\sum\limits}_{i \neq j}
\sum\limits_{x}\sum\limits_{y}[\pi_{x(i),y(j)} -
\pi_{x(i)}\pi_{y(j)}]} {\sigma^2_X}
\end{equation}
Does anyone know how to solve this?
You need to enclose the two summation symbols in \mathop{} and then place \limits after that.
\documentclass{article}
\begin{document}
\begin{equation}
\frac{\mathop{\sum\sum}\limits_{i \neq j}
\sum\limits_{x}\sum\limits_{y}[\pi_{x(i),y(j)} -
\pi_{x(i)}\pi_{y(j)}]}{\sigma^2_X}
\end{equation}
\end{document}
\DeclareMathOperator*{\dsum}{\sum\sum}
– Sigur
Jan 28 '15 at 13:01
If you use lualatex, you can place the \limits outside of the braces:
{\sum\sum}\limits_{i\neq j}
However, this will throw an error if compiled with pdflatex or xelatex, then you need a \mathop:
\mathop{\sum\sum}\limits_{i\neq j}
If you need this more often, you can define a new mathoperator (needs amsmath), if you use the starred Version, the default is the use of \limits.
\DeclareMathOperator*{\ssum}{\sum\sum}
If you are inside a \frac, displaymode is turned of, you can switch it back on:
\begin{equation}
\frac{\displaystyle \ssum_{i\neq j}}{2}
\end{equation}
! Limit controls must follow a math operator. Please improve your answer.
– Henri Menke
Jan 28 '15 at 13:04
\documentclass{...}and ending with\end{document}. – Jan 28 '15 at 13:02