3

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?

1
  • Welcome to TeX.SX! Please help us to help you and add a minimal working example (MWE) that illustrates your problem. It will be much easier for us to reproduce your situation and find out what the issue is when we see compilable code, starting with \documentclass{...} and ending with \end{document}.
    – user31729
    Jan 28, 2015 at 13:02

2 Answers 2

10

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}

enter image description here

2
  • +1. Also, I'd add the new operator double sum \DeclareMathOperator*{\dsum}{\sum\sum}
    – Sigur
    Jan 28, 2015 at 13:01
  • 3
    @Sigur True, even thought I would only declare a new operator if I were to use this construct multiple times. Jan 28, 2015 at 13:06
2

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}
7
  • It does not work. Limit controls must follow a math operator. ...} {\sum\sum}\limits
    – Sigur
    Jan 28, 2015 at 13:04
  • This won't work. ! Limit controls must follow a math operator. Please improve your answer. Jan 28, 2015 at 13:04
  • i compiled it! With lualatex using mathtools and unicode-math
    – MaxNoe
    Jan 28, 2015 at 13:05
  • @MaxNoe, in that case, it works.
    – Sigur
    Jan 28, 2015 at 13:08
  • 3
    @MaxNoe Aha! You found another tex vs. luatex incompatibility. Jan 28, 2015 at 13:08

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .