# Sums in math mode: how to display index under the sigma sign?

I have a fraction with sums above and under the line. How can I convince LaTeX to write the indices of the sums under the sigma instead of next to it?

\begin{displaymath}
\frac{\sum_{s \in S} s^2}{\sum_{p \in P} p^2}
\end{displaymath}


\begin{displaymath}
\frac{\sum\limits_{s \in S} s^2}{\sum\limits_{p \in P} p^2}
\end{displaymath}

• Might be useful. LaTeX insists on the fact that "Limit controls must follow a math operator". BTW the sumlimits option could be given to the amsmath package on loading. That automatically affects the placement of limits when in display math, not inline math. Cfr. tex.stackexchange.com/a/32827/4735 Commented Sep 4, 2017 at 12:59
\begin{displaymath}
\frac{\displaystyle\sum_{s \in S} s^2}{\displaystyle\sum_{p \in P} p^2}
\end{displaymath}

• I think \limits is the better choice since it does not change the size of the summation sign. Commented Mar 18, 2011 at 16:20
• This answer seems to work, too. Unfortunately, it's not possible to mark two answers as correct. Commented Mar 18, 2011 at 16:28
• @Thomas: As I said, the difference between the two answers lies in the size of the summation signs, which is changed by \displaystyle in this case. Commented Mar 18, 2011 at 16:32
• @Michael: It is not the \limits that changed the size of the sigma symbols, it was the use of \displaystyle. I think your first comment should be deleted for confusion reasons. Commented Jun 30, 2011 at 6:25
• @night Read my comment again. I say that \limits is a better choice exactly because it does not change the size. Commented Jun 30, 2011 at 10:08