# 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}

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

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

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I think \limits is the better choice since it does not change the size of the summation sign. –  Michael Ummels Mar 18 '11 at 16:20
This answer seems to work, too. Unfortunately, it's not possible to mark two answers as correct. –  Thomas Mar 18 '11 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. –  Michael Ummels Mar 18 '11 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. –  night owl Jun 30 '11 at 6:25
@night Read my comment again. I say that \limits is a better choice exactly because it does not change the size. –  Michael Ummels Jun 30 '11 at 10:08