Latex: how break a line when summing over two things

Question

Lets say I want to sum over i \in S and i \in T. Currently I use:

\displaystyle \sum_{i \in S, i \in T} i

But this will display the sum-overs one after the other and not one above the other like I want.

How can I do this?

thanks

Answer 1

I don't understand the formula you are trying to typeset, but maybe this is what you want?

\sum_{i \in S \atop i \in T} i

Answer 2

\atop doesn't give the right spacing. You should use amsmath and use \substack:

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{equation}
\sum_{i \in S \atop i \in T} i
\end{equation}

\begin{equation}
\sum_{\substack{i \in S\\ i \in T}} i
\end{equation}

\end{document}

The result from \atop:

The result from \substack: