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Possible Duplicate:
Separate long math text under sum symbol into different lines?

I'm trying to produce a product which has two subindices. Like this:

\[ \phi_n(X)=\prod_{z\in \mu_n \\ z \textup{primitive}} (X-z) \]

That doesn't work: the \\ is not recognized.

marked as duplicate by Caramdir, Martin Scharrer Mar 27 '11 at 18:43

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    See if tex.stackexchange.com/questions/12344 explains what you need. – Andrey Vihrov Mar 27 '11 at 17:15
  • Wonderful, that's exactly what I needed. I guess I didn't search using the correct terms. Thanks! – Bruno Stonek Mar 27 '11 at 17:18
  • If the above linked question solved your problem this question should be closed as duplicate. I can do that if you don't mind. – Martin Scharrer Mar 27 '11 at 17:58
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The line break \\ doesn't work inside math like that. You can use e.g. \shortstack to stack the indices:

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

\[ \phi_n(X)=\prod_{\shortstack{$\scriptstyle z\in \mu_n $\\$\scriptstyle z \textup{primitive}$}} (X-z) \]

\end{document}
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    What's the difference between this and \prod_{\substack{z\in \mu_n \\ z \textup{ primitive}}} (X-z) ? – Bruno Stonek Mar 27 '11 at 17:23
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    @Bruno: I didn't now \substack before. It seems to be the better solution. The \shortstack macro is however also useful in other situations (e.g. textmode) where things have to be stacked. – Martin Scharrer Mar 27 '11 at 17:25

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