# math subarray/substack environment not working

I am trying to get stacked indices using subarray/substack using the following snippet, but I am not seeing stacking beneath the sum symbol. This is the type of output I am expecting,

but this is what I am getting:

Here is the code:

\documentclass{report}
\usepackage{amsmath, amsfonts, amssymb}
\begin{document}
$\frac{\sum_{\begin{subarray}{l}\forall x \\ \mathcal{D}(x)=\mathcal{D}_i\end{subarray}} \sum_{\substack{\forall y\in\Omega(x) \\ \mathcal{D}(y)=\mathcal{D}_j}} 1} {\sum_{\forall x }\mathcal{D}(x)}$
\end{document}


What should I do to get proper stacking?

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So you are after output that resembles this? Issue \displaystyle in the numerator and denominator of \frac. –  Werner Aug 25 at 17:39
@werner, thanks. wish it's possible to accept a solution in a comment too :) –  Jey Aug 25 at 18:16

You need to add the directive \displaystyle at the start of both the numerator and denominator. I would also add \, (thinspace) between the two main terms in the numerator.

\documentclass{report}
\usepackage{amsmath, amsfonts, amssymb}
\begin{document}
$\frac{\displaystyle\sum_{\begin{subarray}{l}\forall x \\ \mathcal{D}(x)=\mathcal{D}_i\end{subarray}} \, \sum_{\substack{\forall y\in\Omega(x) \\ \mathcal{D}(y)=\mathcal{D}_j}} 1} {\displaystyle\sum_{\forall x }\mathcal{D}(x)}$
\end{document}


For more on this subject, you may want to consult the posting Fractions with large elements.

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Or \limits, but perhaps in this case \displaystyle is better. –  egreg Aug 25 at 18:04
@Mico, it worked!! –  Jey Aug 25 at 18:15