# Why 'displaystyle' is not working?

I am typing some mathematical expression related to 'displaystyle'. This is working well for $\displaystyle\sum_{i=1}^nx_i=1$. But it is not working in the following example :

\documentclass[10pt]{article}
\usepackage{amsmath}

\begin{document}
$P \displaystyle\simeq_p Q$
\end{document}


This gives :

But my desired output is :

Anyone can tell me what is the problem here ?

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Thank you in advance. – Soumitra Sen Apr 12 '13 at 14:23
The result is what I expect from \simeq_p. Why do you think it should be different? Setting displaystyle doesn't stack symbols as far as I know. – Martin Scharrer Apr 12 '13 at 14:23
You should usually use \stackrel from the stackrel package or \underset from the amsmath package for such constructions. Cf. tex.stackexchange.com/q/39225/15925 – Andrew Swann Apr 12 '13 at 14:30
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Only limits on \mathop operators (like \sum) change in dispaystyle. Limits on relational infix operators stay in subscript position.
$P \displaystyle\mathrel{\mathop{\simeq}_p} Q$