# How to implement this symbol in LaTeX? [duplicate]

I wanted to do the sigma enclosures in the below shown equation correctly. Actually this is also expectation symbol, right? I don't know it's name.

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## marked as duplicate by yo', m0nhawk, Mico, Svend Tveskæg, Paul GesslerAug 16 '14 at 14:26

You mean \langle, \rangle? – percusse Aug 16 '14 at 13:43
@percusse: Yes. Please put it in answer – dexterdev Aug 16 '14 at 13:44

A mathtools solution:

\documentclass{article}

\usepackage{mathtools}

\DeclarePairedDelimiter\ExpValue{\langle}{\rangle}
\newcommand*\diff{\mathop{}\!\mathit{d}}

\begin{document}

$$\ExpValue*{\sigma_{N}^{2}} = \int_{-\infty}^{\infty} \diff\sigma P(\sigma)\sigma^{2} \simeq \delta_{k}\sum_{i = 1}^{k} P(\sigma_{k})\sigma_{k}^{2}$$

\end{document}


Note that with the starred version of \ExpValue, ther delimiters will scale automatically.

P.S. Are you sure that \simeq shouldn't be \approx? (I'm not sure...)

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If you are typing lot of physics, then you may consider using physics package. It provides lot of goodies including \expval

\documentclass{article}

\usepackage{physics}

\begin{document}
$\expval{\sigma_{N}^{2}} = \int_{-\infty}^{\infty}\mathrm{d}\sigma P(\sigma)\sigma^{2} \simeq \delta_{k}\sum_{i=1}^{k} P(\sigma_{k})\sigma_{k}^{2}$
$\expval{A}{\Psi} \quad \ev{A}{\Psi} \quad$
\end{document}


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Reproduced equation:

\documentclass[varwidth]{standalone}

\begin{document}
$\langle\sigma_{N}^{2}\rangle = \int_{-\infty}^{\infty}\mathrm{d}\sigma P(\sigma)\sigma^{2} \simeq \delta_{k}\sum_{i=1}^{k} P(\sigma_{k})\sigma_{k}^{2}$
\end{document}


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– moewe Aug 16 '14 at 13:51
Please do not use $$ in a LaTeX document to initiate and terminate display-math mode. For more information, see the postings Why is $...$ preferable to $$ ... $$? and What are the differences between $$, \[, align, equation and displaymath? – Mico Aug 16 '14 at 13:52