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

enter image description here

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

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2  
You mean \langle, \rangle? –  percusse Aug 16 at 13:43
    
@percusse: Yes. Please put it in answer –  dexterdev Aug 16 at 13:44

3 Answers 3

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}

enter image description here

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A mathtools solution:

\documentclass{article}

\usepackage{mathtools}

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


\begin{document}

\begin{equation}
 \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{equation}

\end{document}

output

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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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}

enter image description here

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3  
Nowadays, it is generally preferred to use \[...\] instead of $$...$$. –  moewe Aug 16 at 13:51
4  
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 at 13:52

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