How to implement this symbol in LaTeX?

Question

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.

Answer 1

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}

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...)

Answer 2

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}

Answer 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}