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Often in physics, the symbol \sim is used as a unary operator to mean "approximately."

For example, one might say "the energy of these processes is $\sim 4$ eV." However, this looks awkward because \sim is a binary operator and the spacing is incorrect. How can one use \sim act as a unary operator?

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${\sim}4$ or $\mathord{\sim}4$ –  Werner Oct 25 '12 at 21:31
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\sim is a relation symbol, rather than a binary operation; if it were then $\sim 4$ would give no space. It's probably better to define a new command for it as an "Ord": \newcommand{\appx}{{\sim}}. Consider using siunitx for typesetting units of measure. –  egreg Oct 25 '12 at 21:34
    
This question is a duplicate of tex.stackexchange.com/q/55701 –  Philippe Goutet Oct 26 '12 at 5:30
    
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1 Answer

up vote 8 down vote accepted

Just define a new command for it:

\newcommand{\ca}{{\sim}}

I suggest also to use siunitx, if you have units of measure to typeset in your document: it ensures uniform setting.

\documentclass{article}
\newcommand{\ca}{{\sim}}

\usepackage{siunitx}
\sisetup{input-protect-tokens=\ca,input-symbols=\ca}

\begin{document}
\SI{\ca 4}{eV}

$\ca 4$\,eV
\end{document}

The input might seem more difficult, but it's surely rewarding.

enter image description here

If you load amssymb you can change the definition to

\usepackage{amssymb}
\newcommand{\ca}{{\thicksim}}

and the result would be

enter image description here

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