How to write ~2.1 in LaTex?

Question

When I type '~2.1' here, the distance between ~ and 2.1 is perfect (i.e. exactly what I want). How can it be achieved in LaTeX?

With $\sim\,2.1$, the distance between them is too large.

Answer 1

\sim acts like a relation and therefore is surrounded by a \thickmuskip, similar to what you'll find around =:

$a \sim b$
$a = b$

If you add \showoutput to your document, you'll find the spacing inserted in the .log:

....\mathon                                    <- $
....\OML/cmm/m/it/10 a                         <- a
....\glue(\thickmuskip) 2.77771 plus 2.77771   <- \thickmuskip
....\OMS/cmsy/m/n/10 ^^X                       <- \sim
....\penalty 500                               <- ...penalty to limit line breaks
....\glue(\thickmuskip) 2.77771 plus 2.77771   <- \thickmuskip
....\OML/cmm/m/it/10 b                         <- b
....\mathoff                                   <- $
    ----------------------------------------
....\mathon                                    <- $
....\OML/cmm/m/it/10 a                         <- a
....\glue(\thickmuskip) 2.77771 plus 2.77771   <- \thickmuskip
....\OT1/cmr/m/n/10 =                          <- =
....\penalty 500                               <- ...penalty to avoid line breaks
....\glue(\thickmuskip) 2.77771 plus 2.77771   <- \thickmuskip
....\OML/cmm/m/it/10 b                         <- b
....\mathoff                                   <- $

You can change the setting of \sim as a relation to be set as an ordinary math element using {\sim} or \mathord{\sim}, and then add spacing to suit your needs.

\documentclass{article}
\begin{document}
\begin{tabular}{ l l }
  \verb|\sim 2.1|: &
    $\sim 2.1$ \
  \verb|\sim , 2.1|: &
    $\sim , 2.1$ \
  \verb|{\sim} 2.1|: &
    ${\sim} 2.1$ \
  \verb|\mathord{\sim} 2.1|: &
    $\mathord{\sim} 2.1$ \
  \verb|{\sim} , 2.1|: &
    ${\sim} , 2.1$ \
  \verb|\mathord{\sim} , 2.1|: &
    $\mathord{\sim} , 2.1$
\end{tabular}
\end{document}

Answer 2

\documentclass{article}

\begin{document}
%>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
$\sim\!2.1$

$\sim2.1$
%<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
\end{document}