Code for the next symbol

Question

What is the latex code for the symbol in the picture? (That arrow with $~$)

Answer 1

You can choose among various possibilities:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$f_{\mathcal{C}} \overset{\sim}{\leftarrow} f$
$f_{\mathcal{C}} \xleftarrow{\sim} f$
$f_{\mathcal{C}} \mathrel{,\tilde{!\leftarrow}} f$
\end{document}

Take your pick. The less prominent seems the last one. If you choose it, you should better make a new command:

\newcommand{\leftarrowsim}{\mathrel{\,\tilde{\!\leftarrow}}}

and use

$f_{\mathcal{C}} \leftarrowsim f$

Answer 2

You may want to smash the arrow to get better vertical spacing. You can adjust 0.4ex to get more or less vertical space.

\documentclass{article}
\newcommand\isofrom{\stackrel{\sim}{\smash{\longleftarrow}\rule{0pt}{0.4ex}}}
\begin{document}
$f_{\mathcal{C}} \isofrom f$
\end{document}