You could use \ar@{<--}
, but it's conceptually better to originate it from A
:
\documentclass{article}
\usepackage{amsmath}
\usepackage[all,cmtip]{xy}
\begin{document}
\[
\xymatrix{
& A \ar[d]^{i} \ar@{-->}[dl]_-{\exists \tilde{\varphi}} \\
B \ar[r]^{\varphi} &C
}
\]
\end{document}
Note \tilde{\varphi}
, rather than \overset{\sim}{\varphi}
. You can use \widetilde
if the accent seems too short.

Alternatively, you can use tikz-cd
\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz-cd}
\begin{document}
\[
\begin{tikzcd}
{} & A \arrow{d}{i} \arrow[dashed,swap]{dl}{\exists \tilde{\varphi}} \\
B \arrow{r}{\varphi} & C
\end{tikzcd}
\]
\end{document}

@
symbol is a letter? – Bordaigorl Dec 13 '13 at 18:17