# Triangular commutative diagram

I am using the \xymatrix command

this is the code I've written

$\xymatrix{ & A \ar[d]^{i}\\ B \ar@{-->}[ru]^-{\exists \overset{\sim} \varphi}\ar[r]^{\varphi}&C }$


My problem is with the [ru]... The dashed arrow goes upward, whereas I want it to go downward; and I keep getting an error !

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Can you provide a MWE? Which packages have you included? Are you in a point where the @ symbol is a letter? –  Bordaigorl Dec 13 '13 at 18:17
Welcome to TeX.SE! –  Mico Dec 13 '13 at 18:18

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}


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