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I am using the \xymatrix command

this is the code I've written

& 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
up vote 9 down vote accepted

You could use \ar@{<--}, but it's conceptually better to originate it from A:



  & A \ar[d]^{i} \ar@{-->}[dl]_-{\exists \tilde{\varphi}} \\
  B \ar[r]^{\varphi} &C


Note \tilde{\varphi}, rather than \overset{\sim}{\varphi}. You can use \widetilde if the accent seems too short.

enter image description here

Alternatively, you can use tikz-cd



  {} & A \arrow{d}{i} \arrow[dashed,swap]{dl}{\exists \tilde{\varphi}} \\
  B \arrow{r}{\varphi} & C


enter image description here

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