[Tex/LaTex] Commutative diagram with parallel arrows in opposite direction

amsmatharrowscommutative-diagrams

I have a diagram of the following form, written with the package amscd:

\begin{align*}
 \begin{CD}
  W @>\phi>> X \\
  @V{a}V{b}V  @V{c}V{d}V \\
  Y @>\phi>> Z \\
 \end{CD}
\end{align*}

It want the vertical arrows to be parallel arrows in opposite direction, such that a and c point downwards and b and d point upwards. (For those who understand the terms, it is about an isomorphism between chain complexes.)

To my best knowledge, amscd does not provide these features. Can you show me a piece of code that provides this functionality and is similarly easy to use?

Best Answer

First of all don't use align* for a single object and prefer equation*.

Simple commutative diagrams with the CD environment can have arrows pointing

  • right

    @>>>

  • left

    @<<<

  • down

    @VVV

  • up

    @AAA

  • double stem, no arrow head

    @=

  • nothing (for filling blanks)

    @.

However CD doesn't allow for "double arrows" and you have to go to a more specialized package. For instance xy:

\documentclass{article}
\usepackage{amsmath}
\usepackage[all,cmtip]{xy}
\begin{document}
\begin{equation*}
\xymatrix{
   W \ar[r]^{\phi} \ar@<-2pt>[d]_{a} & X \ar@<-2pt>[d]_{c} \\
   Y\ar@<-2pt>[u]_{b} \ar[r]^{\phi} & Z \ar@<-2pt>[u]_{d}
}
\end{equation*}
\end{document}

enter image description here

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