# Reference for a commutative diagram

How to give a number (for later reference) to the following diagram?

\begin{displaymath}
\xymatrix@!C{ \{ \prod_{g \in A} \OO_g  \} \ar@{->>}[r] & {
\{\prod_{g \in A^{\prime}} \OO_g \} }\\ { {\tilde {\T}}^{\prime}_{
{\tilde{\m}}^{\prime} } } \ar@{^{(}->}[u] \ar@{->>}[r]^(0.55){q} &
{\T} \ar@{^{(}->}[u] }
\end{displaymath}


Perhaps, the simplest way might be:

• if you'd like to number this along with other equations, put this inside an equation environment (instead of displaymath)

• if you'd like to number this with a custom number, put this inside an equation* and use \tag{<your-label-here>}.

And, to get the number centered, the trick is to center the number. So, this might go inside a gathered environment.

\documentclass{amsart}
\usepackage{xypic} %generates a warning, but the problem is with the package
\begin{document}

\begin{gathered}
\xymatrix@!C{
\left\{\prod_{g \in A}\mathcal{O}_g\right\} \ar@{->>}[r]
& \left\{\prod_{g \in A^{\prime}} \mathcal{O}_g \right\} \
\widetilde{\mathcal{T}}}^{\prime}_{{\tilde{\mu}}^{\prime} \ar@{^{(}->}[u] \ar@{->>}[r]^(0.55){q}
& {\mathcal{T}} \ar@{^{(}->}[u]
}
\end{gathered}
\end{document}


Now, brace yourself as you see the output:

1. You seem to be having big symbols inside smaller braces. You might want to put this into \left\{...\right\}. (There are other variants here: the constructs \bigl...\bigr and \biggl...\biggr can be used the way one uses \left...\right.)
2. However, I'd suggest that you draw diagrams in Tikz, if you'd like to see beautiful images. (The package tikz-cd is very good at this!)
• @sri You can do that by passing the option [reqno] to your documentclass: eg, \documentclass[reqno]{amsart}. – kan May 7 '13 at 8:40