Turn off numbering on unused equations [duplicate]

Question

Possible Duplicate:
How do I number equations only if they are referred to in the text?

While I am writing a document, I like to use align rather than align* and label them for all equations that I write in case I need to refer to them later in the document. I end up not referencing some of the equations I have labeled. I was wondering if there is an easy way to strip equation numbers for equations that are not called using eqref anywhere in the document?

I can imagine this can be a difficult task where LaTeX will have to keep a list of labels and a list of eqrefs and then unless the document is changed between compilations, "un-number" the ones in its list of labels that are not found in the list of eqrefs.

Well, worth asking anyway! :)

Answer 1

You can use the option showonlyrefs in the package mathtools. See §3.2.2 "Showing only referenced tags" in the manual for details.

\documentclass{article} 

\usepackage{mathtools}

\mathtoolsset{showonlyrefs}

\begin{document}  

\begin{align}\label{eq:1}
  1+1=2
\end{align}
See \eqref{eq:1}.

\begin{align}\label{eq:2}
  2+2=4
\end{align}

\end{document}