# How can I extract equations from one document for use in another?

I have a LaTeX document that includes many equations. I would like to create a separate "quick reference" sheet that includes many of these equations. Instead of copy-and-pasting the equations from the first document to the second document, I would like to be able to reference the equations by name and have them be automatically included (so that if I later change an equation in the first document, that change will be automatically reflected in the second document after I recompile it). What's a good way to do this?

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use package extract:

\usepackage[
active,
generate=file,
extract-env={equation,align}
]{extract}

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can you selectively extract some equations from the file using that method? Otherwise, one can always define a dummy \newenvironment{extracteqn}{\equation}{\endequation} and put extracteqn instead of equation in the options. –  Bruno Le Floch Mar 11 '11 at 8:45
@Bruno: for example: figure-nrs={-2,4-}, extract-cmd=chapter, chapter-nrs={3-5,7} –  Herbert Mar 11 '11 at 11:59
I most definitely have to look into this package. –  Bruno Le Floch Mar 11 '11 at 13:19