# Is it possible to attach a label to a theorem so it can be cited later?

I want to know if it's possible to add some sort of label to a theorem such that I can cite it in the future. I know I can do this by writing explicitly the theorem's number every time I need it, but if you move the theorem to another section then you have to manually correct each reference you have made to that theorem, and this becomes very time consuming and, even, confusing. I want something that does this for me so I don't have to worry about it.

Also, if this is possible I would like to know how to do it with definitions and notes also.

• \label and \ref work for theorems just as they do for sections and equations – David Carlisle Mar 28 '14 at 17:00
• This is one of the strong parts of TeX. Look at the related question in the right hand of this page. – Manuel Mar 28 '14 at 17:07
• For a discussion of various packages that facilitate cross-referencing, see the posting Cross-reference packages: which to use, which conflict?. – Mico Mar 28 '14 at 21:48

You can even use the cleveref and ntheorem (or amsthm) packages. Provided the \newtheoremdeclarations are writtenafter cleveref loading, supposing for instance some_label refers a given theorem, you can write in your .tex file something like :

 According to \cref{some_label}…


and the result will be:

According to theorem 2.1.4 …


So not only the reference number will follow if you move you theorem and you will save some typing, but if you change the theorem to a proposition or a lemma, the name will also change in your sentence without having to modify it. Cleveref is also aware of plural forms, capitalised initials and knows about babel.

B.A.