# Reference to a manually numbered equation

I want my equation's number to be "1'". I have done so through \tag{1'}. But now I don't know how to refer it in my document. Is there a method to call it or numbering equations like 1, 1', 1''?

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I would suggest using \tag{$1^\prime$} rather than \tag{1'} - the \prime is a little cleaner. –  Werner Oct 27 '11 at 18:09
@Werner: No, that is still not good. The number 1 should be obtained by a \ref to equation 1. I'll update my answer to show the technic. –  Leo Liu Oct 27 '11 at 18:11
@LeoLiu: Oh, so the \prime should be added automatically? –  Werner Oct 27 '11 at 18:14
@Werner: I mean the number. There should not be any manual serial numbers in LaTeX. \prime is still typed manaully. –  Leo Liu Oct 27 '11 at 18:23

Use \label and \eqref (or \ref) as usual. I don't think there is any difference.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$$1+1=2 \tag{A}\label{eq:simple}$$

Eq.~\eqref{eq:simple} is simple.

\end{document}


(To make this question more useful)

To make a equation as a variant of another one, you can use this:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$$a+b = b+a \label{eq:comm}$$

$$1+2 = 2+1 \tag{\ref{eq:comm}'}\label{eq:comm:inst} % if hyperref is used, replace \ref with \ref*$$

Commutative law \eqref{eq:comm} and an instance \eqref{eq:comm:inst}.

\end{document}

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when i use them, equations are numbered like 1, 2, 3 and so on. What i need is 1, 1', 1'' when i use \tag, i am unable to refer it back. this is the problem –  Cagatay Oct 27 '11 at 18:04
Note that you have to use both \tag{<stuff>} and \label{<lab>}, and then you can refer to <lab> with either \eqref{<lab>} or (\ref{<lab>}). Also note the brackets around \ref{<lab>} which is automatically inserted when using \eqref{<lab>}. –  Werner Oct 27 '11 at 18:06
Well, then I recover the example I've just deleted. –  Leo Liu Oct 27 '11 at 18:07
that has worked, thanks a lot –  Cagatay Oct 27 '11 at 18:14
If the "equation variant" version is to be used with hyperref, I wonder whether using \tag{\ref*{eq:comm}$'$} would be better. Otherwise only a portion of the tag will be hyperlinked. It may also be personal preference at this point. –  Werner Oct 27 '11 at 18:22