Reference to a manually numbered equation

Question

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''?

Answer 1

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

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}
1+1=2 \tag{A}\label{eq:simple}
\end{equation}

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

\end{document}

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(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}

\begin{equation}
a+b = b+a \label{eq:comm}
\end{equation}

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

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

\end{document}