Number equation with previously used (auto-assigned) number

Question

Assume you have a long text (i.e. a thesis/book/etc) with a lot of equations per chapter. At some point later in the text, you not only want to reference an equation but actually spell it out again, for the convenience of the reader. The same equation has been used before, so you don’t want a new number for it. You could put it in the text without a number. But what would be the best way to reproduce the original equation number for this second (or any subsequent) appearance? Ideally the solution would work with the hyperref package, allowing to place a link to the original equation in the pdf.

Answer 1

The \tag command from the amsmath package can help:

\documentclass[12pt,a4paper,twoside]{book}
\usepackage[latin1]{inputenc}
\usepackage{hyperref}
\usepackage{amsmath}
\begin{document}
This is the first appearance of the equation:
\begin{equation}\label{eqn:einstein}
E=mc^2
\end{equation}
Now comes a lot of text
...
...
Now I want to cite the equation, and repeat it for the reader:
\[
E=mc^2\tag{\ref{eqn:einstein}}
\]
\end{document}