repeating an equation and getting the same equation number?

Question

Let's say I have written an equation in my paper and numbered it:

y = a + bx    (1)

Then, later in the paper, I want to remind the reader by showing the same equation with the same equation number.

Sure, I can repeat the equation, but LaTeX gives it a new equationn number:

y = a + bx     (2)

How do I force LaTeX to use the original equation numbers?

Answer 1

Use the \label, \ref mechanism; once you have \labelled the equation you can use \ref inside \tag to retrieve the number:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}\label{eq:test}
a = b + c.
\end{equation}

\begin{equation}
a = b + c. \tag{\ref{eq:test}}
\end{equation}

\end{document}

As Niel de Beaudrap mentions in his comment, one way to avoid equation numbering confusion for the reader would be to add "revisited" to the tag of the duplicated equation:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}\label{eq:test}
a = b + c.
\end{equation}

\begin{equation}
a = b + c. \tag{\ref{eq:test} revisited}
\end{equation}

\end{document}

Small addition: Use equation* in stead of equation when repeating the number so that LaTeX knows it should not use a new number. This avoids warnings when using e.g. the hyperref package. Thus:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}\label{eq:test}
a = b + c.
\end{equation}

\begin{equation*}
a = b + c. \tag{\ref{eq:test} revisited}
\end{equation*}

\end{document}