repeating an equation and getting the same equation number?

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?

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I'm not sure whether I wouldn't be confused by such thing in a document, because when I search for a specific eq. number, I simply follow the numbers on the border and this makes a mess there. What you want to achieve is similar to including a term "foo" between "bar" and "baz" in a dictionary. I would recommend you to say it only in the text: "Let us recall the equation (1): ..." – yo' Aug 18 '12 at 6:37
Kind of duplicate of this: tex.stackexchange.com/q/33750/586 Same question, except that in that case a LyX answer was asked for. (A LaTeX answer is also given.) – Torbjørn T. Aug 18 '12 at 7:54

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}

$$\label{eq:test} a = b + c.$$

$$a = b + c. \tag{\ref{eq:test}}$$

\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}

$$\label{eq:test} a = b + c.$$

$$a = b + c. \tag{\ref{eq:test} revisited}$$

\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}

$$\label{eq:test} a = b + c.$$

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

\end{document}

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A good way to approach avoiding equation numbering confusion for the reader would be to use \tag{\ref{eq:label} revisited}. – Niel de Beaudrap Aug 18 '12 at 15:59
@NieldeBeaudrap yes, that's a possibility. I've added it to my answer. Thank you. – Gonzalo Medina Aug 18 '12 at 16:37
– sandu Mar 2 '13 at 9:38
Using unstarred align instead of equation* doesn't give me any hyperref warning and skips the number properly [i.e., (1), (1 revisited), (2), ...]. – Alfredo Hernández May 7 '15 at 11:18