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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): ..." –  tohecz 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

1 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}

enter image description here

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}

enter image description here

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}
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5  
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
    
Same as tex.stackexchange.com/questions/33750/… –  sandu Mar 2 '13 at 9:38

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