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I am using align*, but I still need an equation number. I know one solution is to use align and add \nonumber to all lines but the last. Is there a 'lazy' way to do it?

I searched a little and found similar ones suggesting use split: Show equation number only once in align environment

Unfortunately, due to the page break issue I have, I need to use align, because split won't auto page break, as described in Long equation does not page break.

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3 Answers 3

up vote 37 down vote accepted

Use \tag:

\documentclass{article}
\usepackage{amsmath}
\newcommand\numberthis{\addtocounter{equation}{1}\tag{\theequation}}
\begin{document}
\begin{align*}
a &=b \\
  &=c \numberthis \label{eqn}
\end{align*}
Equation \eqref{eqn} shows that $a=c$.
\begin{equation}
d = e
\end{equation}
\end{document}

See page 3 of the amsmath package documentation for details.

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this \numberthis works beautifully. Thank you very much! –  cpp initiator Jan 29 '12 at 22:16
    
Why didn't you use \stepcounter or \refstepcounter instead of \addtocounter? –  Please don't touch Aug 31 '13 at 6:14
    
@PSTikZ --- \refstepcounter isn't needed, as the \ref value is set correctly already. \stepcounter is probably the best choice, to take care of subsidiary counters. I don't think I gave much thought to this when I composed the answer. –  Ian Thompson Sep 2 '13 at 8:25
    
OK. Thank you for responding! –  Please don't touch Sep 2 '13 at 13:44
    
Your example works perfectly for me, thank you. I have a followup question though. How can I make the number on the same line than the last line of the euqation provided that the last line is longer normal align would allow (it does fit in align* though) –  magu_ Mar 24 at 9:56
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Here are minimal working examples that I use. This is what I have as a header

\documentclass[12pt]{amsart}

\usepackage{amsmath,amsfonts,amssymb,epsfig}
\usepackage{graphicx}
\usepackage{hyperref}
%\usepackage{srcltx}
\hypersetup{colorlinks}
\usepackage{color}

Two equations each with separate equation numbers and separate reference labels

\begin{align}
        \dot{a} &  =2\alpha(t)\eta\label{classicalmotion1}\\
        \dot{\eta} &  =-2\gamma(t)a\label{classicalmotion2}%
\end{align}%

Two equations with single equation number and single reference label

\begin{equation}
        \begin{aligned}
        \dot{A} &  =2i\alpha(t)B\\
        \dot{B} &  =2i\gamma(t)A
        \end{aligned}
\label{eqn2.qo}
\end{equation}

I am guessing you know how to do the one without equation numbers.

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Another option is to use align, and then use \nonumber, as mentioned in http://mirrors.dotsrc.org/ctan/info/math/voss/mathmode/Mathmode.pdf.

Example:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
a &=b \nonumber \\
b &=c
\label{eq:test}
\end{align}
Equation \eqref{eq:test} shows that $a=c$.
\end{document}
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The user seems to want to use an align* environment, though. Your approach would involve a lot of \nonumber, if the align contains a lot of lines. –  Jubobs Feb 26 at 9:50
2  
Welcome to TeX.SX! This answer, while technically correct, is already mentioned in the second sentence of the question; the asker is looking for a different solution. –  ChrisS Feb 26 at 9:51
    
I must have misread the question. I did not see that. Sorry. –  martinweiss Feb 27 at 13:03
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protected by Jubobs Feb 26 at 9:51

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