130

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.

152

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.

  • Why didn't you use \stepcounter or \refstepcounter instead of \addtocounter? – kiss my armpit 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
  • 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 '14 at 9:56
  • 1
    @magu_ Please post a new question with an example showing exactly what you want. I think I understand your question, but I'm not 100% sure. Also, other users (who may have better solutions than me) will see the new question, whereas they are unlikely to see new comments here. – Ian Thompson Mar 24 '14 at 12:04
  • 1
    This solution has a problem when combined with the cleveref package: The sorting of \numberthis-references does not work properly. See this question for further detail. If anyone knows a solution fixing this problem, please let me know. – Lukas Sep 30 '16 at 15:32
33

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}
  • 5
    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. – jub0bs Feb 26 '14 at 9:50
  • 8
    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 '14 at 9:51
  • I must have misread the question. I did not see that. Sorry. – martinweiss Feb 27 '14 at 13:03
16

Here are minimal working examples that I use. This is what I have as a header

\documentclass[12pt]{amsart}           
\usepackage{amsmath,amsfonts,amssymb}

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 a single equation number and a single reference label

\begin{equation}
        \begin{aligned}[b]
        \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.

  • thanks! somewhat surprisingly putting the \label(eqn2.qo} before \begin{aligned} changed the position of the eqn number in my case... – Xi'an Oct 26 '14 at 16:38
  • 1
    @Xi'an -- putting the \label before the align might change the position of the equation number, but it will also give an incorrect value with \eqref. – barbara beeton Jul 13 '17 at 1:50
1

Another solution.

\begin{subequations}\label{eqn}
\begin{align*}
a &=b \\
  &=c \tag{\ref{eqn}} 
\end{align*}
\end{subequations}
  • 1
    Try having two equations like that, you will see that this code doesn't work. – user156344 Apr 5 '19 at 13:44
  • 1
    @user156344: tried and it works as well (of course your need to change the equation labels). – user2477759 Sep 10 '19 at 13:58

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