173

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.

Community
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digit plumber
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6 Answers6

194

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.

David Carlisle
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Ian Thompson
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  • Why didn't you use \stepcounter or \refstepcounter instead of \addtocounter? – kiss my armpit Aug 31 '13 at 06:14
  • @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 02 '13 at 08:25
  • 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 09:56
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    @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
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    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 D. Sauer Sep 30 '16 at 15:32
  • Thanks for the answer. I am a bit surprised to learn that this is the best one can do. For me, \numberthis and then \label{...} is still a lot of typing. Maybe I should use another \newcommand... that bundles the two. (I am extremely lazy and sometimes forgets to type \numberthis in \align*, which leads to errors). – Ying Nov 23 '18 at 21:49
  • For completeness, when I change addtocounter to stepcounter I also have to delete the "{1}" in the command. – George Colpitts May 16 '23 at 22:24
63

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}
martinweiss
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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. – jub0bs Feb 26 '14 at 09:50
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    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 09:51
  • I must have misread the question. I did not see that. Sorry. – martinweiss Feb 27 '14 at 13:03
21

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.

yudai-nkt
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Predrag Punosevac
  • 9,273
  • thanks! somewhat surprisingly putting the \label(eqn2.qo} before \begin{aligned} changed the position of the eqn number in my case... – Xi'an ні війні Oct 26 '14 at 16:38
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    @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 01:50
2

Another solution.

\begin{subequations}\label{eqn}
\begin{align*}
a &=b \\
  &=c \tag{\ref{eqn}} 
\end{align*}
\end{subequations}
user2477759
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  • 1
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1

Here is my stupid "lazy" solution:

\newcommand{\NNN}{\nonumber\\}
% . . .
\begin{align}
a & = b \NNN
  & = c \NNN
  & = d \\
v & = w \NNN
  & = x \NNN
  & = y \NNN
  & = z
\end{align}

I wish align* be extended to allow for a \number command. That would be a mirror image, sort of, of align.

Ryo
  • 871
1

Define a new environment based on align* that adds the equation number at the end.

\documentclass{article}
\usepackage[a6paper]{geometry}
\usepackage{amsmath}

\NewDocumentEnvironment{alignb}{b}{%
  \begin{align}
  \refstepcounter{equation} #1 \tag{\theequation}
  \end{align}
}{}


\allowdisplaybreaks


\begin{document}


Check the reference: \eqref{test}


\begin{alignb}
a &= b \
  &= d \
  &= e \
  &= d \
  &= e \
  &= d \
  &= e \
  &= d \
  &= e \
  &= d \
  &= e \
  &= d \
  &= e \
  &= d \
  &= e \
  &= d \
  &= e \
  &= d \
  &= e \
  &= d \
  &= e \
  &= d \
  &= e \
  &= d \
  &= e \
  &= d \
  &= e \
  &= d \
  &= e \
  &= f \label{test}
\end{alignb}


\end{document}

I used a6paper so the picture is smaller, in order to show that page breaks are honored.

enter image description here

egreg
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