align* but show one equation number at the end

Question

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.

Answer 1

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.

Answer 2

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}

Answer 3

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.

Answer 4

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*}
}{\ignorespacesafterend}

\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.

Answer 5

Another solution.

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

Answer 6

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.

Answer 7

The environment {DispWithArrows} of witharrows, which is similar to {align} provides a key tagged-lines to control which lines will be tagged. With tagged-lines=last, only the last line is tagged.

\documentclass{article}
\usepackage{amsmath}
\usepackage{witharrows}

\begin{document}
\begin{DispWithArrows}[tagged-lines=last]
a &=b \\
  &=c \label{eqn}
\end{DispWithArrows}
Equation \eqref{eqn} shows that $a=c$.
\begin{equation}
d = e
\end{equation}
\end{document}