Prevent multiple equation numbers when using \allowdisplaybreak with align

Question

I have a very long equation of the following format

\begingroup
\allowdisplaybreaks
\begin{align}
a &= very long expression here\\
  + & another long expression here\\
  + & another long expression here
\end{align}
\endgroup

Now this equation is labelled thrice, once for each line. How do i make sure that the equation gets labelled only once?

use aligned instead of align, and wrap that environment in \begin{equation} ... \end{equation}. this will result in precisely one equation number. requires amsmath, but then, so does align, so you are apparently already using that. recommended reading: texdoc amsldoc. — barbara beeton, May 04 '13 at 19:16
@barbarabeeton But then there cannot be a page break between the lines, can there? (I assume he wishes that due the \allowdisplaybreaks.) — Torbjørn T., May 04 '13 at 19:34
Another related question: http://tex.stackexchange.com/q/42726/10679 — kan, May 04 '13 at 19:43
Insert a \notag directive at the end of each line (just before the \ line-break instruction) for every line you do not want to get numbered. — Mico, May 04 '13 at 19:58
@TorbjørnT. -- multline doesn't align the parts in any useful way, although it does yield only one number and allow breaks. — barbara beeton, May 04 '13 at 20:58
@TorbjørnT. -- yes, you're correct. i didn't notice the \allowdisplaybreaks. so \notag on the lines that aren't to be numbered, or align* and an explicit \tag are the only feasible approaches. (however, having only one number on an equation that is split across pages is likely to be confusing, and thus not recommended.) — barbara beeton, May 04 '13 at 21:01
@barbarabeeton Agreed, on both comments. (In hindsight, it wasn't a very good suggestion.) — Torbjørn T., May 04 '13 at 21:03
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score 3 · Answer 1 · edited Jun 16 '17 at 16:28

Here is one solution. No need to use \notag on each line that should remain unnumbered if you use this trick; only insert \numberthis at the end of the equation.

enter image description here

\documentclass{article}

\usepackage{amsmath}
\usepackage{lipsum}

\newcommand\numberthis[1][]{%
    \refstepcounter{equation}%
    \ifx#1\empty\else\label{eq:#1}\fi%
    \tag{\theequation}%
}

\begin{document}

\lipsum[1-4]
filler text\\
filler text\\
filler text\\
filler text

{
\Large
\allowdisplaybreaks
\begin{align*}
a = b   &+ c + d + e + f + g + h + i + j\\
        &+ k + l + m + n + o + p + q +r \numberthis\\
        &+ s + t + u + v + w + x + y + z \numberthis[jubobs]
\end{align*}
}

See \ref{eq:jubobs}.
\end{document}

Prevent multiple equation numbers when using \allowdisplaybreak with align

1 Answers1

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