# Prevent multiple equation numbers when using \allowdisplaybreak with align

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 align* and make align* number the last equation. – kan May 4 '13 at 19:08
• Is using multline instead of align an option? – Torbjørn T. May 4 '13 at 19:11
• use aligned instead of align, and wrap that environment in $$...$$. 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 4 '13 at 19:16
• Another related question: tex.stackexchange.com/q/42726/10679 – kan May 4 '13 at 19:43
• @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 4 '13 at 21:01

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.

\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}