Why does \label not work in math mode when using [fleqn] with amsmath?

Question

Try the following

\documentclass[11pt]{article}
\usepackage[fleqn]{amsmath}

\begin{document}
\newcounter{eqnthing}\setcounter{eqnthing}{0}
\[a+b=c\refstepcounter{eqnthing}\label{maththing}\]
\par See(\ref{maththing}).
\refstepcounter{eqnthing}\label{linething}
\par See (\ref{linething}).
\end{document}

The first reference will not work. Then comment out \usepackage[fleqn]{amsmath} and it will work. It also works with \usepackage{amsmath} so the fleqn option appears to be the culprit.

Is this a known bug? Is there a workaround?

Answer 1

Its the way amsmath has redesigned equations, especially labelling inside of them - handled differently when using the fleqn package option. Specifically, amsmath redefines how \label works, as in some cases (most notably within multi-parse items like align and friends) it has to switch it on-and-off in order to avoid multiply defined labels.

You can still access the old/traditional LaTeX \label using \ltx@label, for which I've defined an interface as \ltxlabel below:

\documentclass[11pt]{article}
\usepackage[fleqn]{amsmath}
\makeatletter
\newcommand{\ltxlabel}{\ltx@label}
\makeatother

\newcounter{eqnthing}\setcounter{eqnthing}{0}
\begin{document}

\[a+b=c\refstepcounter{eqnthing}\hfill(\arabic{eqnthing})\ltxlabel{maththing}\]

See (\ref{maththing}).

$a+b=c$\refstepcounter{eqnthing}\hfill(\arabic{eqnthing})\label{linething}

See (\ref{linething}).
\end{document}

Answer 2

I'm not sure why you want to define in a wrong way what's already featured in amsmath, namely \tag.

\documentclass[11pt]{article}
\usepackage[fleqn]{amsmath}

\newcounter{eqnthing}\setcounter{eqnthing}{0}
\newcommand\geofflabel[1]{%
  \refstepcounter{eqnthing}\tag{\theeqnthing}\label{#1}%
}

\begin{document}
\[
a+b=c\geofflabel{maththing}
\]
See~\eqref{maththing}.
\begin{align*}
a&=1\geofflabel{a}\\
b&=11111\\
c&=0\geofflabel{c}
\end{align*}
See \eqref{a}~and~\eqref{c}.
\end{document}

Answer 3

Lookng around on TeX.SX it seems that there are many bugs with the fleqn option such as /equation-spacing-issue-with-fleqn-amsmath-and-frenchb-babel, problem-using-option-fleqn-in-amsmath-package, combined-use-of-fleqn-and-qedhere, ...

I am not sure whether this one is known or not. The problem seems to be that when amsmath closes off a displayed equation it forgets to put the label in when the fleqn option is present. Here is a rough hack that fixes the problem with your MWE...although I haven't tested it extensively so perhaps it creates other problems:

Here is the code:

\documentclass[11pt]{article}
\usepackage[fleqn]{amsmath}
\makeatletter
\def\endmathdisplay@a{%
  \if@eqnsw \gdef\df@tag{\tagform@\theequation}\fi
  \if@fleqn \ifx\df@label\@empty \else \@xp\ltx@label\@xp{\df@label}\fi\@xp\endmathdisplay@fleqn
  \else \ifx\df@tag\@empty \else \veqno \alt@tag \df@tag \fi
    \ifx\df@label\@empty \else \@xp\ltx@label\@xp{\df@label}\fi
  \fi
  \ifnum\dspbrk@lvl>\m@ne
    \postdisplaypenalty -\@getpen\dspbrk@lvl
    \global\dspbrk@lvl\m@ne
  \fi
}
\makeatother

\begin{document}
\newcounter{eqnthing}\setcounter{eqnthing}{0}
\[ a+b=c\refstepcounter{eqnthing}\hfill(\arabic{eqnthing})\label{maththing} \]
\par See (\ref{maththing}).
\refstepcounter{eqnthing}\hfill(\arabic{eqnthing})\label{linething}
\par See (\ref{linething}).
\end{document}

No doubt egreg will come along soon and give a "proper" fix:)