Can I define more than one label inside an equation environment?

Question

I would like to give one equation two separate labels; can I do this? The purpose of this is to have a long section in which I give a long list of equations, explaining them as I go along. I then de-dimentionalise them for which I will need to say something like 'de-dimentionalising equations (1) to (20) yields...'. Thus I need to be able to refer to label equation (1) twice, once as the first equation in the list and secondly as the equation I want to discuss. Of course I could give it only one label, but it would be convenient to give it two labels such that if I reorder/add in equations I can move the 'first equation' label, and then everything will still make sense.

As a concise example, what I want to do is

\begin{equation}
\label{first_equation} \label{special_equation}
a=b
\end{equation}

Edit: The above code does compile, but the inclusion of the amsmath package prevents it from doing so. Is there any way to use the features in this package and have the ability to multi-label?

Answer 1

Yes. Putting several \labels on the same object just gives you different names for referring to it:

\documentclass{article}

\begin{document}

\begin{equation}
  \label{eq:ab}\label{eq:equality}
  a = b.
\end{equation}
The relation between \( a \) and \( b \) is given by~(\ref{eq:ab}).
Using the equality~(\ref{eq:equality}), we see\dots

\end{document}

amsmath

As Mico correctly points out, this does not work if you load the amsmath package. His answer (https://tex.stackexchange.com/a/40185/15925) gives a way to define a new command \ltxlabel that will work fine in AmSmath equation environments. However, it fails in gather and friends. Here is one work around define a \clonelabel command that can be used outside the environment. Now updated to work with hyperref and further updated to work with cleveref:

\documentclass{article}

\usepackage{amsmath}

\makeatletter
\newcommand{\clonelabel}[2]{\@bsphack
  \expandafter\ifx\csname r@#2\endcsname\relax
  \else\protected@write\@auxout{}{\string\newlabel{#1}%
    {\csname r@#2\endcsname}}%
  \fi
  \expandafter\ifx\csname r@#2@cref\endcsname\relax
  \else\protected@write\@auxout{}{\string\newlabel{#1@cref}%
    {\csname r@#2@cref\endcsname}}%
  \fi
  \@esphack}
\makeatother

\usepackage{hyperref}
\usepackage{cleveref}

\begin{document}

\begin{equation}
  \label{eq:equality}
  a = b.
\end{equation}\clonelabel{eq:ab}{eq:equality}
The relation between \( a \) and \( b \) is given by~\eqref{eq:ab}.
Using the equality~\eqref{eq:equality}, we see\dots

\begin{gather}
  \label{eq:inequality}
  p \ge q\\
  \label{eq:ratio}
  r/s = 20
\end{gather}
\clonelabel{eq:pq}{eq:inequality}
\clonelabel{eq:rs}{eq:ratio}
We refer to \eqref{eq:pq}, \eqref{eq:inequality}, \eqref{eq:rs} and
\eqref{eq:ratio}. 

We think \cref{eq:pq} is as interesting as \cref{eq:inequality}.

\end{document}