How do I number equations only if they are referred to in the text?

Question

I have a paper where all equations are numbered, but I want to number equations only if they are actually referred to in the .tex file using \ref{}. How can I make that happen? Alternatively, how can I make all equations that have no \label next to them (because then I know I don't refer to them) not have a number? Do I have to use \nonumber separately for each such equation?

Answer 1

The mathtools package provides a switch showonlyrefs. You could use that to achieve that only those equations would be numbered which are referred to in the text.

Usage:

\mathtoolsset{showonlyrefs}

or

\mathtoolsset{showonlyrefs=true}

or set it to false this way if desired.

Answer 2

mathtools with showonlyrefs will not work with the cleveref package, so if you rely on that package for your cross-references, it would be better to use autonum. If one wants to use hyperref as well, the loading order must be hyperref - cleveref - autonum, as described in the manual.

\documentclass{article}
\usepackage{amsmath}
\usepackage{hyperref}
\usepackage{cleveref}
\usepackage{autonum}
\begin{document}

\begin{equation}
a = b \label{eq:a}
\end{equation}

\begin{equation}
c = d \label{eq:b}
\end{equation}

\begin{equation}
e = f \label{eq:c}
\end{equation}

See \cref{eq:b}.
\end{document}