Suppress equation number when not refered to

Question

Is is possible to use only equation or align environments without the * and then automatically suppress the equation number if there is nothing refering to that equation? It would make the LaTeX-life much easier I think.

Answer 1

see package mathtools and section "3.2.2 Showing only referenced tags". You'll get the documentation by running texdoc mathtools or run texdoctk if you run Linux

Answer 2

I coded up a workaround before I noticed Herbert's answer. Here it is anyway:

\documentclass{article}
% TEX.SE \url{http://tex.stackexchange.com/q/15820/1402}
\usepackage{amsmath}
\usepackage{etoolbox}

\makeatletter
\appto{\equation}{%
  \notag
  \preto{\label}{%
    \refstepcounter{equation}
    \tag{\arabic{equation}}%
  }
}
\makeatother

\begin{document}

\begin{equation}
A = A
\end{equation}

\begin{equation}\label{B}
B = B
\end{equation}

\begin{equation}
C = C
\end{equation}

Proof: See Equation~\ref{B}.

\end{document}

It's my first use of the etoolbox macros to prepend and append code to hooks. This method first appends \notag to every equation beginning, thus suppressing equation numbering; then prepends to \label the code which steps the counter and resets the tag.