How can I add a section reference to equation labels in an unnumbered section?

I have an unnumered section in a document, defining notation for the further contents. Since it's unnumbered, the equations within are labelled: '(1)', etc.

For example:

\documentclass[12pt]{book}
\begin{document}
\chapter*{Notation}
\label{sec:notation}
A dot over any object denotes the time derivative of that object. Hence,\\
$$\frac{\partial}{\partial x^0}\alpha(x) = \frac{1}{c}\frac{\partial}{\partial t}\alpha(x) = \frac{1}{c}\dot{\alpha}(x).$$
\end{document}


generates:

Is there a way I can tell latex to automatically label these as '(N.1)' similar to an appendix?

-

In using an ordinary equation environment, you can prepend what you're interested in adding to the equation number by modifying \theequation - the printing mechanism for the equation counter. For example:

\documentclass{article}
\begin{document}
\section{A numbered section}
$$f(x) = ax^2 + bx + c \label{eqnA}$$
See~(\ref{eqnA}).
\section*{An unnumbered section}
\renewcommand{\theequation}{N.\arabic{equation}}
$$f(x) = ax^2 + bx + c \label{eqnB}$$
See~(\ref{eqnB}).
\end{document}


Note that you cannot update \theequation inside the equation environment, since the counter stepping (and reference setting) is done at the start of the environment. That is, it would be too late to update this inside equation for it to be picked up.

If the change should be temporary, you can either localize it using a group, or reset it manually where you want it to change.

If you're only after updating the reference and not the equation number itself, you could modify \p@equation, since the construction of \@currentlabel - the element that stored counters that are \refstep-ed - has the following form (from latex.ltx):

\def\refstepcounter#1{\stepcounter{#1}%
\protected@edef\@currentlabel
{\csname p@#1\endcsname\csname the#1\endcsname}%
}


Note how \@currentlabel is (roughly) defined as \p@<cntr>\the<cntr>.

-
A local \renewcommand for this section is exactly what I needed. Thanks. –  Duncan Macleod Mar 27 '13 at 19:41