%%
%% The LaTeX Companion, 2ed (second printing August 2004)
%%
%% Example 1-3-5 on page 14.
%%
%% Copyright (C) 2004 Frank Mittelbach, Michel Goossens,
%%    Johannes Braams, David Carlisle, and Chris Rowley
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3
%% of this license or (at your option) any later version.
%%
%% See http://www.latex-project.org/lppl.txt for details.
%%

\documentclass{ttctexa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{135.0pt}
\StartShownPreambleCommands
\makeatletter % `@' now normal "letter"
\@addtoreset{equation}{section}
\makeatother  % `@' is restored as "non-letter"
\renewcommand\theequation{\oldstylenums{\thesection}%
                   .\oldstylenums{\arabic{equation}}}
\StopShownPreambleCommands
\begin{document}
\section{Equations\ldots}
\begin{equation} (a+b)^2 = a^2 + 2ab + b^2\end{equation}
\begin{equation} (a-b)^2 = a^2 - 2ab + b^2\end{equation}

\section{\ldots per section}
\begin{equation} (a+b)(a-b) = a^2 -  b^2  \end{equation}
\end{document}