% subeqnary.sty - Copyright (C) 1996 by Frank Holzwarth to
% establish a new environment "subeqnarray" that locally numbers
% the lines of an equation array with the same number (default
% chapter.equation) and a running alphabetic suffix.
% \label-ing works ok. There is no "subeqnarray*" but \nonumber
% can be used.
%
% This is a hacked again version of:
% modeqary.sty - Copyright (C) 1991 by James Darrell McCauley
% modify freely - but this is just a hack, so you'd be better
% off going back to the original source (see below)

% parts taken from latex.tex
% LATEX VERSION 2.09 <4 Aug 1988>
% Copyright (C) 1988 by Leslie Lamport

% In response to:
%Hi netters,
%       In book style I want to number the equations within the eqnarray
%environment differently.  All I want to do is to number the equations
%some thing like 1.8a, 1.8b....etc instead of 1.8, 1.9 ...etc.
%Is there a way to this ?
%The result I am getting now with eqnarray;
%a= c + d               (1.1)
%e= f - d               (1.2)
%Instead I want to get;
%a= c + d               (1.1a)
%e= f - d               (1.1b)

% SEE EOF FOR THE TEST PROGRAM THAT I USED.

\newcounter{eqsubcnt}
\def\thesubequation{\thechapter.\arabic{equation}\alph{eqsubcnt}}

\def\@eqnsubnum{{\rm (\thesubequation)}}

% Here's the eqnarray environment:
%  Default is for left-hand side of equations to be flushleft.
%  To make them flushright, \let\@eqnsel = \hfil

\newcount\@eqcnt
\newcount\@eqpen
\newif\if@eqnsw\@eqnswtrue

\@centering = 0pt plus 1000pt % Changed 11/4/85 to produce warning message
                              % if line extends into margin.  Doesn't warn
                              % about formula overprinting equation number.

\def\subeqnarray{\stepcounter{equation}\let\@currentlabel=\thesubequation
\def\@eqncr{{\ifnum0=`}\fi\@ifstar{\global\@eqpen\@M
    \@yeqncr}{\global\@eqpen\interdisplaylinepenalty \@yeqncr}}
%
\def\@yeqncr{\@ifnextchar [{\@xeqncr}{\@xeqncr[\z@]}}
%
\def\@xeqncr[##1]{\ifnum0=`{\fi}\@@eqncr
   \noalign{\penalty\@eqpen\vskip\jot\vskip ##1\relax}}
%
\def\@@eqncr{\let\@tempa\relax
    \ifcase\@eqcnt \def\@tempa{& & &}\or \def\@tempa{& &}
      \else \def\@tempa{&}\fi
     \@tempa \if@eqnsw\@eqnsubnum\stepcounter{eqsubcnt}\fi
     \global\@eqnswtrue\global\@eqcnt\z@\cr}
\setcounter{eqsubcnt}{1}
\global\@eqnswtrue\m@th
\global\@eqcnt\z@
\let\\\@eqncr
\@ifundefined{mathindent}{\tabskip\@centering
$$\halign to\displaywidth}{\tabskip\mathindent
 \abovedisplayskip\topsep\ifvmode\advance\abovedisplayskip\partopsep\fi
 \belowdisplayskip\abovedisplayskip
 \belowdisplayshortskip\abovedisplayskip
 \abovedisplayshortskip\abovedisplayskip
 $$\halign to\linewidth}\bgroup\@eqnsel\hskip\@centering
  $\displaystyle\tabskip\z@
  {##}$&\global\@eqcnt\@ne \hskip 2\arraycolsep \hfil${##}$\hfil
  &\global\@eqcnt\tw@ \hskip 2\arraycolsep $\displaystyle{##}$\hfil
   \tabskip\@centering&\llap{##}\tabskip\z@\cr}

\def\endsubeqnarray{\@@eqncr\egroup
      \global\advance\c@equation\m@ne$$\global\@ignoretrue
      \stepcounter{equation}}

\let\@eqnsel=\relax

\def\nonumber{\global\@eqnswfalse}


\endinput

\documentstyle[modeqnarray]{book}
\begin{document}
\chapter{Introduction}
\begin{eqnarray}
a & = & c + d \\
e & = & f - d
\end{eqnarray}

\begin{equation}
O_i = \sum_{j=1}^N I_j W_{ij}
\end{equation}
\end{document}

--
James Darrell McCauley, Grad Res Asst, Spatial Analysis Lab
Dept of Ag Engr, Texas A&M Univ, College Station, TX 77843-2117, USA
(jdm5548@diamond.tamu.edu, jdm5548@tamagen.bitnet)