# Revisiting: “How to put equation numbers on the left or the right, depending on whether the page is odd or even”

The original question/request was for equation numbers to be on the right side for even pages, and the left side for odd pages; that is, near the inside margins. Jagath'' gave a good answer, but the solution is mostly incomprehensible for my small mind. I tried switching odd and even, left and right, but I am unable to get equation numbers to appear in or near the outer margins.

\documentclass{book}
\usepackage{lipsum}
\usepackage{amsmath}
\usepackage{bm}
\newcommand{\Cdot}{\bm{\cdot}}

% Inner margin numbering (I want outer margin numbering)
\makeatletter
\def\mathlabel#1{\@bsphack
\protected@write\@auxout{}%
{\string\newlabel{#1}{{\@currentlabel}{\thepage}}}%
\@esphack}
\def\eqnWrite{\@bsphack
\protected@write\@auxout{}%
{\string\EqnStat{\theequation}{\thepage}}%
\@esphack}%
\def\EqnStat#1#2{%
\expandafter\gdef\csname eqn@#1\endcsname{#2}%
}
\newcommand\@reqnnum{\hb@xt@.01\p@{}%
\rlap{\normalfont\normalcolor%
\hskip -\displaywidth(\theequation)}}
\def\equation{\let\mathlabel\label$$\refstepcounter{equation}} \def\endequation{\eqno\eqnWrite\@ifundefined{eqn@\theequation}{\hbox{\@eqnnum}}%% {\expandafter\ifodd\csname eqn@\theequation\endcsname\hbox{\@reqnnum}% \else\hbox{\@eqnnum}\fi}$$\@ignoretrue}
\def\@@eqncr{\let\reserved@a\relax%
\ifcase\@eqcnt \def\reserved@a{& & &}\or \def\reserved@a{& &}%
\or \def\reserved@a{&}\else%
\let\reserved@a\@empty%
\@latex@error{Too many columns in eqnarray environment}\@ehc\fi%
\reserved@a \if@eqnsw\eqnWrite%
\@ifundefined{eqn@\theequation}{\@eqnnum}%
{\expandafter\ifodd\csname eqn@\theequation\endcsname\@reqnnum%
\else\@eqnnum\fi}\stepcounter{equation}\fi%
\global\@eqnswtrue\global\@eqcnt\z@\cr}
\makeatother

\begin{document}
\lipsum[1]
$$E^2 = (mc^2)^2 + (pc)^2$$
\lipsum[2]
\begin{subequations}
$$A = B + C$$
$$B = C + D$$
$$D = E + F$$
\end{subequations}
$$\begin{array}{|cc|} d & e \\ f & g \end{array}$$
\begin{align}
F &= A\times B\qquad\text{ ALIGN FAILS!}\\
G\Cdot D &= H\Cdot P\times Q + B\Cdot V
\end{align}
\lipsum[3-4]
$$E^2 = (mc^2)^2 + (pc)^2$$
\lipsum[2]
\begin{subequations}
$$A = B + C$$
$$B = C + D$$
$$D = E + F$$
\end{subequations}
$$\begin{array}{|cc|} d & e \\ f & g \end{array}$$
\begin{align}
F &= A\times B\\
G + D &= P\times Q + B
\end{align}

\end{document}


There are two \ifodd tests in the code: you have to switch \@reqnnum and \@eqnnum in their definitions.

\documentclass[twoside]{book}
\usepackage{lipsum}
\usepackage{amsmath}
\usepackage{bm}
\newcommand{\Cdot}{\bm{\cdot}}

% Inner margin numbering (I want outer margin numbering)
\makeatletter
\def\mathlabel#1{\@bsphack
\protected@write\@auxout{}%
{\string\newlabel{#1}{{\@currentlabel}{\thepage}}}%
\@esphack}
\def\eqnWrite{\@bsphack
\protected@write\@auxout{}%
{\string\EqnStat{\theequation}{\thepage}}%
\@esphack}%
\def\EqnStat#1#2{%
\expandafter\gdef\csname eqn@#1\endcsname{#2}%
}
\newcommand\@reqnnum{\hb@xt@.01\p@{}%
\rlap{\normalfont\normalcolor%
\hskip -\displaywidth(\theequation)}}
\def\equation{\let\mathlabel\label$$\refstepcounter{equation}} \def\endequation{\eqno\eqnWrite\@ifundefined{eqn@\theequation}{\hbox{\@eqnnum}}%% {\expandafter\ifodd\csname eqn@\theequation\endcsname\hbox{\@eqnnum}% <- Here \else\hbox{\@reqnnum}\fi}$$\@ignoretrue}
\def\@@eqncr{\let\reserved@a\relax%
\ifcase\@eqcnt \def\reserved@a{& & &}\or \def\reserved@a{& &}%
\or \def\reserved@a{&}\else%
\let\reserved@a\@empty%
\@latex@error{Too many columns in eqnarray environment}\@ehc\fi%
\reserved@a \if@eqnsw\eqnWrite%
\@ifundefined{eqn@\theequation}{\@eqnnum}%
{\expandafter\ifodd\csname eqn@\theequation\endcsname\@eqnnum% <- Here
\else\@reqnnum\fi}\stepcounter{equation}\fi%
\global\@eqnswtrue\global\@eqcnt\z@\cr}
\makeatother

\begin{document}
\lipsum[1]
$$E^2 = (mc^2)^2 + (pc)^2$$
\lipsum[2]
\begin{subequations}
$$A = B + C$$
$$B = C + D$$
$$D = E + F$$
\end{subequations}
$$\begin{array}{|cc|} d & e \\ f & g \end{array}$$
\begin{align}
F &= A\times B\qquad\text{ ALIGN FAILS!}\\
G\Cdot D &= H\Cdot P\times Q + B\Cdot V
\end{align}
\lipsum[3-4]
$$E^2 = (mc^2)^2 + (pc)^2$$
\lipsum[2]
\begin{subequations}
$$A = B + C$$
$$B = C + D$$
$$D = E + F$$
\end{subequations}
$$\begin{array}{|cc|} d & e \\ f & g \end{array}$$
\begin{align}
F &= A\times B\\
G + D &= P\times Q + B
\end{align}

\end{document}


I don't know how to solve the align issue…