%% 
%%  Example 07-07-4 
%%  Copyright (C) 2017 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
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\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{328.62788pt}
\setlength\parindent{0pt}
\usepackage[T1]{fontenc}
\usepackage{kpfonts}
%StartShownPreambleCommands
\usepackage{amsmath}
\makeatletter
\@fleqntrue
\let\old@mathmargin=\@mathmargin
\@mathmargin=-1sp
\let\oldmathindent=\mathindent
\let\mathindent=\@mathmargin
\newsavebox\myendhook % for the tabulars
\def\tagform@#1{{(\maketag@@@{\ignorespaces#1\unskip\@@italiccorr)}
  \makebox[0pt][r]{% after the equation number
    \makebox[0.5\textwidth][l]{\usebox{\myendhook}}}%
  \global\sbox\myendhook{}}}% empty box
\makeatother
\sbox{\myendhook}{%
\footnotesize
 \begin{tabular}{@{}ll}
  $a_0$    & Bohrscher Radius ($\mathrm{= 0{,}53\,\mbox{\AA}}$)\\
  $e$      & Elementarladung\\
  $N_{si}$ & Anzahl der Siliziumatome\\
           & pro Einheitsvolumen\\
  $m$      & Atomgewicht\\
  $Z$      & Kernladungszahl
 \end{tabular}}
%StopShownPreambleCommands
\begin{document}
text text text text text text text text text text text text text text
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%
\begin{equation}
\varepsilon = \frac{E\cdot 4\cdot\pi\cdot\varepsilon_{0}\cdot a_0\cdot
  \left(Z_i^{\frac{2}{3}}+Z_{Si}^{\frac{2}{3}}\right)^{-\frac{1}{2}}}{Z_i\cdot Z_{Si}
  \cdot e2 \cdot\left(1+ \frac{m_i}{m_{Si}} \right)}\,;
\end{equation}
%
\sbox\myendhook{abc}
\begin{equation} a2+b2=c2 \end{equation}
\begin{equation} z = 9 \end{equation}
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\end{document}
