%% 
%%  Ein Beispiel der DANTE-Edition
%%  
%%  1. Auflage
%% 
%%  Beispiel 09-07-50 auf Seite 463.
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%%  Copyright (C) 2011 Herbert Voss
%% 
%%  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.
%% 
%% 
%% ==== 
% Show page(s) 1
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%% 
\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{166.324pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\setcounter{equation}{67}
\renewcommand\theequation{6.\arabic{equation}}
\newcommand*\clap[1]{\hbox to 0pt{\hss#1\hss}}
\newcommand*\mathclap{\mathpalette\mathclapinternal}
\newcommand*\mathclapinternal[2]{\clap{$\mathsurround=0pt#1{#2}$}}
\StartShownPreambleCommands
\usepackage{amsmath}
\StopShownPreambleCommands
\begin{document}
\begin{align}
  X = \sum_{\mathclap{1\le i\le j\le n}}X_{ij}
\end{align}
\end{document}