%%
%% Ein Beispiel der DANTE-Edition
%% Mathematiksatz mit LaTeX
%% 3. Auflage
%% Beispiel 06-12-5 auf Seite 118.
%% Copyright (C) 2018 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
%%
%%
\documentclass[10pt]{screxa}
\pagestyle{empty}
\setlength\textwidth{170.40707pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\setcounter{equation}{63}
\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}
