%% 
%%  Der Mathematiksatz mit LaTeX, 1. Auflage 2009
%% 
%%  Example 6-12-8 on page 118.
%% 
%%  Copyright (C) 2009 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.
%% 

\documentclass[]{ttctexa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{170.59192pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\setcounter{equation}{66}
\renewcommand\theequation{6.\arabic{equation}}
\StartShownPreambleCommands
\usepackage{amsmath}
\StopShownPreambleCommands
\begin{document}
\delimitershortfall=-1pt % sonst Klammer zu klein
\begin{align}
\rule[2.5pt]{1cm}{0.2pt} % mathematische Achse
\left[
  \begin{array}{@{}c@{}}
    \displaystyle
    \sum_{\substack{1\le i\le p\\1\le j\le q\\1\le k\le r}}%
      a_{ij}b_{jk}c_{ki}
  \end{array}
\right] = A
\rule[2.5pt]{1cm}{0.2pt} % mathematische Achse
\end{align}
\end{document}