%% 
%%  Der Mathematiksatz mit LaTeX, 1. Auflage 2009
%% 
%%  Example 6-12-10 on page 119.
%% 
%%  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{355.65944pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\setcounter{equation}{68}
\renewcommand\theequation{6.\arabic{equation}}
\StartShownPreambleCommands
\usepackage{amsmath}
\StopShownPreambleCommands
\begin{document}
\begin{align}
\prod_{j\ge0}\biggl(\sum_{k\ge0}a_{jk}z^k\biggr) = \sum_{n\ge0}z^n\,
  \Biggl(\sum_{\scriptstyle k_0,k_1,\ldots\ge0\atop\scriptstyle
    k_0+k_1+\cdots=n}a_{0k_0}a_{1k_1}\ldots\,\Biggr)
\end{align}
\end{document}