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%%  An UIT Edition example
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%%  Example 10-09-10 on page 220.
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%%  Copyright (C) 2010 Herbert Voss
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%%  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.
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%%  See http://www.latex-project.org/lppl.txt for details.
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\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{375.57637pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\setcounter{equation}{31}
\renewcommand\theequation{10.\arabic{equation}}
\StartShownPreambleCommands
\usepackage{amsmath}
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\begin{document}
\begin{align}
 \sum_{i=1}^\infty \frac{1}{i} &= 1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}
  +\frac{1}{5}+\frac{1}{6}+\frac{1}{7}+\frac{1}{8}+\frac{1}{9}
  +\cdots+\frac{1}{16}+\cdots\\
 & \ge 1+\frac{1}{2}+\underbrace{\frac{1}{4}+\frac{1}{4}}_{\frac{1}{2}}
    +\underbrace{\frac{1}{8}+\frac{1}{8}+\frac{1}{8}+\frac{1}{8}}_{\frac{1}{2}}
    +\underbrace{\frac{1}{16}+\cdots+\frac{1}{16}}_{\frac{1}{2}}+\cdots\\
 &= 1+\frac{1}{2}+\frac{1}{2}+\frac{1}{2}+\frac{1}{2}+\cdots\to\infty
    \quad\rule{1ex}{1ex}
\end{align}
\end{document}