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%%  Der Mathematiksatz mit LaTeX, 1. Auflage 2009
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%%  Copyright (C) 2009 Herbert Voss
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\begin{document}
The Euclidean algorithm: if $a>b$ are integers then $\gcd(a,b)=\gcd(a\bmod b,b)$.
If $\prod_{i=1}^n p^{e_i}_i$ is the prime factorization of $x$ then
\[
  S(x)=\sum_{d\vert x}d=\prod_{i=1}^n{p^{e_i+1}_i-1\over p_i-1}.
\]
\end{document}