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Am. Math. Monthly June 2008, 524. For a natural number n that is not
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natural numbers with no common factors, then n/r=r, hence
a/b=r=n/r=nb/a. Since a and b have no common factors, this implies
that there exists a natural number k such that a=bk (looking
at the denominators!) and therefore r=a/b=k is a natural number.

20419 Rajendra Bhatia/J. Dias da Silva: Infinite divisibility of
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18813 Günter Köhler/Jürgen Spilker: Endziffernblöcke von Potenzen.
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5008 Walter Kraemer: Numerische Berechnung von Schranken fuer \pi.
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10817 Dana Mackenzie: Fractions to make an Egyptian scribe blanch.
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3544 Kenji Nagasaka/Etienne Fouvry (ed.): Analytic number theory.
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5080 Wilhelm Singhof: Einige Beziehungen zwischen stabiler Homotopietheorie 
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N. Sloane/S. Plouffe: The encyclopedia of integer sequences.
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2058 L. Toth: The unitary analogue of Pillai's arithmetical function.
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