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A109395
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Denominator of phi(n)/n = Prod_{p|n} (1-1/p); phi(n)=A000010(n), the Euler totient function.
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6
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1, 2, 3, 2, 5, 3, 7, 2, 3, 5, 11, 3, 13, 7, 15, 2, 17, 3, 19, 5, 7, 11, 23, 3, 5, 13, 3, 7, 29, 15, 31, 2, 33, 17, 35, 3, 37, 19, 13, 5, 41, 7, 43, 11, 15, 23, 47, 3, 7, 5, 51, 13, 53, 3, 11, 7, 19, 29, 59, 15, 61, 31, 7, 2, 65, 33, 67, 17, 69, 35, 71, 3, 73, 37, 15, 19, 77, 13, 79, 5, 3
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OFFSET
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1,2
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COMMENTS
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a(n)=2 iff n=2^k (k>0); otherwise a(n) is odd. If p is prime, a(p)=p; the converse is false, e.g.: a(15)=15. It is remarkable that this sequence often coincides with A006530, the largest prime P dividing n. Theorem: a(n)=P if and only if for every prime p<P in n there is some prime q in n with p|(q-1). - Franz Vrabec, Aug 30 2005
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LINKS
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T. D. Noe, Table of n, a(n) for n = 1..1000
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FORMULA
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a(n)=n/gcd(n, phi(n))=n/A009195(n).
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EXAMPLE
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a(10)=10/gcd(10,phi(10))=10/gcd(10,4)=10/2=5.
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MATHEMATICA
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Table[Denominator[EulerPhi[n]/n], {n, 81}] (* From Alonso del Arte, Sep 03 2011 *)
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PROG
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(PARI) a(n)=n/gcd(n, eulerphi(n)) \\ Charles R Greathouse IV, Feb 20 2013
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CROSSREFS
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Cf. A076512 for the numerator.
Sequence in context: A197862 A006530 A102095 * A145254 A163457 A072593
Adjacent sequences: A109392 A109393 A109394 * A109396 A109397 A109398
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KEYWORD
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nonn,frac
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AUTHOR
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Franz Vrabec, Aug 26 2005
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STATUS
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approved
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