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A109395 Denominator of phi(n)/n = Prod_{p|n} (1-1/p); phi(n)=A000010(n), the Euler totient function. 17
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

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

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..16384 (terms 1..1000 from T. D. Noe)

Antti Karttunen, Data supplement: n, a(n) computed for n = 1..65537

FORMULA

a(n) = n/gcd(n, phi(n)) = n/A009195(n).

From Antti Karttunen, Feb 09 2019: (Start)

a(n) = denominator of A173557(n)/A007947(n).

a(2^n) = 2 for all n >= 1.

(End)

EXAMPLE

a(10) = 10/gcd(10,phi(10)) = 10/gcd(10,4) = 10/2 = 5.

MATHEMATICA

Table[Denominator[EulerPhi[n]/n], {n, 81}] (* Alonso del Arte, Sep 03 2011 *)

PROG

(PARI) a(n)=n/gcd(n, eulerphi(n)) \\ Charles R Greathouse IV, Feb 20 2013

(PARI)

A007947(n) = factorback(factorint(n)[, 1]); \\ From A007947

A173557(n) = my(f=factor(n)[, 1]); prod(k=1, #f, f[k]-1); \\ From A173557

A109395(n) = denominator(A173557(n)/A007947(n)); \\ Antti Karttunen, Feb 09 2019

CROSSREFS

Cf. A076512 for the numerator.

Cf. A000010, A009195, A054741, A318304, A318305, A323170.

Sequence in context: A323616 A102095 A331959 * A145254 A163457 A285708

Adjacent sequences:  A109392 A109393 A109394 * A109396 A109397 A109398

KEYWORD

nonn,frac

AUTHOR

Franz Vrabec, Aug 26 2005

STATUS

approved

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Last modified February 23 02:33 EST 2020. Contains 332159 sequences. (Running on oeis4.)