A318060 a(n) is the denominator of sigma(sigma(n))/n.

1, 1, 3, 1, 5, 3, 7, 1, 9, 10, 11, 3, 13, 7, 1, 1, 17, 9, 19, 5, 1, 22, 23, 1, 25, 13, 3, 7, 29, 2, 31, 4, 33, 17, 35, 9, 37, 19, 13, 20, 41, 1, 43, 11, 15, 46, 47, 3, 49, 25, 17, 52, 53, 3, 11, 7, 19, 29, 59, 1, 61, 31, 3, 1, 65, 66, 67, 17, 23, 70

Offset: 1

Author

Michel Marcus, Aug 14 2018

Keywords

• nonn
• frac

Examples

			For n=2, sigma(sigma(n)) = 4, so a(2) = 1.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000203 (sigma), A051027.

Cf. A019278 (where a(n) = 1), A318059 (numerator).

Programs

Maple

seq(denom((numtheory:-sigma@@2)(n)/n),n=1..200); # Robert Israel, Aug 15 2018

Mathematica

Table[(DivisorSigma[1,DivisorSigma[1,n]])/n,{n,70}]//Denominator (* Harvey P. Dale, Mar 30 2023 *)

PARI

a(n) = denominator(sigma(sigma(n))/n);