A318059 a(n) is the numerator of sigma(sigma(n))/n.

1, 2, 7, 2, 12, 14, 15, 3, 14, 39, 28, 14, 24, 30, 4, 2, 39, 28, 42, 24, 3, 91, 60, 7, 32, 48, 10, 30, 72, 13, 63, 13, 124, 60, 124, 28, 60, 84, 40, 117, 96, 6, 84, 56, 56, 195, 124, 14, 80, 64, 65, 171, 120, 20, 39, 45, 62, 117, 168, 8, 96, 126, 10, 2, 224, 403, 126, 78, 84, 403

Offset: 1

Author

Michel Marcus, Aug 14 2018

Keywords

• nonn
• frac

Examples

			Fractions begin with 1, 2, 7/3, 2, 12/5, 14/3, 15/7, 3, 14/9, 39/10, 28/11, 14/3, ...
		

Links

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

Crossrefs

Cf. A000203 (sigma), A051027.

Cf. A098223 (when denominator is 1), A318060 (denominator).

Programs

Maple

seq(numer((numtheory:-sigma@@2)(n)/n),n=1..100); # Robert Israel, Aug 14 2018

Mathematica

a[n_] := Numerator[DivisorSigma[1, DivisorSigma[1, n]]/n]; Array[a, 100] (* Amiram Eldar, Apr 04 2024 *)

PARI

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

Formula

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