A072869 a(n) = sigma(sigma(n)-n), where sigma = A000203, sum of the divisors of n.

1, 1, 4, 1, 12, 1, 8, 7, 15, 1, 31, 1, 18, 13, 24, 1, 32, 1, 36, 12, 24, 1, 91, 12, 31, 14, 56, 1, 96, 1, 32, 24, 42, 14, 72, 1, 36, 18, 93, 1, 120, 1, 90, 48, 42, 1, 140, 15, 44, 32, 72, 1, 144, 18, 127, 24, 63, 1, 280, 1, 54, 42, 104, 20, 168, 1, 90, 40, 114, 1, 168, 1, 90, 57

Offset: 2

Author

Benoit Cloitre, Jul 27 2002

Keywords

• easy
• nonn

Links

• Antti Karttunen, Table of n, a(n) for n = 2..16384
• Index entries for sequences related to sums of divisors

Crossrefs

Cf. A000203 (sigma(n)), A001065 (sigma(n) - n), A051027.

Cf. A072868 (fixed points).

Programs

Mathematica

Table[DivisorSigma[1,DivisorSigma[1,n]-n],{n,2,80}] (* Harvey P. Dale, Sep 23 2013 *)

PARI

A072869(n) = sigma(sigma(n)-n); \\ Antti Karttunen, Oct 30 2017

Formula

a(n) = A000203(A001065(n)). - Michel Marcus, Nov 26 2013

For p prime, a(p) = 1. - Michel Marcus, Nov 19 2014

a(k) = k if k is one greater than a Mersenne prime. - Jarrod G. Sage, Dec 07 2024