A067065 Numbers k such that sigma(sigma(sigma(k))) == 6*sigma(k).

20, 26, 41, 44, 65, 83, 132, 133, 140, 182, 188, 195, 249, 287, 299, 420, 546, 564, 620, 644, 764, 806, 861, 897, 1001, 1115, 1169, 1271, 28644, 32172, 35052, 39116, 40796, 41478, 42315, 47492, 50162, 51513, 52143, 53745, 54033, 54483, 56427, 56642

Offset: 1

Author

Benoit Cloitre, Feb 17 2002

Keywords

• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..270 (terms 1..146 from Harry J. Smith)

Crossrefs

Cf. A000203 (sigma), A066971.

Programs

Mathematica

Select[Range[60000],Nest[DivisorSigma[1,#]&,#,3]==6*DivisorSigma[1,#]&] (* Harvey P. Dale, Oct 04 2016 *)

PARI

isok(k) = { sigma(sigma(sigma(k))) == 6*sigma(k) } \\ Harry J. Smith, May 03 2010

Extensions

Definition corrected and more terms added by Harry J. Smith, May 03 2010