A066930 Numbers k such that phi(sigma(k)) divides sigma(phi(k)).

1, 7, 9, 29, 71, 97, 109, 121, 139, 142, 175, 183, 194, 215, 225, 242, 244, 261, 278, 311, 344, 349, 355, 430, 497, 509, 516, 533, 556, 571, 605, 622, 631, 647, 673, 709, 729, 791, 817, 859, 911, 923, 1021, 1066, 1112, 1119, 1142, 1207, 1243, 1262, 1277

Offset: 1

Author

Benoit Cloitre, Jan 26 2002

Keywords

• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from Harry J. Smith)

Crossrefs

Cf. A000010 (phi), A000203 (sigma), A033632 (subsequence), A062401, A062402.

Programs

Mathematica

Select[Range[1300],Divisible[DivisorSigma[1,EulerPhi[#]],EulerPhi[ DivisorSigma[ 1,#]]]&] (* Harvey P. Dale, Dec 15 2021 *)

PARI

isok(k) = { sigma(eulerphi(k)) % eulerphi(sigma(k)) == 0 } \\ Harry J. Smith, Apr 08 2010