A320513 Squarefree k > 1 with sigma(sigma(sigma(k))) < 3*k + 1.

3, 97, 217, 241, 337, 577, 889, 1249, 1393, 1501, 3361, 3697, 3937, 4039, 4801, 5587, 6961, 7471, 8191, 9481, 10657, 14449, 16177, 16561, 17887, 18049, 18157, 22459, 23527, 23761, 24301, 25273, 25537, 29617, 29977, 30607, 31249, 32257, 33607, 37537, 37981

Offset: 1

Author

Peter Luschny, Oct 14 2018

Keywords

• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000203, A320514.

Programs

Maple

with(numtheory, sigma):
isA320513 := n -> (n > 1) and issqrfree(n) and (sigma(sigma(sigma(n))) < 3*n+1):
S := select(isA320513, [$1..100000]);

Mathematica

Rest[Select[Range[100000], SquareFreeQ[#] && DivisorSigma[1, DivisorSigma[1, DivisorSigma[1, #]]] < 3*# + 1 &]] (* Vaclav Kotesovec, Oct 14 2018 *)

PARI

isok(n) = (n>1) && issquarefree(n) && (sigma(sigma(sigma(n))) < 3*n + 1); \\ Michel Marcus, Oct 14 2018