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

%I A320513 #17 Jun 17 2020 04:17:53
%S A320513 3,97,217,241,337,577,889,1249,1393,1501,3361,3697,3937,4039,4801,
%T A320513 5587,6961,7471,8191,9481,10657,14449,16177,16561,17887,18049,18157,
%U A320513 22459,23527,23761,24301,25273,25537,29617,29977,30607,31249,32257,33607,37537,37981
%N A320513 Squarefree k > 1 with sigma(sigma(sigma(k))) < 3*k + 1.
%H A320513 Amiram Eldar, <a href="/A320513/b320513.txt">Table of n, a(n) for n = 1..10000</a>
%p A320513 with(numtheory, sigma):
%p A320513 isA320513 := n -> (n > 1) and issqrfree(n) and (sigma(sigma(sigma(n))) < 3*n+1):
%p A320513 S := select(isA320513, [$1..100000]);
%t A320513 Rest[Select[Range[100000], SquareFreeQ[#] && DivisorSigma[1, DivisorSigma[1, DivisorSigma[1, #]]] < 3*# + 1 &]] (* _Vaclav Kotesovec_, Oct 14 2018 *)
%o A320513 (PARI) isok(n) = (n>1) && issquarefree(n) && (sigma(sigma(sigma(n))) < 3*n + 1); \\ _Michel Marcus_, Oct 14 2018
%Y A320513 Cf. A000203, A320514.
%K A320513 nonn
%O A320513 1,1
%A A320513 _Peter Luschny_, Oct 14 2018