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%I #14 May 25 2020 17:17:31
%S 3,97,241,337,577,1249,3361,3697,4801,6961,8191,10657,14449,16561,
%T 18049,23761,25537,31249,32257,37537,48049,49297,64081,65521,77617,
%U 79201,89041,120049,126001,138337,140449,153457,171697,174049,181201,193441,208657,216481
%N Prime k with sigma(sigma(sigma(k))) < 3*k + 1.
%H Robert Israel, <a href="/A320517/b320517.txt">Table of n, a(n) for n = 1..1000</a>
%p with(numtheory,sigma):
%p isA320517 := n -> isprime(n) and (sigma(sigma(sigma(n))) < 3*n + 1):
%p S := select(isA320517, [$1..300000]);
%t Select[Range[250000], PrimeQ[#] && DivisorSigma[1, DivisorSigma[1, DivisorSigma[1, #]]] < 3*# + 1 &] (* _Vaclav Kotesovec_, Oct 14 2018 *)
%o (PARI) isok(n) = isprime(n) && (sigma(sigma(sigma(n))) < 3*n + 1); \\ _Michel Marcus_, Oct 14 2018
%Y Cf. A000203, A320513.
%K nonn
%O 1,1
%A _Peter Luschny_, Oct 14 2018
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