A320517 - OEIS

%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