OFFSET
1,1
LINKS
Robert Israel, Table of n, a(n) for n = 1..1000
MAPLE
with(numtheory, sigma):
isA320517 := n -> isprime(n) and (sigma(sigma(sigma(n))) < 3*n + 1):
S := select(isA320517, [$1..300000]);
MATHEMATICA
Select[Range[250000], PrimeQ[#] && DivisorSigma[1, DivisorSigma[1, DivisorSigma[1, #]]] < 3*# + 1 &] (* Vaclav Kotesovec, Oct 14 2018 *)
PROG
(PARI) isok(n) = isprime(n) && (sigma(sigma(sigma(n))) < 3*n + 1); \\ Michel Marcus, Oct 14 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Peter Luschny, Oct 14 2018
STATUS
approved