OFFSET
1,1
COMMENTS
Conjecturally a subsequence of A085497.
This conjecture is false, the first counterexample is a(113) = 2257 = 37 * 61 which is the least composite term in this sequence. - Amiram Eldar, Jun 17 2020
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
MAPLE
isA320515 := n -> (n > 1) and issqrfree(n) and (sigma(sigma(n)) < 2*n+1):
select(isA320515, [$1..1000]);
MATHEMATICA
Rest[Select[Range[1000], SquareFreeQ[#] && DivisorSigma[1, DivisorSigma[1, #]] < 2*# + 1 &]] (* Vaclav Kotesovec, Oct 14 2018 *)
PROG
(PARI) isok(n) = (n>1) && issquarefree(n) && (sigma(sigma(n)) < 2*n + 1); \\ Michel Marcus, Oct 14 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Peter Luschny, Oct 14 2018
STATUS
approved