This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A320515 #18 Jun 17 2020 05:49:30 %S A320515 2,13,37,43,61,67,73,97,109,151,157,163,181,193,211,229,241,277,283, %T A320515 313,331,337,373,397,409,421,433,457,487,523,541,547,577,601,613,631, %U A320515 661,673,691,709,733,751,757,787,823,829,853,877,883,907,937,997 %N A320515 Squarefree k > 1 with sigma(sigma(k)) < 2*k + 1. %C A320515 Conjecturally a subsequence of A085497. %C A320515 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 %H A320515 Amiram Eldar, <a href="/A320515/b320515.txt">Table of n, a(n) for n = 1..10000</a> %p A320515 isA320515 := n -> (n > 1) and issqrfree(n) and (sigma(sigma(n)) < 2*n+1): %p A320515 select(isA320515, [$1..1000]); %t A320515 Rest[Select[Range[1000], SquareFreeQ[#] && DivisorSigma[1, DivisorSigma[1, #]] < 2*# + 1 &]] (* _Vaclav Kotesovec_, Oct 14 2018 *) %o A320515 (PARI) isok(n) = (n>1) && issquarefree(n) && (sigma(sigma(n)) < 2*n + 1); \\ _Michel Marcus_, Oct 14 2018 %Y A320515 Cf. A085497, A000668. %K A320515 nonn %O A320515 1,1 %A A320515 _Peter Luschny_, Oct 14 2018