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 A320514 #15 Jun 17 2020 04:18:04 %S A320514 1,2,4,9,13,16,25,37,43,49,61,64,67,73,81,97,109,121,151,157,163,169, %T A320514 181,193,211,225,229,241,277,283,289,313,331,337,361,373,397,409,421, %U A320514 433,457,487,523,529,541,547,577,601,613,625,631,661,673,691,709,729,733,751,757,787,823,829,841,853,877 %N A320514 Numbers k such that sigma(sigma(k)) < 2*k + 1. %H A320514 Amiram Eldar, <a href="/A320514/b320514.txt">Table of n, a(n) for n = 1..10000</a> %p A320514 isA320514 := n -> sigma(sigma(n)) < 2*n+1: %p A320514 select(isA320514, [$1..1000]); %t A320514 Select[Range[1000], DivisorSigma[1, DivisorSigma[1, #]] < 2*# + 1 &] (* _Vaclav Kotesovec_, Oct 14 2018 *) %o A320514 (PARI) isok(n) = sigma(sigma(n)) < 2*n + 1; \\ _Michel Marcus_, Oct 14 2018 %Y A320514 Cf. A000203, A320515. %K A320514 nonn %O A320514 1,2 %A A320514 _Peter Luschny_, Oct 14 2018