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 A227240 #26 Jul 28 2025 17:12:40
%S A227240 1,3,5,7,11,23,29,41,53,77,83,89,103,113,131,143,173,179,191,233,239,
%T A227240 251,281,293,359,419,431,443,491,509,533,593,641,653,659,667,683,719,
%U A227240 743,761,807,809,817,911,953,1013,1019,1031,1049,1073,1103,1223,1229,1289,1409
%N A227240 Numbers k such that sigma(k) divides sigma(2*k) and sigma(2*k + 1).
%C A227240 Numbers such that 2*k and/or 2*k + 1 is also in the sequence: 1, 3, 5, 11, 41, 89, 179, 359, 509, 719, 743, ... (Cf. A007700).
%H A227240 G. C. Greubel, <a href="/A227240/b227240.txt">Table of n, a(n) for n = 1..5000</a>
%t A227240 Select[Range[1000], IntegerQ[DivisorSigma[1, 2#]/DivisorSigma[1, #]] && IntegerQ[DivisorSigma[1, 2# + 1]/DivisorSigma[1, #]] &] (* _Alonso del Arte_, Jul 15 2013 *)
%t A227240 Select[Range[1500],And@@Divisible[{DivisorSigma[1,2#],DivisorSigma[1,2#+1]}, DivisorSigma[1,#]]&] (* _Harvey P. Dale_, Feb 25 2016 *)
%o A227240 (PARI) isok(n) = my(sn=sigma(n)); !(sigma(2*n) % sn) && !(sigma(2*n+1) % sn); \\ _Michel Marcus_, Oct 02 2017
%Y A227240 Cf. A000203, A007700, A058072, A058073.
%K A227240 nonn
%O A227240 1,2
%A A227240 _Alex Ratushnyak_, Jul 03 2013