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 A356453 #21 Aug 07 2024 14:11:12 %S A356453 3,6,9,10,12,15,18,21,24,27,28,30,33,36,39,40,42,45,48,50,51,54,57,60, %T A356453 63,66,69,70,72,75,77,78,81,84,87,90,93,96,99,102,105,108,110,111,114, %U A356453 117,120,123,126,129,130,132,133,135,136,138,140,141,144,147,150,153,154,155 %N A356453 Numbers k such that 2*k^2 is not in A014567; complement of A356449. %C A356453 Numbers k such that k and sigma(2*k^2) are not coprime, sigma = A000203. %C A356453 Includes all multiples of 3 since 3 divides sigma(m) if m is twice a square (cf. A065766). %H A356453 Jianing Song, <a href="/A356453/b356453.txt">Table of n, a(n) for n = 1..8914</a> (all terms <= 20000) %e A356453 10 is a term since 10 and sigma(2*10^2) = 465 have a common factor 5. %t A356453 Select[Range[155],GCD[#, DivisorSigma[1,2#^2]]>1 &] (* _Stefano Spezia_, Aug 07 2024 *) %o A356453 (PARI) isA356453(n) = gcd(n, sigma(2*n^2))>1 %Y A356453 Cf. A014567, A000203, A356448, A356449, A356452, A356454, A356456 (all multiples of 3 removed), A065766. %K A356453 nonn,easy %O A356453 1,1 %A A356453 _Jianing Song_, Aug 07 2022