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 A137621 #11 Jun 01 2025 16:18:56
%S A137621 1,2,4,3,6,8,9,5,10,12,7,14,15,16,18,20,21,11,22,24,25,13,26,27,28,30,
%T A137621 32,33,17,34,35,36,19,38,39,40,42,44,45,23,46,48,49,50,51,52,54,55,56,
%U A137621 57,29,58,60,31,62,63,64,65,66,68,69,70,72,37,74,75,76,77,78,80,81,41
%N A137621 Divisors of nonprimes not occurring earlier.
%C A137621 Permutation with inverse A137622; A137623(n) = a(a(n)).
%C A137621 a(A120632(n))=A000040(n); a(A120632(n)+1)=A100484(n); a(A137624(n))=A018252(n).
%H A137621 Reinhard Zumkeller, <a href="/A137621/b137621.txt">Table of n, a(n) for n = 1..10001</a>
%H A137621 <a href="/index/Per#IntegerPermutation">Index entries for sequences that are permutations of the natural numbers</a>
%e A137621 1;2,4;3,6;8;9;5,10;12;7,14;15;16;18;20;21;11,22;24; .. .
%t A137621 s={};Do[If[!PrimeQ[n],s=Join[s,Complement[Divisors[n],s]]],{n,81}];s (* _James C. McMahon_, Jun 01 2025 *)
%K A137621 nonn
%O A137621 1,2
%A A137621 _Reinhard Zumkeller_, Jan 30 2008