A137621 Divisors of nonprimes not occurring earlier.

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, 32, 33, 17, 34, 35, 36, 19, 38, 39, 40, 42, 44, 45, 23, 46, 48, 49, 50, 51, 52, 54, 55, 56, 57, 29, 58, 60, 31, 62, 63, 64, 65, 66, 68, 69, 70, 72, 37, 74, 75, 76, 77, 78, 80, 81, 41

Offset: 1

Author

Reinhard Zumkeller, Jan 30 2008

Keywords

• nonn

Comments

Permutation with inverse A137622; A137623(n) = a(a(n)).

a(A120632(n))=A000040(n); a(A120632(n)+1)=A100484(n); a(A137624(n))=A018252(n).

Examples

			1;2,4;3,6;8;9;5,10;12;7,14;15;16;18;20;21;11,22;24; .. .
		

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..10001
• Index entries for sequences that are permutations of the natural numbers

Programs

Mathematica

s={};Do[If[!PrimeQ[n],s=Join[s,Complement[Divisors[n],s]]],{n,81}];s (* James C. McMahon, Jun 01 2025 *)