A066248 a(n) = if n+1 is prime then A049084(n+1)*2 else A066246(n+1)*2 - 1.

2, 4, 1, 6, 3, 8, 5, 7, 9, 10, 11, 12, 13, 15, 17, 14, 19, 16, 21, 23, 25, 18, 27, 29, 31, 33, 35, 20, 37, 22, 39, 41, 43, 45, 47, 24, 49, 51, 53, 26, 55, 28, 57, 59, 61, 30, 63, 65, 67, 69, 71, 32, 73, 75, 77, 79, 81, 34, 83, 36, 85, 87, 89, 91, 93, 38, 95, 97, 99, 40, 101, 42

Offset: 1

Author

Reinhard Zumkeller, Dec 09 2001

Keywords

• nonn

Comments

Permutation of natural numbers; inverse: A066249.

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000
• Index entries for sequences that are permutations of the natural numbers.

Crossrefs

Cf. A066249, A049084, A066246, A026238, A066247, A066250, A066252.

Programs

Mathematica

a[n_] := If[PrimeQ[n+1], 2 * PrimePi[n+1], 2 * (n - PrimePi[n+1]) - 1]; Array[a, 100] (* Amiram Eldar, Mar 19 2025 *)

Formula

a(n) = A026238(n+1)*2 - A066247(n+1).