A167419 Exchange adjacent nonprimes and primes.

2, 1, 4, 3, 6, 5, 8, 7, 9, 11, 10, 13, 12, 14, 15, 17, 16, 19, 18, 20, 21, 23, 22, 24, 25, 26, 27, 29, 28, 31, 30, 32, 33, 34, 35, 37, 36, 38, 39, 41, 40, 43, 42, 44, 45, 47, 46, 48, 49, 50, 51, 53, 52, 54, 55, 56, 57, 59, 58, 61, 60, 62, 63, 64, 65, 67, 66, 68, 69, 71, 70, 73, 72

Offset: 1

Author

Giovanni Teofilatto, Nov 03 2009

Keywords

• easy
• nonn

Comments

If we have consecutive numbers, one prime and one nonprime, swap them. So after the initial 2,1,4,3,6,5,8,7 we have "if n is prime, a(n) = n-1; if n+1 is prime, a(n) = n+1, otherwise a(n) = n".

Links

• Stefano Spezia, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A014681.

Programs

Mathematica

a[n_]:=If[PrimeQ[n],n-1,If[PrimeQ[n+1],n+1,n]]; Join[{2,1,4,3,6,5,8,7},Array[a,65,9]] (* Stefano Spezia, May 05 2023 *)

Extensions

Edited by Franklin T. Adams-Watters, Nov 04 2009

42 and 64 inserted by Stefano Spezia, May 05 2023