A163578 If n is prime, a(n) is next composite greater than a(n-1), else if n is composite, a(n) is next prime greater than a(n-1) else 2.

2, 4, 6, 7, 8, 11, 12, 13, 17, 19, 20, 23, 24, 29, 31, 37, 38, 41, 42, 43, 47, 53, 54, 59, 61, 67, 71, 73, 74, 79, 80, 83, 89, 97, 101, 103, 104, 107, 109, 113, 114, 127, 128, 131, 137, 139, 140, 149, 151, 157, 163, 167, 168, 173, 179, 181, 191, 193, 194, 197, 198, 199

Offset: 1

Author

Gerald Hillier, Jul 31 2009

Keywords

• nonn
• changed

Examples

			1 is not prime, a(1)=2.
2 is prime, a(2)=4.
3 is prime, a(3)=6.
4 is composite, a(4)=7.
		

Links

• Paolo Xausa, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A159559, A178663.

Programs

Mathematica

Join[{2, 4}, FoldList[If[PrimeQ[#2], #+1, NextPrime[#]] &, 6, Range[4, 100]]] (* Paolo Xausa, Sep 03 2025 *)

Extensions

More terms from Paolo Xausa, Sep 03 2025