A178663 a(1)=1. For n>1, a(n) is the smallest number greater than a(n-1) such that exactly one of n and a(n) is prime and the other is composite.

1, 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

1,2

Comments

Equals A163578 from the second term on. - R. J. Mathar, Jun 07 2010

Links

Table of n, a(n) for n=1..62.

Example

a(6) cannot equal 9 because both 6 and 9 are composite.

Mathematica

a[n_] := a[n] = Block[{k = a[n - 1]}, If[ PrimeQ@n, k++; While[PrimeQ@k, k++ ], k = NextPrime@k]; k]; a[1] = 1; Array[a, 62] (* Robert G. Wilson v, Jun 04 2010 *)

Crossrefs

Sequence in context: A139490 A030375 A081712 * A084395 A047291 A100349

Adjacent sequences: A178660 A178661 A178662 * A178664 A178665 A178666

Keyword

nonn

Author

J. Lowell, Jun 01 2010

Extensions

a(9) onwards from Robert G. Wilson v, Jun 04 2010

Status

approved