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
Keywords
Examples
a(6) cannot equal 9 because both 6 and 9 are composite.
Programs
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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 *)
Extensions
a(9) onwards from Robert G. Wilson v, Jun 04 2010
Comments