A178663 - OEIS

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.

%I #11 Aug 02 2015 12:58:15

%S 1,4,6,7,8,11,12,13,17,19,20,23,24,29,31,37,38,41,42,43,47,53,54,59,

%T 61,67,71,73,74,79,80,83,89,97,101,103,104,107,109,113,114,127,128,

%U 131,137,139,140,149,151,157,163,167,168,173,179,181,191,193,194,197,198,199

%N 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.

%C Equals A163578 from the second term on. - _R. J. Mathar_, Jun 07 2010

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

%t 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 *)

%K nonn

%O 1,2

%A _J. Lowell_, Jun 01 2010

%E a(9) onwards from _Robert G. Wilson v_, Jun 04 2010