A231507 a(n) is smallest number greater than a(n-1) such that a(n)+a(n-1) is composite.

4, 5, 7, 8, 10, 11, 13, 14, 16, 17, 18, 20, 22, 23, 25, 26, 28, 29, 31, 32, 33, 35, 37, 38, 39, 41, 43, 44, 46, 47, 48, 50, 52, 53, 55, 56, 58, 59, 60, 61, 62, 63, 65, 67, 68, 70, 71, 72, 73, 74, 76, 77, 78, 80, 81, 83, 85, 86, 88, 89, 91, 92, 93, 94, 95, 97

Offset: 1

Author

Neil Fernandez, Nov 09 2013

Keywords

• nonn

Examples

			a(1) = 4, the first composite. So the smallest a(2) could possibly be 5. 4+5=9, which is composite, so a(2) = 5. a(3) cannot be 6, because 5+6=11, which is prime. But 5+7=12 is composite, so a(3) = 7.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A062042, A073627.

Programs

Mathematica

nxt[n_]:=Module[{k=n+1},While[PrimeQ[n+k],k++];k]; NestList[nxt,4,70] (* Harvey P. Dale, Jul 18 2014 *)