A127957 Numbers k such that (2^prime(k) + 1)/3 is composite.

10, 12, 13, 15, 16, 17, 19, 20, 21, 23, 24, 25, 27, 28, 29, 30, 32, 33, 34, 35, 36, 37, 38, 40, 41, 42, 44, 45, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 66, 67, 68, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89

Offset: 1

Author

Artur Jasinski, Feb 09 2007

Keywords

• nonn

Links

• G. C. Greubel, Table of n, a(n) for n = 1..1175

Crossrefs

Cf. A000979, A000978, A124400, A126614, A127955, A127956.

Programs

Mathematica

a = {}; Do[c = (2^Prime[x] + 1)/3; If[PrimeQ[c] == False, AppendTo[a, x]], {x, 2, 300}]; a
Select[Range[2,100],!PrimeQ[(2^Prime[#]+1)/3]&] (* Harvey P. Dale, Nov 24 2013 *)

PARI

isok(n) = (n!=1) && !isprime((2^prime(n)+1)/3); \\ Michel Marcus, Jul 07 2018