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A127957 Numbers k such that (2^prime(k) + 1)/3 is composite. 13

%I

%S 10,12,13,15,16,17,19,20,21,23,24,25,27,28,29,30,32,33,34,35,36,37,38,

%T 40,41,42,44,45,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,

%U 66,67,68,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89

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

%H G. C. Greubel, <a href="/A127957/b127957.txt">Table of n, a(n) for n = 1..1175</a>

%t a = {}; Do[c = (2^Prime[x] + 1)/3; If[PrimeQ[c] == False, AppendTo[a, x]], {x, 2, 300}]; a

%t Select[Range[2,100],!PrimeQ[(2^Prime[#]+1)/3]&] (* _Harvey P. Dale_, Nov 24 2013 *)

%o (PARI) isok(n) = (n!=1) && !isprime((2^prime(n)+1)/3); \\ _Michel Marcus_, Jul 07 2018

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

%K nonn

%O 1,1

%A _Artur Jasinski_, Feb 09 2007

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Last modified July 20 01:17 EDT 2019. Contains 325168 sequences. (Running on oeis4.)