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A127958 Numbers x such that 1 + Sum_{k=1..n} 2^(2k-1) is not prime for n=1,2,...,x.

%I A127958 #11 Sep 16 2018 02:26:39
%S A127958 4,7,10,12,13,14,16,17,18,19,20,22,23,24,25,26,27,28,29,31,32,33,34,
%T A127958 35,36,37,38,40,41,42,43,44,45,46,47,48,49,51,52,53,54,55,56,57,58,59,
%U A127958 60,61,62,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,84,85
%N A127958 Numbers x such that 1 + Sum_{k=1..n} 2^(2k-1) is not prime for n=1,2,...,x.
%C A127958 Numbers x such that 1 + Sum_{k=1..n} 2^(2k-1) is prime for n=1,2,...,x gives A127936.
%H A127958 G. C. Greubel, <a href="/A127958/b127958.txt">Table of n, a(n) for n = 1..10000</a>
%t A127958 a = {}; Do[If[PrimeQ[1 + Sum[2^(2n - 1), {n, 1, x}]] == False, AppendTo[a, x]], {x, 1, 1000}]; a
%o A127958 (PARI) isok(x) = !isprime(1+sum(k=1, x, 2^(2*k-1))); \\ _Michel Marcus_, May 09 2018
%Y A127958 Cf. A000979, A000978, A124400, A126614, A127955, A127957, A127936.
%K A127958 nonn
%O A127958 1,1
%A A127958 _Artur Jasinski_, Feb 09 2007