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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. 11

%I

%S 4,7,10,12,13,14,16,17,18,19,20,22,23,24,25,26,27,28,29,31,32,33,34,

%T 35,36,37,38,40,41,42,43,44,45,46,47,48,49,51,52,53,54,55,56,57,58,59,

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

%C Numbers x such that 1 + Sum_{k=1..n} 2^(2k-1) is prime for n=1,2,...,x gives A127936.

%H G. C. Greubel, <a href="/A127958/b127958.txt">Table of n, a(n) for n = 1..10000</a>

%t a = {}; Do[If[PrimeQ[1 + Sum[2^(2n - 1), {n, 1, x}]] == False, AppendTo[a, x]], {x, 1, 1000}]; a

%o (PARI) isok(x) = !isprime(1+sum(k=1, x, 2^(2*k-1))); \\ _Michel Marcus_, May 09 2018

%Y Cf. A000979, A000978, A124400, A126614, A127955, A127957, A127936.

%K nonn

%O 1,1

%A _Artur Jasinski_, Feb 09 2007

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Last modified August 6 11:55 EDT 2020. Contains 336246 sequences. (Running on oeis4.)