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A228374 Numbers n such that 2*prime(n) - prime(n+1) is not prime. 1

%I

%S 1,2,5,6,7,8,10,11,12,13,14,17,18,19,20,22,23,24,25,26,27,28,29,30,31,

%T 32,33,34,35,36,38,39,41,42,43,44,45,46,48,49,50,52,53,54,57,58,59,60,

%U 61,62,63,64,65,66,67,68,69,70,72,73,75,76,77,78,79,80,81,82

%N Numbers n such that 2*prime(n) - prime(n+1) is not prime.

%C Complement of A216075.

%H Vincenzo Librandi, <a href="/A228374/b228374.txt">Table of n, a(n) for n = 1..1000</a>

%e 7 is in the sequence because 2*17-19=15 and 15 is not prime.

%t f[n_]: = (2 Prime[n] - Prime[n + 1]); Select[Range[110], ! PrimeQ[f[#]]&]

%t Flatten[Position[2#[[1]]-#[[2]]&/@Partition[Prime[Range[100]],2,1],_?(!PrimeQ[#]&)]] (* _Harvey P. Dale_, Jan 12 2016 *)

%o (MAGMA) [n: n in [1..100]| not IsPrime(2*NthPrime(n) - NthPrime(n+1))];

%Y Cf. A216075.

%K nonn,easy

%O 1,2

%A _Vincenzo Librandi_, Aug 21 2013

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Last modified September 28 04:24 EDT 2021. Contains 347698 sequences. (Running on oeis4.)