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%I #10 Sep 08 2022 08:46:05
%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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