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%I #14 Sep 08 2022 08:45:17
%S 1,5,8,11,14,18,20,23,26,29,33,35,39,41,44,48,50,53,56,63,65,68,74,78,
%T 81,83,86,89,95,98,105,111,113,116,119,125,128,131,134,138,140,146,
%U 153,155,158,165,168,173,176,179,183,186,189,191,194,198,200,204,209,215
%N Numbers n such that 2n+1 is prime and 2n-1 is not prime.
%H Robert Israel, <a href="/A104279/b104279.txt">Table of n, a(n) for n = 1..10000</a>
%e a(1)=1 because 3 is prime and 1 is not prime.
%e a(2)=5 because 11 is prime and 9 is not prime.
%p filter:= n -> isprime(2*n+1) and not isprime(2*n-1):
%p select(filter, [$1..300]); # _Robert Israel_, Apr 27 2020
%t Select[ Range[218], PrimeQ[2# + 1] && !PrimeQ[2# - 1] &] (* _Robert G. Wilson v_, Apr 18 2005 *)
%o (Magma) [n: n in [1..220]| not IsPrime(2*n-1) and IsPrime(2*n+1)] // _Vincenzo Librandi_, Jan 28 2011
%o (PARI) is(n)=isprime(2*n+1) && !isprime(2*n-1) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A040040 and others.
%K easy,nonn
%O 1,2
%A _Alexandre Wajnberg_, Apr 17 2005
%E More terms from _Robert G. Wilson v_, Apr 18 2005
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