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Nonprimes n such that exactly one of 2n-1 and 2n+1 is prime.
2

%I #10 Aug 27 2024 18:39:57

%S 1,4,8,10,12,14,16,18,20,22,24,26,27,33,34,35,39,40,42,44,45,48,49,50,

%T 52,55,56,57,63,64,65,66,68,70,74,76,78,81,82,84,86,87,91,95,98,100,

%U 105,106,111,112,115,116,117,119,121,125,126,128,129,132,134,136,138,140

%N Nonprimes n such that exactly one of 2n-1 and 2n+1 is prime.

%H Harvey P. Dale, <a href="/A172458/b172458.txt">Table of n, a(n) for n = 1..1000</a>

%t Select[Range[150],!PrimeQ[#]&&Total[Boole[PrimeQ[2 #+{1,-1}]]]==1&] (* _Harvey P. Dale_, Aug 27 2024 *)

%K nonn,easy,less

%O 1,2

%A _Juri-Stepan Gerasimov_, Feb 03 2010

%E Edited by _Charles R Greathouse IV_, Mar 25 2010