A241264 - OEIS

%I #18 Sep 08 2022 08:46:07

%S 4,13,20,24,26,30,31,32,35,36,37,38,40,41,43,47,49,51,54,55,60,62,63,

%T 64,67,70,71,72,73,75,76,78,79,81,82,84,85,88,89,92,94,97,98,100,102,

%U 105,108,109,111,112,113,114,115,117,118,119,120,122,123,124,125

%N Numbers k such that 2*k^2 + 2*k - 41 is not a prime.

%H Vincenzo Librandi, <a href="/A241264/b241264.txt">Table of n, a(n) for n = 1..5000</a>

%e 13 is in this sequence because 2*13^2 + 2*13 - 41 = 323 = 17*19.

%t Select[Range[4, 200], ! PrimeQ[2 #^2 + 2 # - 41] &]

%o (Magma) [n: n in [1..150] | not IsPrime(2*n^2+2*n-41)];

%Y Complement of A155771.

%K nonn,easy

%O 1,1

%A _Vincenzo Librandi_, Apr 20 2014