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This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A143834 Numbers k such that 2k^2 - 1 is not prime.

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%I A143834 #11 Sep 08 2022 08:45:37
%S A143834 1,5,9,12,14,16,19,20,23,26,27,29,30,31,32,33,35,37,40,44,47,48,51,53,
%T A143834 54,55,57,58,60,61,65,66,67,68,70,71,72,74,75,77,78,79,82,83,84,86,88,
%U A143834 89,90,93,94,96,97,99,100,101,103,104,105,106,107,110,111,114,116,117
%N A143834 Numbers k such that 2k^2 - 1 is not prime.
%C A143834 Complement of A066049.
%t A143834 p = 2; a = {}; Do[k = p x^2 - 1; If[PrimeQ[k],NULL, AppendTo[a, x]], {x, 1, 1000}]; a
%t A143834 Select[Range[120],!PrimeQ[2#^2-1]&] (* _Harvey P. Dale_, Mar 14 2018 *)
%o A143834 (Magma) [n: n in [1..120]| not IsPrime(2*n^2-1)] // _Vincenzo Librandi_, Jan 28 2011
%Y A143834 Cf. A066436, A066049, A090686, A090684, A143826, A143827, A143828, A143829, A143830, A143831, A143833.
%K A143834 nonn
%O A143834 1,2
%A A143834 _Artur Jasinski_, Sep 02 2008