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 A045751 Numbers k such that 4*k + 1 is not prime. 14
 0, 2, 5, 6, 8, 11, 12, 14, 16, 17, 19, 20, 21, 23, 26, 29, 30, 31, 32, 33, 35, 36, 38, 40, 41, 42, 44, 46, 47, 50, 51, 52, 53, 54, 55, 56, 59, 61, 62, 63, 65, 66, 68, 71, 72, 74, 75, 76, 77, 80, 81, 82, 83, 85, 86, 89, 90, 91, 92, 94, 95, 96, 98, 101, 103, 104, 106, 107, 109 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Terms (except 0) can be written as 4xy +- (x+y) for x > 0, y > 0. - Ron R Spencer, Jul 28 2016 Numbers k such that (4*k)!/(4*k + 1) is an integer. - Peter Bala, Jan 25 2017 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 EXAMPLE Distribution of the positive terms in the following triangular array:    2;    *,   6;    5,   *,  12;    *,  11,   *,  20;    8,   *,  19,   *,  30;    *,  16,   *,  29,   *,  42;   11,   *,  26,   *,  41,   *,  56;    *,  21,   *,  38,   *,  55,   *,  72;   14,   *,  33,   *,  52,   *,  71,   *,  90;    *,  26,   *,  47,   *,  68,   *,  89,   *, 110;   17,   *,  40,   *,  63,   *,  86,   *, 109,   *, 132; etc., where * marks the noninteger values of (2*h*k + k + h)/2 with h >= k >= 1. - Vincenzo Librandi, Jan 14 2013 MAPLE for n from 0 to 100 do if irem(factorial(4*n), 4*n+1) = 0 then print(n); end if; end do: # Peter Bala, Jan 25 2017 MATHEMATICA Select[Range[0, 200], ! PrimeQ[4 # + 1] &] PROG (MAGMA) [n: n in [0..220]| not IsPrime(4*n+1)]; // Vincenzo Librandi, Jan 28 2011 (PARI) is(n)=!isprime(4*n+1) \\ Charles R Greathouse IV, Jul 29 2016 CROSSREFS Cf. A014076, A153170, A095277, A153088, A153329, A153343. Sequence in context: A024798 A108857 A132142 * A047267 A058591 A247062 Adjacent sequences:  A045748 A045749 A045750 * A045752 A045753 A045754 KEYWORD nonn,easy AUTHOR EXTENSIONS More terms from Erich Friedman STATUS approved

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Last modified May 26 17:02 EDT 2020. Contains 334630 sequences. (Running on oeis4.)