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A007640 Numbers k such that 2*k^2 - 2*k + 19 is composite.
(Formerly M5068)
1

%I M5068

%S 19,20,22,25,29,34,38,39,40,45,47,48,55,56,57,58,60,61,63,64,65,68,71,

%T 74,76,77,78,82,83,85,90,91,93,94,95,96,97,102,104,107,110,112,113,

%U 114,115,117,119,122,123,124,126,127,129,130,131,133,134,135,137,139

%N Numbers k such that 2*k^2 - 2*k + 19 is composite.

%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

%H Vincenzo Librandi, <a href="/A007640/b007640.txt">Table of n, a(n) for n = 1..3400</a>

%t Select[Range[140], CompositeQ[2#^2 - 2# + 19] &] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Jun 05 2017 *)

%t Select[Range[200], !PrimeQ[2 #^2 - 2 # + 19] &] (* _Vincenzo Librandi_, Jun 08 2017 *)

%o (MAGMA) [n: n in [0..200] | not IsPrime(2*n^2-2*n+19)]; // _Vincenzo Librandi_, Jun 08 2017

%K nonn

%O 1,1

%A _N. J. A. Sloane_, _Mira Bernstein_, _Robert G. Wilson v_

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Last modified July 31 18:46 EDT 2021. Contains 346376 sequences. (Running on oeis4.)