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A007640 Numbers k such that 2*k^2 - 2*k + 19 is composite.
(Formerly M5068)
1
19, 20, 22, 25, 29, 34, 38, 39, 40, 45, 47, 48, 55, 56, 57, 58, 60, 61, 63, 64, 65, 68, 71, 74, 76, 77, 78, 82, 83, 85, 90, 91, 93, 94, 95, 96, 97, 102, 104, 107, 110, 112, 113, 114, 115, 117, 119, 122, 123, 124, 126, 127, 129, 130, 131, 133, 134, 135, 137, 139 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

REFERENCES

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

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..3400

MATHEMATICA

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

Select[Range[200], !PrimeQ[2 #^2 - 2 # + 19] &] (* Vincenzo Librandi, Jun 08 2017 *)

PROG

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

CROSSREFS

Sequence in context: A018824 A289469 A093680 * A265201 A274340 A241849

Adjacent sequences:  A007637 A007638 A007639 * A007641 A007642 A007643

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Mira Bernstein, Robert G. Wilson v

STATUS

approved

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Last modified June 16 18:25 EDT 2021. Contains 345066 sequences. (Running on oeis4.)