A007638 Numbers k such that 3*k^2 - 3*k + 23 is composite.
23, 24, 28, 31, 39, 44, 45, 46, 47, 50, 52, 56, 57, 60, 63, 67, 69, 70, 71, 79, 80, 85, 86, 88, 89, 90, 92, 93, 96, 97, 102, 107, 108, 112, 115, 116, 118, 119, 121, 122, 123, 126, 128, 131, 134, 137, 138, 139, 143, 144, 145, 147, 148, 151, 153, 156, 157, 161, 162
Offset: 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..3100
- H. Dubner, Generalized Cullen numbers, J. Rec. Math., 21 (No. 3, 1989), 190-191. (Annotated scanned copy)
Programs
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Magma
[n: n in [0..200]| not IsPrime(3*n^2-3*n+23)]; // Vincenzo Librandi, Jun 08 2017
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Mathematica
Select[Range[200], !PrimeQ[3 #^2 - 3 # + 23] &] (* Vincenzo Librandi, Jun 08 2017 *)
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PARI
is(n)=!isprime(3*n^2 - 3*n + 23) \\ Charles R Greathouse IV, Oct 23 2015