A090686 -id:A090686 - cp's OEIS frontend

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

1, 5, 9, 12, 14, 16, 19, 20, 23, 26, 27, 29, 30, 31, 32, 33, 35, 37, 40, 44, 47, 48, 51, 53, 54, 55, 57, 58, 60, 61, 65, 66, 67, 68, 70, 71, 72, 74, 75, 77, 78, 79, 82, 83, 84, 86, 88, 89, 90, 93, 94, 96, 97, 99, 100, 101, 103, 104, 105, 106, 107, 110, 111, 114, 116, 117

Offset: 1

Artur Jasinski, Sep 02 2008

nonn

Complement of A066049.

Cf. A066436, A066049, A090686, A090684, A143826, A143827, A143828, A143829, A143830, A143831, A143833.

[n: n in [1..120]| not IsPrime(2*n^2-1)] // Vincenzo Librandi, Jan 28 2011

p = 2; a = {}; Do[k = p x^2 - 1; If[PrimeQ[k],NULL, AppendTo[a, x]], {x, 1, 1000}]; a
Select[Range[120],!PrimeQ[2#^2-1]&] (* Harvey P. Dale, Mar 14 2018 *)

A089238 Numbers k such that 3*k^2/2 - 1 is a prime.

Original entry on oeis.org

2, 4, 6, 10, 14, 16, 20, 24, 26, 34, 40, 44, 54, 56, 60, 64, 70, 76, 86, 90, 104, 110, 116, 120, 124, 126, 130, 136, 140, 146, 150, 154, 164, 166, 174, 186, 194, 200, 204, 230, 240, 244, 260, 264, 286, 300, 306, 336, 346, 354, 374, 386, 394, 400, 414, 416, 420

Offset: 1

Giovanni Teofilatto, Dec 22 2003

nonn

All terms must be even numbers. - Harvey P. Dale, Jul 03 2017

M. Cerasoli, F. Eugeni and M. Protasi, Elementi di Matematica Discreta, Bologna 1988
Emanuele Munarini and Norma Zagaglia Salvi, Matematica Discreta,UTET, CittaStudiEdizioni, Milano 1997

Cf. A090686.

Select[Range[0,500,2],PrimeQ[(3#^2)/2-1]&] (* Harvey P. Dale, Jul 03 2017 *)

forstep(n=2,1000,2, if(isprime(3*n^2/2-1), print1(n,", "))) - Rick L. Shepherd, May 06 2008

Corrected and extended by Rick L. Shepherd, May 06 2008

cp's OEIS Frontend

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

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