A143829 Numbers n such that 10n^2 - 1 is prime.

3, 6, 9, 12, 21, 30, 33, 36, 45, 48, 60, 69, 72, 75, 81, 87, 99, 108, 111, 114, 117, 120, 123, 126, 129, 153, 165, 168, 174, 177, 183, 201, 204, 207, 222, 234, 243, 252, 267, 279, 282, 285, 294, 303, 312, 315, 318, 339, 345, 348, 369, 378, 381, 384, 393, 396

Offset: 1

Author

Artur Jasinski, Sep 02 2008

Keywords

• nonn

Crossrefs

A066436, A066049, A090686, A090684, A143826, A143827, A143828

Programs

Mathematica

p = 10; a = {}; Do[k = p x^2 - 1; If[PrimeQ[k], AppendTo[a, x]], {x, 1, 1000}]; a
Select[Range[500],PrimeQ[10#^2-1]&] (* Harvey P. Dale, Nov 11 2020 *)

PARI

is(n)=isprime(10*n^2-1) \\ Charles R Greathouse IV, Feb 17 2017