A143834 - OEIS

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

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

Complement of A066049.

LINKS

Table of n, a(n) for n=1..66.

MATHEMATICA

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 *)

PROG

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