A120382 Even numbers k such that 3*k-1 and 3*k+1 are not prime.

40, 48, 62, 68, 72, 82, 96, 100, 108, 114, 138, 142, 158, 172, 176, 178, 184, 194, 208, 212, 222, 232, 238, 260, 264, 268, 272, 278, 282, 290, 298, 300, 308, 320, 334, 342, 348, 352, 358, 360, 378, 380, 382, 386, 392, 402, 414, 418, 422, 424, 438, 444, 446, 448, 450

Offset: 1

Author

Pierre CAMI, Jun 29 2006

Keywords

• easy
• nonn

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A060461.

Programs

Maple

remove(k -> isprime(3*k-1) or isprime(3*k+1), [seq(k,k=2..1000,2)]); # Robert Israel, Sep 30 2024

Mathematica

Select[Range[2,450,2],!PrimeQ[3#-1]&&!PrimeQ[3#+1]&] (* James C. McMahon, Sep 30 2024 *)
Select[2*Range[300],NoneTrue[3#+{1,-1},PrimeQ]&] (* Harvey P. Dale, May 27 2025 *)

PARI

isok(n) = !(n%2) && !isprime(3*n-1) && !isprime(3*n+1); \\ Michel Marcus, Sep 15 2019

Formula

a(n) = 2*A060461(n). - Michel Marcus, Sep 15 2019

Extensions

More terms from Michel Marcus, Sep 15 2019