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

%I #16 Sep 30 2024 18:40:30

%S 40,48,62,68,72,82,96,100,108,114,138,142,158,172,176,178,184,194,208,

%T 212,222,232,238,260,264,268,272,278,282,290,298,300,308,320,334,342,

%U 348,352,358,360,378,380,382,386,392,402,414,418,422,424,438,444,446,448,450

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

%H Robert Israel, <a href="/A120382/b120382.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = 2*A060461(n). - _Michel Marcus_, Sep 15 2019

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

%t Select[Range[2,450,2],!PrimeQ[3#-1]&&!PrimeQ[3#+1]&] (* _James C. McMahon_, Sep 30 2024 *)

%o (PARI) isok(n) = !(n%2) && !isprime(3*n-1) && !isprime(3*n+1); \\ _Michel Marcus_, Sep 15 2019

%Y Cf. A060461.

%K easy,nonn

%O 1,1

%A _Pierre CAMI_, Jun 29 2006

%E More terms from _Michel Marcus_, Sep 15 2019