login
Numbers n for which neither 2n-3 nor 2n+3 are primes.
1

%I #7 Nov 12 2014 13:04:16

%S 6,9,12,15,18,21,24,26,27,30,33,36,39,42,44,45,48,51,54,57,59,60,61,

%T 63,66,69,72,75,78,79,81,84,86,87,90,93,96,99,102,103,105,106,108,109,

%U 111,114,117,120,123,125,126,128,129,131,132,135,138,141,144,146

%N Numbers n for which neither 2n-3 nor 2n+3 are primes.

%e Not 5 because 7 and 13 are prime, but 6 because neither 9 nor 15 are primes.

%t Select[Range[200],NoneTrue[2#+{3,-3},PrimeQ]&] (* The program uses the NoneTrue function from Mathematica version 10 *) (* _Harvey P. Dale_, Nov 12 2014 *)

%o (PARI) isok(n) = !isprime(2*n-3) && !isprime(2*n+3) \\ _Michel Marcus_, Jul 11 2013

%Y Cf. A104278.

%K easy,nonn

%O 2,1

%A _W. Neville Holmes_, Jul 11 2007

%E Missing term 24 added by _Michel Marcus_, Jul 11 2013