A130771 Numbers n not divisible by 3 for which neither 2n-3 nor 2n+3 are primes.

26, 44, 59, 61, 79, 86, 103, 106, 109, 125, 128, 131, 146, 149, 151, 161, 163, 166, 169, 179, 184, 187, 194, 205, 224, 236, 239, 254, 257, 265, 266, 268, 271, 274, 278, 281, 289, 293, 296, 304, 313, 316, 326, 334, 341, 346, 350, 355, 359, 364, 367, 376, 389, 391

Offset: 1

Author

W. Neville Holmes, Jul 14 2007

Keywords

• easy
• nonn

Comments

This is A130699 with the trivial multiples of 3 removed.

Examples

			26 because 2*26+3=55 is not a prime, nor is 2*26-3=49.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..2500

Crossrefs

Cf. A130699, A001651.

Programs

Mathematica

Select[Range[400],!Divisible[#,3]&&NoneTrue[2#+{3,-3},PrimeQ]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Sep 19 2018 *)

PARI

isok(n) = (n % 3) && !isprime(2*n-3) && !isprime(2*n+3) \\ Michel Marcus, Jul 11 2013

Extensions

Missing term 169 added by Michel Marcus, Jul 11 2013