A275669 Numbers k such that 3*k-1 is composite.

3, 5, 7, 9, 11, 12, 13, 15, 17, 19, 21, 22, 23, 25, 26, 27, 29, 31, 32, 33, 35, 37, 39, 40, 41, 42, 43, 45, 47, 48, 49, 51, 52, 53, 54, 55, 57, 59, 61, 62, 63, 65, 67, 68, 69, 70, 71, 72, 73, 74, 75, 77, 79

Offset: 1

Author

Ron R Spencer, Aug 04 2016

Keywords

• nonn
• easy

Comments

Numbers n that can be expressed as 3xy+x-y for x>0, y>0.

Examples

			a(1) = 3 because 3*3-1=8 is composite.
a(2) = 5 because 3*5-1=14 is composite.
1, 2, 4 give 2, 5, 11 which are primes, so 1, 2 and 4 are not terms.
		

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..4100

Crossrefs

Cf. A003627.

Programs

Magma

[n: n in [2..90]| not IsPrime(3*n-1)]; // Vincenzo Librandi, Jun 09 2017

Mathematica

Select[Range@ 120, CompositeQ[3 # - 1] &] (* Michael De Vlieger, Aug 06 2016 *)
Select[Range[200], !PrimeQ[3 # - 1] &] (* Vincenzo Librandi, Jun 09 2017 *)

PARI

isok(n) = !isprime(3*n-1); \\ Michel Marcus, Aug 04 2016