OFFSET
1,1
COMMENTS
Numbers n that can be expressed as 3xy+x-y for x>0, y>0.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..4100
EXAMPLE
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.
MATHEMATICA
Select[Range@ 120, CompositeQ[3 # - 1] &] (* Michael De Vlieger, Aug 06 2016 *)
Select[Range[200], !PrimeQ[3 # - 1] &] (* Vincenzo Librandi, Jun 09 2017 *)
PROG
(PARI) isok(n) = !isprime(3*n-1); \\ Michel Marcus, Aug 04 2016
(Magma) [n: n in [2..90]| not IsPrime(3*n-1)]; // Vincenzo Librandi, Jun 09 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Ron R Spencer, Aug 04 2016
STATUS
approved