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A275669
Numbers k such that 3*k-1 is composite.
1
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
1,1
COMMENTS
Numbers n that can be expressed as 3xy+x-y for x>0, y>0.
LINKS
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
Cf. A003627.
Sequence in context: A107315 A340855 A230078 * A191275 A260392 A186346
KEYWORD
nonn,easy
AUTHOR
Ron R Spencer, Aug 04 2016
STATUS
approved