Number of primitive prime factors of 3^n - 1.
1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 2, 2, 2, 2, 1, 1, 2, 2, 1, 2, 1, 3, 2, 3, 2, 3, 1, 1, 3, 2, 1, 2, 2, 3, 1, 3, 2, 2, 2, 3, 1, 4, 3, 5, 2, 2, 2, 3, 2, 2, 2, 3, 3, 2, 1, 2, 2, 1, 1, 3, 2, 3, 2, 2, 1, 1, 1, 4, 3, 3, 2, 3, 4, 3, 2, 3, 2, 4, 2, 2, 1, 3
1
T. D. Noe, Plot of 87 terms
Wikipedia, Cunningham project
(Mma) pp = {}; Table[f = Transpose[FactorInteger[3^n - 1]][[1]]; p = Complement[f, pp]; pp = Union[pp, p]; Length[p], {n, 87}]
Cf. A112505 (number of primitive prime factors of 10^n - 1).
Cf. A129733 (primitive prime factors of 3^n - 1).
Cf. S000002-S000007, S000360 (3^n-1 has one primitive prime).
Cf. A085021 (number of primitive prime factors of 2^n - 1).
nonn,hard
T. D. Noe, May 14 2014