Number of primitive prime factors of 10^n + 1.
1, 1, 1, 2, 2, 1, 1, 1, 2, 2, 2, 3, 1, 2, 3, 3, 5, 3, 1, 1, 2, 3, 3, 4, 1, 3, 2, 2, 2, 2, 3, 1, 4, 2, 3, 2, 3, 3, 2, 4, 2, 2, 2, 3, 2, 3, 3, 3, 4, 2, 4, 2, 2, 1, 3, 4, 3, 2, 4, 3, 1, 2, 2, 2, 3, 2, 3, 1, 2, 3, 5, 2, 2, 3, 6, 1, 4, 6, 3, 3, 4, 4, 2, 6, 3, 3, 2
0
T. D. Noe, Plot of 87 terms
Wikipedia, Cunningham project
(Mma) pp = {}; Table[f = Transpose[FactorInteger[10^n + 1]][[1]]; p = Complement[f, pp]; pp = Union[pp, p]; Length[p], {n, 0, 72}]
Cf. A086257 (number of primitive prime factors of 2^n + 1), S000375 (10^n+1 has one primitive prime).
Cf. S000008-S000014, S000052 (primitive prime factors of 10^n + 1).
nonn,hard
T. D. Noe, May 14 2014
Offset corrected. - T. D. Noe, Nov 20 2014