Irregular table of primitive prime factors of 9^n - 1.
2, 5, 7, 13, 41, 11, 61, 73, 547, 1093, 17, 193, 19, 37, 757, 1181, 23, 67, 661, 3851, 6481, 398581, 797161, 29, 16493, 31, 271, 4561, 21523361, 103, 307, 1021, 1871, 34511, 530713, 1597, 2851, 101917, 363889, 42521761, 43, 2269, 368089, 5501, 570461, 47
1
T. D. Noe, Plot of irregular table up to n = 100
T. D. Noe, Table up to n = 100
(Mma) pp = {}; t = Table[f = Transpose[FactorInteger[9^n - 1]][[1]]; p = Complement[f, pp]; pp = Union[pp, p]; p, {n, 30}]; Flatten[t]
Cf. S000007 (number of primitive prime factors of 9^n - 1).
nonn,tabf
T. D. Noe, May 14 2014