Irregular table of primitive prime factors of 7^n - 1.
2, 3, 19, 5, 2801, 43, 29, 4733, 1201, 37, 1063, 11, 191, 1123, 293459, 13, 181, 16148168401, 113, 911, 31, 159871, 17, 169553, 14009, 2767631689, 117307, 419, 4534166740403, 281, 4021, 11898664849, 23, 10746341, 47, 3083, 31479823396757, 73, 193, 409, 2551
1
T. D. Noe, Plot of irregular table up to n = 100
T. D. Noe, Table up to n = 100
Wikipedia, Cunningham project
(Mma) pp = {}; t = Table[f = Transpose[FactorInteger[7^n - 1]][[1]]; p = Complement[f, pp]; pp = Union[pp, p]; p, {n, 30}]; Flatten[t]
Cf. S000005 (number of primitive prime factors of 7^n - 1).
nonn,tabf
T. D. Noe, May 14 2014