Irregular table of primitive prime factors of 5^n - 1.
2, 3, 31, 13, 11, 71, 7, 19531, 313, 19, 829, 521, 12207031, 601, 305175781, 29, 449, 181, 1741, 17, 11489, 409, 466344409, 5167, 191, 6271, 3981071, 41, 9161, 379, 519499, 23, 67, 5281, 8971, 332207361361, 390001, 101, 251, 401, 9384251, 5227, 38923, 109
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[5^n - 1]][[1]]; p = Complement[f, pp]; pp = Union[pp, p]; p, {n, 30}]; Flatten[t]
Cf. S000003 (number of primitive prime factors of 5^n - 1).
nonn,tabf
T. D. Noe, May 14 2014