Irregular table of primitive prime factors of 6^n - 1.
5, 7, 43, 37, 311, 31, 55987, 1297, 19, 2467, 11, 101, 23, 3154757, 13, 97, 3433, 760891, 29, 197, 1171, 1201, 17, 98801, 239, 409, 1123, 30839, 46441, 191, 638073026189, 241, 6781, 1822428931, 51828151, 47, 139, 3221, 7505944891, 1678321, 18198701, 40185601
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[6^n - 1]][[1]]; p = Complement[f, pp]; pp = Union[pp, p]; p, {n, 30}]; Flatten[t]
Cf. S000004 (number of primitive prime factors of 6^n - 1).
nonn,tabf
T. D. Noe, May 14 2014