Irregular table of primitive prime factors of 7^n + 1.
2, 5, 43, 1201, 11, 191, 13, 181, 113, 911, 17, 169553, 117307, 281, 4021, 23, 10746341, 73, 193, 409, 53, 228511817, 13564461457, 6568801, 353, 47072139617, 29078814248401, 13841169553, 351121, 4058036683, 41, 810221830361, 51031, 309079, 661, 1409
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. S000012 (number of primitive prime factors of 7^n + 1).
nonn,tabf
T. D. Noe, May 14 2014