Irregular table of primitive prime factors of 5^n + 1.
2, 3, 13, 7, 313, 521, 601, 29, 449, 17, 11489, 5167, 41, 9161, 23, 67, 5281, 390001, 5227, 38923, 234750601, 61, 7621, 2593, 29423041, 3061, 41540861, 37, 6597973, 761, 19609, 213029, 241, 632133361, 43, 127, 7603, 89, 1030330938209, 47, 42272797713043
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. S000010 (number of primitive prime factors of 5^n + 1).
nonn,tabf
T. D. Noe, May 14 2014