Irregular table of primitive prime factors of 3^n + 1.
2, 5, 7, 41, 61, 73, 547, 17, 193, 19, 37, 1181, 67, 661, 6481, 398581, 29, 16493, 31, 271, 21523361, 103, 307, 1021, 530713, 2851, 101917, 42521761, 43, 2269, 5501, 570461, 23535794707, 97, 577, 769, 151, 22996651, 53, 4795973261, 19441, 19927, 430697
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[3^n + 1]][[1]]; p = Complement[f, pp]; pp = Union[pp, p]; p, {n, 30}]; Flatten[t]
Cf. S000008 (number of primitive prime factors of 3^n + 1).
nonn,tabf
T. D. Noe, May 14 2014