Irregular table of primitive prime factors of 4^n + 1.
5, 17, 13, 257, 41, 241, 29, 113, 65537, 37, 109, 61681, 397, 2113, 97, 673, 53, 157, 1613, 15790321, 61, 1321, 641, 6700417, 137, 953, 26317, 433, 38737, 229, 457, 525313, 4278255361, 1429, 14449, 353, 2931542417, 277, 1013, 1657, 30269, 193, 22253377, 101
1
T. D. Noe, Plot of irregular table up to n = 100
T. D. Noe, Table up to n = 100
(Mma) pp = {}; t = Table[f = Transpose[FactorInteger[4^n + 1]][[1]]; p = Complement[f, pp]; pp = Union[pp, p]; p, {n, 30}]; Flatten[t]
Cf. A158895 (primitive prime factors of 2^n + 1), S000009 (number of primitive prime factors of 4^n + 1).
nonn,tabf
T. D. Noe, May 14 2014