Numbers n such that 2^n+1 has only one primitive prime factor.
0, 1, 2, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 15, 16, 17, 19, 20, 21, 23, 27, 28, 31, 39, 40, 43, 45, 49, 60, 61, 63, 75, 79, 85, 87, 92, 96, 101, 104, 117, 127, 140, 148, 156, 161, 167, 171, 183, 187, 191, 199, 205, 207, 275, 295, 300, 301, 313, 345, 347, 356
1
Numbers n such that A086257(n) = 1.
T. D. Noe, Plot of 123 terms
T. D. Noe, Table of 123 terms
Eric W. Weisstein, MathWorld: Primitive Prime Factor
(Mma) d = 2; f2[n_] := Cyclotomic[2*n, d]/Cyclotomic[n, d]; Join[{0}, Select[Range[1000], PrimePowerQ[f2[#]/GCD[f2[#], #]] &]]
Cf. A086257 (number of primitive prime factors of 2^n + 1).
nonn,hard
T. D. Noe, Nov 21 2014