Numbers n such that 10^n+1 has only one primitive prime factor.
0, 1, 2, 5, 6, 7, 12, 18, 19, 24, 31, 53, 60, 67, 75, 98, 147, 160, 293, 327, 369, 586, 641, 702, 713, 726, 876, 906, 918, 922, 931, 1067, 1116, 1132, 1875, 2137, 2177, 3011, 3261, 3341, 4348, 4775, 6546, 6685, 7357, 7728, 9334, 9455, 9620
1
Numbers n such that S000015(n) = 1.
T. D. Noe, Plot of 49 terms
T. D. Noe, Table of 49 terms
Eric W. Weisstein, MathWorld: Primitive Prime Factor
(Mma) d = 10; f2[n_] := Cyclotomic[2*n, d]/Cyclotomic[n, d]; Join[{0}, Select[Range[1000], PrimePowerQ[f2[#]/GCD[f2[#], #]] &]]
Cf. S000015.
nonn,hard
T. D. Noe, Nov 21 2014