Numbers n such that 7^n-1 has only one primitive prime factor.
2, 3, 4, 5, 6, 8, 13, 18, 21, 28, 30, 34, 36, 46, 48, 50, 54, 55, 58, 63, 76, 84, 94, 105, 122, 131, 148, 149, 224, 280, 288, 296, 332, 352, 456, 528, 531, 581, 650, 654, 730, 740, 759, 1026, 1047, 1065, 1460, 1660, 1699, 1959, 2067, 2260, 2380, 2665, 2890
1
Numbers n such that S000005(n) = 1.
T. D. Noe, Plot of 69 terms
T. D. Noe, Table of 69 terms
Eric W. Weisstein, MathWorld: Primitive Prime Factor
(Mma) d = 7; Select[Range[1000], PrimePowerQ[Cyclotomic[#, d]/GCD[Cyclotomic[#, d], #]] &]
Cf. A161508 (2^n-1 case), S000005, S000360-S000366.
nonn,hard
T. D. Noe, Nov 20 2014