A161509 The unique primitive prime factor of 2^n-1 for the n in A161508.

3, 7, 5, 31, 127, 17, 73, 11, 13, 8191, 43, 151, 257, 131071, 19, 524287, 41, 337, 683, 241, 2731, 262657, 331, 2147483647, 65537, 599479, 43691, 174763, 61681, 5419, 2796203, 4432676798593, 87211, 15790321, 2305843009213693951, 715827883

Offset: 1

Author

T. D. Noe, Jun 17 2009

Keywords

• nonn

Comments

For these primes p, the binary expansion of 1/p has a unique period length. The binary analog of A007615.

Links

• Max Alekseyev, Table of n, a(n) for n = 1..179 (terms for n=1..100 from T. D. Noe)

Crossrefs

Cf. A144755 (sorted).

Programs

Mathematica

Reap[Do[c=Cyclotomic[n,2]; q=c/GCD[c,n]; If[PrimePowerQ[q], Sow[FactorInteger[q][[1,1]]]],{n,100}]][[2,1]]