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A161509 The unique primitive prime factor of 2^n-1 for the n in A161508. 4
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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

LINKS

T. D. Noe, Table of n, a(n) for n=1..100

MATHEMATICA

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

CROSSREFS

Cf. A144755 (sorted).

Sequence in context: A005420 A212953 A161818 * A108974 A106853 A083778

Adjacent sequences:  A161506 A161507 A161508 * A161510 A161511 A161512

KEYWORD

nonn

AUTHOR

T. D. Noe, Jun 17 2009

STATUS

approved

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Last modified August 26 02:19 EDT 2019. Contains 326324 sequences. (Running on oeis4.)