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 A086251 Number of primitive prime factors of 2^n - 1. 9
 0, 1, 1, 1, 1, 0, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 2, 3, 1, 1, 1, 1, 1, 2, 2, 2, 1, 2, 1, 2, 1, 3, 2, 2, 1, 3, 2, 1, 2, 3, 3, 3, 1, 3, 1, 2, 2, 2, 2, 1, 1, 2, 2, 1, 2, 2, 3, 1, 2, 3, 2, 3, 2, 2, 3, 1, 1, 3, 1, 3, 2, 2, 2, 1, 1, 2, 2, 1, 1, 3, 4, 1, 2, 3, 2, 2, 1, 3, 3, 2, 3, 2, 2, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,11 COMMENTS A prime factor of 2^n - 1 is called primitive if it does not divide 2^r - 1 for any r < n. Equivalently, p is a primitive prime factor of 2^n - 1 if ord(2,p) = n. Zsigmondy's theorem says that there is at least one primitive prime factor for n > 1, except for n=6. See A086252 for those n that have a record number of primitive prime factors. Number of odd primes p such that A002326((p-1)/2) = n. Number of occurrences of number n in A014664. - Thomas Ordowski, Sep 12 2017 LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..1200 (first 500 terms from T. D. Noe) Eric Weisstein's World of Mathematics, Zsigmondy Theorem FORMULA a(n) = Sum{d|n} mu(n/d) A046800(d), inverse Mobius transform of A046800. a(n) <= A182590(n). - Thomas Ordowski, Sep 14 2017 a(n) = A001221(A064078(n)). - Thomas Ordowski, Oct 26 2017 EXAMPLE a(11) = 2 because 2^11 - 1 = 23*89 and both 23 and 89 have order 11. MATHEMATICA Join[{0}, Table[cnt=0; f=Transpose[FactorInteger[2^n-1]][[1]]; Do[If[MultiplicativeOrder[2, f[[i]]]==n, cnt++ ], {i, Length[f]}]; cnt, {n, 2, 200}]] PROG (PARI) a(n) = sumdiv(n, d, moebius(n/d)*omega(2^d-1)); \\ Michel Marcus, Sep 12 2017 CROSSREFS Cf. A046800, A046051 (number of prime factors, with repetition, of 2^n-1), A086252. Sequence in context: A245690 A146291 A327538 * A177121 A329314 A092931 Adjacent sequences:  A086248 A086249 A086250 * A086252 A086253 A086254 KEYWORD hard,nonn AUTHOR T. D. Noe, Jul 14 2003 STATUS approved

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Last modified May 26 09:37 EDT 2020. Contains 334620 sequences. (Running on oeis4.)