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 A086257 Number of primitive prime factors of 2^n+1. 6
 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 2, 1, 1, 1, 2, 1, 2, 2, 3, 1, 1, 2, 2, 1, 2, 2, 3, 2, 2, 2, 3, 1, 1, 2, 2, 1, 2, 1, 4, 2, 2, 1, 3, 3, 2, 2, 2, 2, 2, 2, 2, 3, 1, 1, 4, 1, 2, 3, 2, 2, 2, 2, 2, 2, 2, 2, 4, 1, 3, 3, 4, 1, 2, 3, 4, 5, 2, 1, 4, 1, 3, 3, 3, 3, 1, 2, 3, 2, 1, 4, 3, 2, 4, 1, 4, 2, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,15 COMMENTS A prime factor of 2^n+1 is called primitive if it does not divide 2^r+1 for any r 0, a(n) = A086251(2*n). - Max Alekseyev, Sep 06 2022 EXAMPLE a(14) = 2 because 2^14+1 = 5*29*113 and 29 and 113 do not divide 2^r+1 for r < 14. MATHEMATICA nMax=200; pLst={}; Table[f=Transpose[FactorInteger[2^n+1]][[1]]; f=Complement[f, pLst]; cnt=Length[f]; pLst=Union[pLst, f]; cnt, {n, 0, nMax}] CROSSREFS Excluding a(0) = 1, forms a bisection of A086251. Cf. A046799 (number of distinct prime factors of 2^n+1), A054992 (number of prime factors, with repetition, of 2^n+1), A086258. Sequence in context: A214054 A330739 A076398 * A161098 A354999 A136177 Adjacent sequences: A086254 A086255 A086256 * A086258 A086259 A086260 KEYWORD hard,nonn AUTHOR T. D. Noe, Jul 14 2003 STATUS approved

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Last modified December 11 13:16 EST 2023. Contains 367727 sequences. (Running on oeis4.)