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 A000374 Number of cycles (mod n) under doubling map. 16
 1, 1, 2, 1, 2, 2, 3, 1, 3, 2, 2, 2, 2, 3, 5, 1, 3, 3, 2, 2, 6, 2, 3, 2, 3, 2, 4, 3, 2, 5, 7, 1, 5, 3, 6, 3, 2, 2, 5, 2, 3, 6, 4, 2, 8, 3, 3, 2, 5, 3, 8, 2, 2, 4, 5, 3, 5, 2, 2, 5, 2, 7, 13, 1, 7, 5, 2, 3, 6, 6, 3, 3, 9, 2, 8, 2, 6, 5, 3, 2, 5, 3, 2, 6, 12, 4, 5, 2, 9, 8, 10, 3, 14, 3, 5, 2, 3, 5, 8, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Number of cycles of the function f(x) = 2x mod n. Number of irreducible factors in the factorization of the polynomial x^n-1 over the integers mod 2. - T. D. Noe, Apr 16 2003 REFERENCES R. Lidl and H. Niederreiter, Finite Fields, Addison-Wesley, 1983, p. 65. LINKS T. D. Noe, Table of n, a(n) for n=1..10000 FORMULA a(n) = Sum_{d|m} phi(d)/ord(2, d), where m is n with all factors of 2 removed. - T. D. Noe, Apr 19 2003 EXAMPLE a(14) = 3 because (1) the function 2x mod 14 has the three cycles (0),(2,4,8),(6,12,10) and (2) the factorization of x^14-1 over integers mod 2 is (1+x)^2 (1+x+x^3)^2 (1+x^2+x^3)^2, which has three unique factors. Note that the length of the cycles is the same as the degree of the factors. MATHEMATICA Table[Length[FactorList[x^n - 1, Modulus -> 2]] - 1, {n, 100}] CountFactors[p_, n_] := Module[{sum=0, m=n, d, f, i}, While[Mod[m, p]==0, m/=p]; d=Divisors[m]; Do[f=d[[i]]; sum+=EulerPhi[f]/MultiplicativeOrder[p, f], {i, Length[d]}]; sum]; Table[CountFactors[2, n], {n, 100}] CROSSREFS Cf. A000005, A023135-A023142. Cf. A081844 (number of irreducible factors of x^(2n+1) - 1 over GF(2)). Cf. A037226 (number of primitive irreducible factors of x^(2n+1) - 1 over integers mod 2). Sequence in context: A210868 A176853 A261787 * A256757 A277314 A120562 Adjacent sequences:  A000371 A000372 A000373 * A000375 A000376 A000377 KEYWORD nonn AUTHOR STATUS approved

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