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 A059892 a(n) = |{m : multiplicative order of 10 mod m is equal to n}|. 23
 3, 3, 5, 6, 9, 53, 9, 36, 12, 33, 9, 186, 21, 33, 111, 144, 9, 564, 3, 330, 239, 273, 3, 1756, 84, 165, 76, 714, 93, 16167, 21, 5952, 111, 177, 363, 4288, 21, 15, 99, 5724, 45, 48807, 45, 4314, 1140, 183, 9, 14192, 36, 2940, 495, 1338, 45, 11572, 747, 11484 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The multiplicative order of a mod m, gcd(a,m)=1, is the smallest natural number d for which a^d = 1 (mod m). The number of unit fractions 1/k having a decimal expansion of period n and with k coprime to 10. - T. D. Noe, May 18 2007 Also, number of primitive factors of 10^n - 1 (cf. A003060). - Max Alekseyev, May 03 2022 a(n) is odd if and only if n is squarefree. Proof: Note that 10^d - 1 == 3 (mod 4) for d >= 2, so 10^d - 1 is a square if and only if d = 1. From the formula we can see that a(n) is odd if and only if mu(n) is nonzero, or n is squarefree. - Jianing Song, Jun 15 2021 LINKS FORMULA a(n) = Sum_{d|n} mu(n/d)*tau(10^d-1), (mu(n) = Moebius function A008683, tau(n) = number of divisors of n A000005). MAPLE with(numtheory): a:= n-> add(mobius(n/d)*tau(10^d-1), d=divisors(n)): seq(a(n), n=1..30); # Alois P. Heinz, Oct 12 2012 MATHEMATICA f[n_, d_] := MoebiusMu[n/d]*Length[Divisors[10^d - 1]]; a[n_] := Total[(f[n, #] & ) /@ Divisors[n]]; Table[a[n], {n, 1, 56}] (* Jean-François Alcover, Mar 21 2011 *) PROG (PARI) j=[]; for(n=1, 10, j=concat(j, sumdiv(n, d, moebius(n/d)*numdiv(10^d-1)))); j (Python) from sympy import divisors, mobius, divisor_count def a(n): return sum(mobius(n//d)*divisor_count(10**d - 1) for d in divisors(n)) # Indranil Ghosh, Apr 23 2017 CROSSREFS Number of primitive factors of b^n - 1: A059499 (b=2), A059885(b=3), A059886 (b=4), A059887 (b=5), A059888 (b=6), A059889 (b=7), A059890 (b=8), A059891 (b=9), this sequence (b=10). Cf. A000005, A008683, A002329, A007138, A005422, A057951, A058946, A070528. Column k=10 of A212957. Sequence in context: A187679 A048274 A351012 * A241090 A091607 A276434 Adjacent sequences: A059889 A059890 A059891 * A059893 A059894 A059895 KEYWORD nonn,nice AUTHOR Vladeta Jovovic, Feb 06 2001 EXTENSIONS More terms from Jason Earls, Aug 06 2001. Terms to a(280) in b-file from T. D. Noe, Oct 01 2013 a(281)-a(322) in b-file from Ray Chandler, May 03 2017 a(323)-a(352) in b-file from Max Alekseyev, May 03 2022 STATUS approved

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Last modified January 30 05:55 EST 2023. Contains 359939 sequences. (Running on oeis4.)