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 A059907 a(n) = |{m : multiplicative order of n mod m = 2}|. 6
 0, 1, 2, 2, 5, 2, 6, 4, 6, 3, 12, 2, 10, 6, 8, 4, 13, 2, 18, 6, 10, 4, 16, 4, 12, 9, 12, 4, 26, 2, 20, 6, 8, 12, 20, 4, 15, 6, 16, 4, 32, 2, 24, 10, 10, 6, 20, 4, 26, 9, 18, 4, 26, 6, 32, 12, 12, 4, 28, 2, 20, 10, 12, 18, 25, 4, 24, 6, 26, 4, 52, 2, 18, 10, 12, 18, 26, 4, 40, 8, 14, 5, 28 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 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). LINKS Alois P. Heinz, Table of n, a(n) for n = 1..10000 FORMULA a(n) = tau(n^2-1)-tau(n-1), where tau(n) = number of divisors of n A000005. Generally, if b(n, r) = |{m : multiplicative order of n mod m = r}| then b(n, r) = Sum_{d|r} mu(d)*tau(n^(r/d)-1), where mu(n) = Moebius function A008683. EXAMPLE a(2) = |{3}| = 1, a(3) = |{4,8}| = 2, a(4) = |{5,15}| = 2, a(5) = |{3,6,8,12,24}| = 5, a(6) = |{7,35}| = 2, a(7) = |{4,8,12,16,24,48}| = 6,... MAPLE with(numtheory):f := n->tau(n^2-1)-tau(n-1):for n from 1 to 100 do printf(`%d, `, f(n)) od: CROSSREFS Cf. A059908-A059916, A059499, A059885-A059892, A002326, A053446-A053453, A055205, A048691, A048785. Row n=2 of A212957. - Alois P. Heinz, Oct 24 2012 Sequence in context: A294339 A185291 A018216 * A024931 A256612 A305797 Adjacent sequences:  A059904 A059905 A059906 * A059908 A059909 A059910 KEYWORD easy,nonn AUTHOR Vladeta Jovovic, Feb 08 2001 STATUS approved

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Last modified October 13 18:57 EDT 2019. Contains 327981 sequences. (Running on oeis4.)