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A091813
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Number of positive squarefree integers k<=n satisfying gcd_*(k,n)=1, where gcd_*(k,n) is the greatest divisor of k that is also a unitary divisor of n.
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1
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1, 1, 2, 3, 3, 2, 5, 6, 6, 3, 7, 6, 8, 5, 6, 11, 11, 8, 12, 10, 8, 9, 15, 12, 16, 10, 17, 14, 17, 8, 19, 20, 13, 13, 15, 23, 23, 15, 17, 21, 26, 11, 28, 26, 24, 18, 30, 23, 31, 20, 21, 29, 32, 22, 25, 29, 23, 23, 36, 23, 37, 25, 34, 39, 30, 18, 41, 39, 29, 22, 44, 45, 45, 30, 35, 44
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OFFSET
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1,3
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REFERENCES
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Steven R. Finch, Mathematical Constants II, Cambridge University Press, 2018, section 1.7, Unitarism and Infinitarism, pp. 49-56.
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LINKS
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EXAMPLE
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a(4)=3 because each of 1, 2, 3 are squarefree and gcd_*(2,4)=1. The latter follows since 2 is not a unitary divisor of 4. a(5)=3 because 4 is not squarefree.
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MATHEMATICA
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udiv[n_] := Select[Divisors[n], GCD[#, n/#] == 1 &]; uGCD[a_, b_] := Max[Intersection[Divisors[a], udiv[b]]]; a[n_] := Sum[MoebiusMu[k]^2 * Boole[uGCD[k, n] == 1], {k, 1, n}]; Array[a, 76] (* Amiram Eldar, Oct 01 2019 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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