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A368216
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Number of divisors of n that are antiharmonic numbers (A020487).
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0
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1, 1, 1, 2, 1, 1, 1, 2, 2, 1, 1, 2, 1, 1, 1, 3, 1, 2, 1, 3, 1, 1, 1, 2, 2, 1, 2, 2, 1, 1, 1, 3, 1, 1, 1, 4, 1, 1, 1, 3, 1, 1, 1, 2, 2, 1, 1, 3, 2, 3, 1, 2, 1, 2, 1, 2, 1, 1, 1, 3, 1, 1, 2, 4, 1, 1, 1, 2, 1, 1, 1, 4, 1, 1, 2, 2, 1, 1, 1, 4, 3, 1, 1, 2, 1, 1, 1, 2
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OFFSET
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1,4
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COMMENTS
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Differs from A046951 for n = 20, 40, 50, 60, 80, ....
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LINKS
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FORMULA
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a(p^k) = floor((k + 2)/2), p prime, k >= 1.
a(p*q) = 1, for p, q prime, p <> q.
Asymptotic mean: Limit_{m->oo} (1/m) * Sum_{k=1..m} a(k) = Sum_{k>=1} 1/A020487(k) = 1.784... . - Amiram Eldar, Jan 26 2024
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EXAMPLE
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a(1) = 1 because 1 has only one divisor 1 = A020487(1) antiharmonic number.
a(4) = 2 because 4 has divisors 1 = A020487(1) and 4 = A020487(2), antiharmonic numbers.
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MATHEMATICA
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a[n_] := DivisorSum[n, 1 &, Divisible[DivisorSigma[2, #], DivisorSigma[1, #]] &]; Array[a, 100] (* Amiram Eldar, Jan 21 2024 *)
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PROG
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(Magma) f:=func<n|DivisorSigma(2, n) mod DivisorSigma(1, n) eq 0>; [#[d:d in Divisors(k)|f(d)]:k in [1..100]];
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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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