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A349258
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a(n) is the number of prime powers (not including 1) that are infinitary divisors of n.
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7
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0, 1, 1, 1, 1, 2, 1, 3, 1, 2, 1, 2, 1, 2, 2, 1, 1, 2, 1, 2, 2, 2, 1, 4, 1, 2, 3, 2, 1, 3, 1, 3, 2, 2, 2, 2, 1, 2, 2, 4, 1, 3, 1, 2, 2, 2, 1, 2, 1, 2, 2, 2, 1, 4, 2, 4, 2, 2, 1, 3, 1, 2, 2, 3, 2, 3, 1, 2, 2, 3, 1, 4, 1, 2, 2, 2, 2, 3, 1, 2, 1, 2, 1, 3, 2, 2, 2, 4, 1, 3, 2, 2, 2, 2, 2, 4, 1, 2, 2, 2, 1, 3, 1, 4, 3
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OFFSET
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1,6
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COMMENTS
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The total number of prime powers (not including 1) that divide n is A001222(n).
For each n, all the prime powers that are infinitary divisors of n are "Fermi-Dirac primes" (A050376).
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LINKS
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FORMULA
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Additive with a(p^e) = 2^A000120(e) - 1.
a(n) <= A037445(n) - 1, with equality if and only if n is a prime power (including 1, A000961).
Sum_{k=1..n} a(k) ~ n * (log(log(n)) + B + C), where B is Mertens's constant (A077761) and C = Sum_{p prime} f(1/p) = 0.28135949730844648114..., where f(x) = -(x+1) + (1-x) * Product_{k>=0} (1 + 2*x^(2^k)). - Amiram Eldar, Sep 29 2023
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EXAMPLE
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12 has 4 infinitary divisors, 1, 3, 4 and 12. Two of these divisors, 3 and 4 = 2^2 are prime powers. Therefore a(12) = 2.
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MATHEMATICA
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f[p_, e_] := 2^DigitCount[e, 2, 1] - 1; a[1] = 0; a[n_] := Plus @@ f @@@ FactorInteger[n]; Array[a, 100]
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PROG
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(PARI) A349258(n) = if(1==n, 0, vecsum(apply(x->(2^hammingweight(x))-1, factor(n)[, 2]))); \\ Antti Karttunen, Nov 12 2021
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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