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A019555
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Smallest number whose cube is divisible by n.
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16
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1, 2, 3, 2, 5, 6, 7, 2, 3, 10, 11, 6, 13, 14, 15, 4, 17, 6, 19, 10, 21, 22, 23, 6, 5, 26, 3, 14, 29, 30, 31, 4, 33, 34, 35, 6, 37, 38, 39, 10, 41, 42, 43, 22, 15, 46, 47, 12, 7, 10, 51, 26, 53, 6, 55, 14, 57, 58, 59, 30, 61, 62, 21, 4, 65, 66, 67, 34, 69, 70, 71, 6, 73, 74, 15, 38, 77, 78
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OFFSET
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1,2
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COMMENTS
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This can be thought as an "upper 3rd root" of a positive integer. Upper k-th roots were studied by Broughan (2002, 2003, 2006). The sequence of "lower 3rd root" of positive integers is given by A053150. - Petros Hadjicostas, Sep 15 2019
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LINKS
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FORMULA
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Replace any cubic factors in n by their cube roots.
Multiplicative with a(p^e) = p^ceiling(e/3). - R. J. Mathar, May 29 2011
Dirichlet g.f.: zeta(3*s-1) * Product_{p prime} (1 + p^(1 - s) + p^(1 - 2*s)).
Dirichlet g.f.: zeta(3*s-1) * zeta(s-1) * Product_{p prime} (1 - p^(2 - 3*s) + p^(1 - 2*s) - p^(2 - 2*s)).
Sum_{k=1..n} a(k) ~ c * zeta(5) * n^2 / 2, where c = Product_{p prime} (1 - 1/p^2 + 1/p^3 - 1/p^4) = 0.684286924186862318141968725791218083472312736723163777284618226290055... (End)
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MAPLE
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f:= n -> mul(t[1]^ceil(t[2]/3), t = ifactors(n)[2]):
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MATHEMATICA
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cubes=Range[85]^3; Table[Position[Divisible[cubes, i], True, 1, 1][[1, 1]], {i, 85}] (* Harvey P. Dale, Jan 12 2011 *)
f[p_, e_] := p^Ceiling[e/3]; a[1] = 1; a[n_] := Times @@ f @@@ FactorInteger[n]; Array[a, 100] (* Amiram Eldar, Jan 06 2024 *)
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PROG
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(PARI) for(n=1, 100, print1(direuler(p=2, n, (1 + p*X + p*X^2)/(1 - p*X^3))[n], ", ")) \\ Vaclav Kotesovec, Aug 30 2021
(PARI) a(n) = {my(f = factor(n)); prod(i = 1, #f~, f[i, 1]^ceil(f[i, 2]/3)); } \\ Amiram Eldar, Jan 06 2024
(Sage) [prod([t[0]^(ceil(t[1]/3)) for t in factor(n)]) for n in range(1, 79)] # Danny Rorabaugh, Sep 22 2015
(Python 3.8+)
from math import prod
from sympy import factorint
def A019555(n): return prod(p**((q%3 != 0)+(q//3)) for p, q in factorint(n).items()) # Chai Wah Wu, Aug 18 2021
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CROSSREFS
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KEYWORD
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nonn,easy,mult
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AUTHOR
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R. Muller
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EXTENSIONS
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STATUS
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approved
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