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A369889 The sum of squarefree divisors of the cubefree numbers. 1
1, 3, 4, 3, 6, 12, 8, 4, 18, 12, 12, 14, 24, 24, 18, 12, 20, 18, 32, 36, 24, 6, 42, 24, 30, 72, 32, 48, 54, 48, 12, 38, 60, 56, 42, 96, 44, 36, 24, 72, 48, 8, 18, 72, 42, 54, 72, 80, 90, 60, 72, 62, 96, 32, 84, 144, 68, 54, 96, 144, 72, 74, 114, 24, 60, 96, 168 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
The number of squarefree divisors of the n-th cubefree number is A366536(n).
LINKS
FORMULA
a(n) = A048250(A004709(n)).
Sum_{j=1..n} a(j) ~ c * n^2, where c = zeta(3)^2/(2*zeta(5)) = 0.6967413068... .
In general, the formula holds for the sum of squarefree divisors of the k-free numbers with c = zeta(k)^2/(2*zeta(2*k-1))..., for k >= 2.
MATHEMATICA
f[p_, e_] := p + 1; s[1] = 1; s[n_] := Times @@ f @@@ FactorInteger[n]; cubefreeQ[n_] := Max[FactorInteger[n][[;; , 2]]] < 3; s /@ Select[Range[100], cubefreeQ]
(* or *)
f[p_, e_] := If[e > 2, 0, p + 1]; s[1] = 1; s[n_] := Times @@ f @@@ FactorInteger[n]; Select[Array[s, 100], # > 0 &]
PROG
(PARI) lista(kmax) = {my(f, s, p, e); for(k = 1, kmax, f = factor(k); s = prod(i = 1, #f~, p = f[i, 1]; e = f[i, 2]; if(e < 3, p + 1, 0)); if(s > 0, print1(s, ", "))); }
CROSSREFS
Sequence in context: A340323 A046897 A109506 * A365481 A000113 A365347
KEYWORD
nonn,easy
AUTHOR
Amiram Eldar, Feb 15 2024
STATUS
approved

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Last modified May 19 06:35 EDT 2024. Contains 372666 sequences. (Running on oeis4.)