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A329376
Multiplicative with a(p^e) = p when e = 2, otherwise a(p^e) = 1.
3
1, 1, 1, 2, 1, 1, 1, 1, 3, 1, 1, 2, 1, 1, 1, 1, 1, 3, 1, 2, 1, 1, 1, 1, 5, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 6, 1, 1, 1, 1, 1, 1, 1, 2, 3, 1, 1, 1, 7, 5, 1, 2, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 3, 1, 1, 1, 1, 2, 1, 1, 1, 3, 1, 1, 5, 2, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 3, 1, 2, 1, 1, 1, 1, 1, 7, 3, 10, 1, 1, 1, 1, 1
OFFSET
1,4
COMMENTS
Product of those distinct prime factors that occur exactly twice in the prime factorization of n, that is, whose exponent is 2.
FORMULA
Multiplicative with a(p^e) = p when e = 2, otherwise a(p^e) = 1.
a(n) <= A000196(n).
From Amiram Eldar, Feb 11 2023: (Start)
a(n) <= sqrt(n), with equality if and only if n is in A062503.
a(n) = 1 if and only if n is in A337050. (End)
From Vaclav Kotesovec, May 31 2024: (Start)
Dirichlet g.f.: zeta(2*s-1) * zeta(s) * Product_{p prime} (1 - 1/p^(5*s-1) + 1/p^(5*s-2) + 1/p^(4*s-1) - 1/p^(4*s-2) - 1/p^(3*s-1) + 1/p^(3*s) - 1/p^(2*s)).
Let f(s) = Product_{p prime} (1 - 1/p^(5*s-1) + 1/p^(5*s-2) + 1/p^(4*s-1) - 1/p^(4*s-2) - 1/p^(3*s-1) + 1/p^(3*s) - 1/p^(2*s)).
Sum_{k=1..n} a(k) ~ f(1) * n * (log(n) + 3*gamma - 1 + f'(1)/f(1)) / 2, where
f(1) = Product_{p prime} (1 - 3/p^2 + 3/p^3 - 1/p^4) = 0.33718787379158997196169281615215824494915412775816393888028828465611936...,
f'(1) = f(1) * Sum_{p prime} (9*p^2 - 12*p + 5) * log(p) / (p^4 - 3*p^2 + 3*p - 1) = f(1) * 3.78385641685861932254178374972226733621783278751462026270346293...
and gamma is the Euler-Mascheroni constant A001620. (End)
MATHEMATICA
f[p_, e_] := If[e == 2, p, 1]; a[n_] := Times @@ f @@@ FactorInteger[n]; Array[a, 100] (* Amiram Eldar, Feb 11 2023 *)
PROG
(PARI) A329376(n) = { my(f = factor(n)); prod(i=1, #f~, f[i, 1]^(2 == f[i, 2])); };
(PARI) for(n=1, 100, print1(direuler(p=2, n, (1 + p*X^2 + X + X^2/(-1 + 1/X)))[n], ", ")) \\ Vaclav Kotesovec, May 31 2024
CROSSREFS
Row 3 of array A106177, and the square roots of its row 9.
Sequence in context: A211005 A162154 A134505 * A336643 A334039 A076933
KEYWORD
nonn,easy,mult
AUTHOR
Antti Karttunen, Nov 16 2019
STATUS
approved