OFFSET
0,3
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..1000
FORMULA
a(n) = Sum_{k=1..n} Sum_{d|k} phi(k/d) * binomial(d+n-2, n-1).
a(n) = [x^n] (1/(1 - x)) * Sum_{k >= 1} phi(k) * x^k/(1 - x^k)^n.
a(n) ~ 2^(2*n-1) / sqrt(Pi*n). - Vaclav Kotesovec, Jun 11 2021
a(n) = Sum_{k=1..n} phi(k) * binomial(floor(n/k)+n-1,n). - Seiichi Manyama, Sep 13 2024
MAPLE
a:= n-> coeff(series((1/(1-x))* add(numtheory[phi](k)
*x^k/(1-x^k)^n, k=1..n), x, n+1), x, n):
seq(a(n), n=0..26); # Alois P. Heinz, Jun 11 2021
MATHEMATICA
a[n_] := Sum[DivisorSum[k, EulerPhi[k/#] * Binomial[n + # - 2, n - 1] &], {k, 1, n}]; Array[a, 30, 0] (* Amiram Eldar, Jun 11 2021 *)
PROG
(PARI) a(n) = sum(k=1, n, sumdiv(k, d, eulerphi(k/d)*binomial(d+n-2, n-1)));
(PARI) a(n) = sum(k=1, n, eulerphi(k)*binomial(n\k+n-1, n)); \\ Seiichi Manyama, Sep 13 2024
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jun 11 2021
STATUS
approved