OFFSET
1,2
LINKS
Vaclav Kotesovec, Table of n, a(n) for n = 1..1650
FORMULA
a(n) = Sum_{d|n} phi(n/d) * binomial(d+n-1, n).
a(n) = [x^n] Sum_{k >= 1} phi(k) * x^k/(1 - x^k)^(n+1).
a(n) ~ 2^(2*n - 1) / sqrt(Pi*n). - Vaclav Kotesovec, May 23 2021
MATHEMATICA
a[n_] := DivisorSum[n, EulerPhi[n/#] * Binomial[n + # - 1, n] &]; Array[a, 25] (* Amiram Eldar, Apr 18 2021 *)
PROG
(PARI) a(n) = sumdiv(n, d, eulerphi(n/d)*binomial(d+n-1, n));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Apr 17 2021
STATUS
approved