OFFSET
1,2
LINKS
Seiichi Manyama, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = Sum_{k=1..n} phi(n/gcd(k, n))^3.
G.f.: Sum_{k>=1} phi(k)^4 * x^k/(1 - x^k).
From Amiram Eldar, Nov 13 2022: (Start)
Multiplicative with a(p^e) = 1 + ((p-1)^3*(p^(4*e)-1))/(p^3 + p^2 + p + 1).
Sum_{k=1..n} a(k) ~ c * n^5, where c = (zeta(5)/5) * Product_{p prime} (1 - 4/p^2 + 6/p^3 - 4/p^4 + 1/p^5) = 0.05936545607... . (End)
MATHEMATICA
a[n_] := DivisorSum[n, EulerPhi[#]^4 &]; Array[a, 40] (* Amiram Eldar, Mar 13 2021 *)
PROG
(PARI) a(n) = sumdiv(n, d, eulerphi(d)^4);
(PARI) a(n) = sum(k=1, n, eulerphi(n/gcd(k, n))^3);
(PARI) my(N=66, x='x+O('x^N)); Vec(sum(k=1, N, eulerphi(k)^4*x^k/(1-x^k)))
CROSSREFS
KEYWORD
nonn,mult
AUTHOR
Seiichi Manyama, Mar 13 2021
STATUS
approved