OFFSET
1,2
LINKS
Seiichi Manyama, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = Sum_{d|n} phi(n/d) * phi(d)^3.
a(n) = Sum_{k=1..n} phi(gcd(k,n))*phi(n/gcd(k,n))^2. - Richard L. Ollerton, May 10 2021
Sum_{k=1..n} a(k) ~ c * n^3, where c = (zeta(3)/4) * Product_{p prime} (1 - 3/p^2 + 3/p^3 - 2/p^4 + 3/p^6 - 3/p^7 + 1/p^8) = 0.093622450005... . - Amiram Eldar, Nov 15 2022
MATHEMATICA
a[n_] := DivisorSum[n, EulerPhi[n/#] * EulerPhi[#]^3 &]; Array[a, 50] (* Amiram Eldar, Mar 15 2021 *)
Table[Sum[EulerPhi[GCD[k, n]]^3, {k, n}], {n, 50}] (* Harvey P. Dale, Jul 15 2021 *)
PROG
(PARI) a(n) = sum(k=1, n, eulerphi(gcd(k, n))^3);
(PARI) a(n) = sumdiv(n, d, eulerphi(n/d)*eulerphi(d)^3);
CROSSREFS
KEYWORD
nonn,mult
AUTHOR
Seiichi Manyama, Mar 15 2021
STATUS
approved