OFFSET
1,2
FORMULA
a(n) = Sum_{d|n} phi(n/d)*5^(d - 1) = A054612(n)/5.
G.f.: Sum_{k>=1} phi(k) * x^k / (1 - 5*x^k).
MATHEMATICA
a[n_] := Sum[5^(GCD[k, n] - 1), {k, 1, n}]; Array[a, 24] (* Amiram Eldar, Apr 17 2021 *)
PROG
(PARI) a(n) = sum(k=1, n, 5^(gcd(k, n)-1));
(PARI) a(n) = sumdiv(n, d, eulerphi(n/d)*5^(d-1));
(PARI) my(N=40, x='x+O('x^N)); Vec(sum(k=1, N, eulerphi(k)*x^k/(1-5*x^k)))
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Apr 17 2021
STATUS
approved