OFFSET
1,2
FORMULA
G.f.: (-1/4) * Sum_{k>0} phi(5*k) * log(1-5*x^k)/(5*k).
MATHEMATICA
a[n_] := DivisorSum[n, 5^(n/#-1)*EulerPhi[5*#]/(4*n) &]; Array[a, 25] (* Amiram Eldar, Jul 14 2023 *)
PROG
(PARI) a(n) = sumdiv(n, d, 5^(n/d-1)*eulerphi(5*d))/(4*n);
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jul 13 2023
STATUS
approved