OFFSET
1,2
FORMULA
G.f.: Sum_{k>=1} phi(k) * x^k / ((1 - x^k) * (1 - 2*x^k)).
a(n) = Sum_{k=1..n} (2^gcd(n,k) - 1).
MATHEMATICA
Table[Sum[EulerPhi[n/d] (2^d - 1), {d, Divisors[n]}], {n, 1, 35}]
nmax = 35; CoefficientList[Series[Sum[EulerPhi[k] x^k/((1 - x^k) (1 - 2 x^k)), {k, 1, nmax}], {x, 0, nmax}], x] // Rest
PROG
(PARI) a(n) = sumdiv(n, d, eulerphi(n/d)*(2^d - 1)); \\ Michel Marcus, Sep 17 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Sep 17 2021
STATUS
approved