OFFSET
1,2
FORMULA
G.f.: Sum_{k>=1} mu(6 * k) * x^k / (1 - x^k)^2, where mu() is the Moebius function (A008683).
Multiplicative with a(p^e) = p^e if p = 2 or 3, and (p-1)*p^(e-1) otherwise.
PROG
(PARI) a(n) = eulerphi(6*n)/2;
(PARI) my(N=80, x='x+O('x^N)); Vec(sum(k=1, N, moebius(6*k)*x^k/(1-x^k)^2))
CROSSREFS
KEYWORD
nonn,mult,easy
AUTHOR
Seiichi Manyama, May 10 2024
STATUS
approved