OFFSET
1,4
FORMULA
G.f.: Sum_{k>=1} mu(k)^2 * k * x^(k^2) / (1 - x^k).
MATHEMATICA
Table[DivisorSum[n, # &, # <= Sqrt[n] && SquareFreeQ[#] &], {n, 1, 90}]
nmax = 90; CoefficientList[Series[Sum[MoebiusMu[k]^2 k x^(k^2)/(1 - x^k), {k, 1, nmax}], {x, 0, nmax}], x] // Rest
PROG
(PARI) a(n) = sumdiv(n, d, if ((d^2<=n) && issquarefree(d), d)); \\ Michel Marcus, Apr 03 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Apr 03 2020
STATUS
approved