OFFSET
1,4
LINKS
FORMULA
EXAMPLE
a(8) = 2 because 8 has 4 divisors {1, 2, 4, 8} among which 2 are noncubes {2, 4}.
MATHEMATICA
nmax = 105; Rest[CoefficientList[Series[Sum[(x^k - x^k^3)/((1 - x^k) (1 - x^k^3)), {k, 1, nmax}], {x, 0, nmax}], x]]
PROG
(PARI) a(n) = sumdiv(n, d, !ispower(d, 3)); \\ Michel Marcus, Aug 21 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Aug 21 2017
STATUS
approved