OFFSET
0,5
COMMENTS
Number of ordered ways of writing n as sum of three squarefree numbers (A005117).
LINKS
Eric Weisstein's World of Mathematics, Squarefree
FORMULA
G.f.: (Sum_{k>=1} mu(k)^2*x^k)^3.
EXAMPLE
a(4) = 3 because we have [2, 1, 1], [1, 2, 1] and [1, 1, 2].
MATHEMATICA
nmax = 72; CoefficientList[Series[(Sum[MoebiusMu[k]^2 x^k, {k, 1, nmax}])^3, {x, 0, nmax}], x]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Dec 29 2016
STATUS
approved