OFFSET
0,2
FORMULA
G.f.: -x / Sum_{k>=1} k^2*(-x)^k/(1 - (-x)^k).
G.f.: 1 / (d/dx) log(Product_{k>=1} (1 - (-x)^k)^k).
MATHEMATICA
a[0] = 1; a[n_] := a[n] = Sum[(-1)^(k + 1) DivisorSigma[2, k + 1] a[n - k], {k, 1, n}]; Table[a[n], {n, 0, 27}]
nmax = 27; CoefficientList[Series[-x/Sum[k^2 (-x)^k/(1 - (-x)^k), {k, 1, nmax + 1}], {x, 0, nmax}], x]
nmax = 27; CoefficientList[Series[1/D[Log[Product[(1 - (-x)^k)^k, {k, 1, nmax + 1}]], x], {x, 0, nmax}], x]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Mar 30 2019
STATUS
approved