OFFSET
0,3
FORMULA
G.f.: A(x) = Sum_{n>=1} a(n)*x^n = x * Product_{n>=1} 1/(1 - x^n)^(a(n)+1).
Recurrence: a(n+1) = (1/n) * Sum_{k=1..n} ( Sum_{d|k} d*(a(d) + 1) ) * a(n-k+1).
EXAMPLE
G.f.: A(x) = x + 2*x^2 + 6*x^3 + 17*x^4 + 52*x^5 + 161*x^6 + 524*x^7 + 1739*x^8 + 5929*x^9 + 20562*x^10 + ...
MATHEMATICA
terms = 28; A[_] = 0; Do[A[x_] = x Exp[Sum[(A[x^k] + DivisorSigma[1, k] x^k)/k, {k, 1, terms}]] + O[x]^(terms + 1) // Normal, terms + 1]; CoefficientList[A[x], x]
a[n_] := a[n] = SeriesCoefficient[x Product[1/(1 - x^k)^(a[k] + 1), {k, 1, n - 1}], {x, 0, n}]; Table[a[n], {n, 0, 28}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, May 08 2019
STATUS
approved