OFFSET
1,5
FORMULA
G.f.: x + x^2 * Product_{n>=1} (1 - x^n)^a(n).
a(n+2) = -(1/n) * Sum_{k=1..n} ( Sum_{d|k} d * a(d) ) * a(n-k+2).
MAPLE
a:= proc(n) option remember; `if`(n<3, 1, -add(add(a(n-k)*
d*a(d), d=numtheory[divisors](k)), k=1..n-2)/(n-2))
end:
seq(a(n), n=1..42); # Alois P. Heinz, Jun 11 2021
MATHEMATICA
nmax = 41; A[_] = 0; Do[A[x_] = x + x^2 Exp[Sum[-A[x^k]/k, {k, 1, nmax}]] + O[x]^(nmax + 1) // Normal, nmax + 1]; CoefficientList[A[x], x] // Rest
a[1] = a[2] = 1; a[n_] := a[n] = -(1/(n - 2)) Sum[Sum[d a[d], {d, Divisors[k]}] a[n - k], {k, 1, n - 2}]; Table[a[n], {n, 1, 41}]
CROSSREFS
KEYWORD
sign
AUTHOR
Ilya Gutkovskiy, Jun 11 2021
STATUS
approved