OFFSET
0,3
FORMULA
E.g.f.: 1 / (1 - x * exp(x^2/2) * (1 + sqrt(Pi/2) * erf(x/sqrt(2)))), where erf() is the error function.
MATHEMATICA
a[0] = 1; a[n_] := a[n] = Sum[Binomial[n, k] k!! a[n - k], {k, 1, n}]; Table[a[n], {n, 0, 20}]
nmax = 20; CoefficientList[Series[1/(1 - x Exp[x^2/2] (1 + Sqrt[Pi/2] Erf[x/Sqrt[2]])), {x, 0, nmax}], x] Range[0, nmax]!
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Jun 26 2020
STATUS
approved