E.g.f.: -exp(x)*LambertW(-x).

Terms

a(0) =0a(1) =1a(2) =4a(3) =18a(4) =116a(5) =1060a(6) =12702a(7) =187810a(8) =3296120a(9) =66897288a(10) =1540762010a(11) =39693752494