E.g.f.: exp(2 * (x + x^2 / 2 + x^3 / 3)).

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =108a(5) =552a(6) =3144a(7) =19392a(8) =129168a(9) =920736a(10) =6958944a(11) =55582848a(12) =466824384a(13) =4104798336a(14) =37688879232a(15) =360236187648