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

Terms

a(0) =0a(1) =0a(2) =2a(3) =6a(4) =28a(5) =280a(6) =3486a(7) =50624a(8) =877080a(9) =17677440a(10) =404537050a(11) =10360548352a(12) =293676213876