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

Terms

a(0) =0a(1) =0a(2) =2a(3) =6a(4) =44a(5) =360a(6) =4206a(7) =59584a(8) =1021432a(9) =20329344a(10) =461596090a(11) =11756157952a(12) =331835099364