E.g.f. tan(x)/(1-tan(x)-tan(x)^2).

Terms

a(0) =0a(1) =1a(2) =2a(3) =14a(4) =88a(5) =856a(6) =8912a(7) =115184a(8) =1644928a(9) =26916736a(10) =484527872a(11) =9646440704a(12) =208891091968