E.g.f. 1/(1-arctan(x)).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =8a(5) =24a(6) =128a(7) =624a(8) =1920a(9) =8448a(10) =137472a(11) =1203456a(12) =835584a(13) =-23073792a(14) =1024616448a(15) =15237156864a(16) =-88263327744