E.g.f. sqrt(1+arctan(2*x)).

Terms

a(0) =1a(1) =1a(2) =-1a(3) =-5a(4) =17a(5) =249a(6) =-1489a(7) =-27453a(8) =237537a(9) =6037041a(10) =-68649441a(11) =-2107439157a(12) =29789919345