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

Terms

a(0) =0a(1) =1a(2) =6a(3) =24a(4) =192a(5) =1920a(6) =23040a(7) =322560a(8) =5160960a(9) =92897280a(10) =1857945600a(11) =40874803200a(12) =980995276800