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

Terms

a(0) =0a(1) =1a(2) =0a(3) =6a(4) =24a(5) =240a(6) =2160a(7) =25200a(8) =322560a(9) =4717440a(10) =76204800a(11) =1357171200a(12) =26345088000a(13) =554204851200