Expansion of e.g.f.: (1-x^3)/(1-x-x^3).

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =48a(5) =360a(6) =2880a(7) =30240a(8) =362880a(9) =4717440a(10) =68947200a(11) =1117670400a(12) =19639065600a(13) =373621248000