Expansion of e.g.f. x^3*(1-x)/(1-2*x).

Terms

a(0) =0a(1) =0a(2) =0a(3) =6a(4) =24a(5) =240a(6) =2880a(7) =40320a(8) =645120a(9) =11612160a(10) =232243200a(11) =5109350400a(12) =122624409600