E.g.f. (1+x^3-x^4)/(1-x).

Terms

a(0) =1a(1) =1a(2) =2a(3) =12a(4) =24a(5) =120a(6) =720a(7) =5040a(8) =40320a(9) =362880a(10) =3628800a(11) =39916800a(12) =479001600a(13) =6227020800a(14) =87178291200