a(n) = (n+1)! - (n)!!.

Terms

a(0) =0a(1) =1a(2) =4a(3) =21a(4) =112a(5) =705a(6) =4992a(7) =40215a(8) =362496a(9) =3627855a(10) =39912960a(11) =478991205a(12) =6226974720a(13) =87178156065