a(n) = n! - n^2 + 2n - 1.

Terms

a(0) =1a(1) =1a(2) =2a(3) =15a(4) =104a(5) =695a(6) =5004a(7) =40271a(8) =362816a(9) =3628719a(10) =39916700a(11) =479001479a(12) =6227020656a(13) =87178291031