a(n) = n! - n(n-1)/2.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =18a(5) =110a(6) =705a(7) =5019a(8) =40292a(9) =362844a(10) =3628755a(11) =39916745a(12) =479001534a(13) =6227020722a(14) =87178291109