a(n) = floor(n*n!/2 + 1).

Terms

a(0) =1a(1) =3a(2) =10a(3) =49a(4) =301a(5) =2161a(6) =17641a(7) =161281a(8) =1632961a(9) =18144001a(10) =219542401a(11) =2874009601a(12) =40475635201a(13) =610248038401