Sum_{0<j<k<=n} (k!-j!).

Terms

a(0) =1a(1) =10a(2) =73a(3) =520a(4) =3967a(5) =33334a(6) =309661a(7) =3166468a(8) =35416555a(9) =430546642a(10) =5655609529a(11) =79856902816