a(n) = Sum_{k=1..n} ((n-k)!*(k-1)! - (-1)^k).

Terms

a(0) =2a(1) =2a(2) =6a(3) =16a(4) =65a(5) =312a(6) =1813a(7) =12288a(8) =95617a(9) =840960a(10) =8254081a(11) =89441280a(12) =1060369921a(13) =13649610240a(14) =189550368001