a(n) = Sum_{k=0..n} k!*(n - k)!.

Terms

a(0) =1a(1) =2a(2) =5a(3) =16a(4) =64a(5) =312a(6) =1812a(7) =12288a(8) =95616a(9) =840960a(10) =8254080a(11) =89441280a(12) =1060369920a(13) =13649610240a(14) =189550368000