a(n) = (n!/2) * Sum_{k=0..n-2} 1/k!.

Terms

a(0) =0a(1) =0a(2) =1a(3) =6a(4) =30a(5) =160a(6) =975a(7) =6846a(8) =54796a(9) =493200a(10) =4932045a(11) =54252550a(12) =651030666a(13) =8463398736a(14) =118487582395