a(n) = (n-1)! + ((n+1)/2)*a(n-1), a(1)=0.

Terms

a(0) =1a(1) =4a(2) =16a(3) =72a(4) =372a(5) =2208a(6) =14976a(7) =115200a(8) =996480a(9) =9607680a(10) =102366720a(11) =1195568640a(12) =15193785600a(13) =208728576000