a(n) = floor(n^2/4)*n!.

Terms

a(0) =0a(1) =0a(2) =2a(3) =12a(4) =96a(5) =720a(6) =6480a(7) =60480a(8) =645120a(9) =7257600a(10) =90720000a(11) =1197504000a(12) =17244057600a(13) =261534873600