a(n) = n! * C(n,2).

Terms

a(0) =2a(1) =18a(2) =144a(3) =1200a(4) =10800a(5) =105840a(6) =1128960a(7) =13063680a(8) =163296000a(9) =2195424000a(10) =31614105600a(11) =485707622400