a(n) = n! / 3.

Terms

a(0) =2a(1) =8a(2) =40a(3) =240a(4) =1680a(5) =13440a(6) =120960a(7) =1209600a(8) =13305600a(9) =159667200a(10) =2075673600a(11) =29059430400a(12) =435891456000