a(n) = n!/6.

Terms

a(0) =1a(1) =4a(2) =20a(3) =120a(4) =840a(5) =6720a(6) =60480a(7) =604800a(8) =6652800a(9) =79833600a(10) =1037836800a(11) =14529715200a(12) =217945728000