a(n) = n! / (6 * floor(n/3)).

Terms

a(0) =1a(1) =4a(2) =20a(3) =60a(4) =420a(5) =3360a(6) =20160a(7) =201600a(8) =2217600a(9) =19958400a(10) =259459200a(11) =3632428800a(12) =43589145600a(13) =697426329600