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

Terms

a(0) =1a(1) =4a(2) =20a(3) =60a(4) =420a(5) =3360a(6) =10080a(7) =100800a(8) =1108800a(9) =3326400a(10) =43243200a(11) =605404800a(12) =1816214400a(13) =29059430400a(14) =494010316800