a(n) = 6*(2*n)!/(n+2).A064335a(n) = 6*(2*n)!/(n+2).Terms a(0) =3a(1) =4a(2) =36a(3) =864a(4) =40320a(5) =3110400a(6) =359251200a(7) =58118860800External referencesoeis: A064335