a(n) = C(3n,n) - C(3n,n-4).

Terms

a(0) =1a(1) =3a(2) =15a(3) =84a(4) =494a(5) =2988a(6) =18411a(7) =114950a(8) =724845a(9) =4606095a(10) =29451240a(11) =189264672a(12) =1221417360a(13) =7910510312a(14) =51388786107a(15) =334716829674