a(n) = binomial(3*n, n - 1).

Terms

a(0) =1a(1) =6a(2) =36a(3) =220a(4) =1365a(5) =8568a(6) =54264a(7) =346104a(8) =2220075a(9) =14307150a(10) =92561040a(11) =600805296a(12) =3910797436a(13) =25518731280a(14) =166871334960