a(n) = 3^n*Catalan(n).

Terms

a(0) =1a(1) =3a(2) =18a(3) =135a(4) =1134a(5) =10206a(6) =96228a(7) =938223a(8) =9382230a(9) =95698746a(10) =991787004a(11) =10413763542a(12) =110546105292