a(n) = 3/(n + 1) * Catalan(2*n).

Terms

a(0) =3a(1) =3a(2) =14a(3) =99a(4) =858a(5) =8398a(6) =89148a(7) =1002915a(8) =11785890a(9) =143291610a(10) =1790214660a(11) =22870640910a(12) =297670187844