a(n) = Sum_{i=1..n-1} a(i)*a(n-1-i), with a(0) = 1, a(1) = 3.

Terms

a(0) =1a(1) =3a(2) =3a(3) =12a(4) =30a(5) =111a(6) =363a(7) =1353a(8) =4917a(9) =18777a(10) =71769a(11) =280506a(12) =1103556a(13) =4395009a(14) =17622309a(15) =71220828a(16) =289510662a(17) =1183627137a(18) =4862148753a(19) =20061888924a(20) =83100910530a(21) =345457823493