a(0) = a(1) = 1; a(n) = 3 * Sum_{k=0..n-2} a(k) * a(n-k-2).

Terms

a(0) =1a(1) =1a(2) =3a(3) =6a(4) =21a(5) =54a(6) =189a(7) =558a(8) =1944a(9) =6210a(10) =21681a(11) =72576a(12) =254988a(13) =878850a(14) =3112101a(15) =10935000a(16) =39030660a(17) =139001346a(18) =499808232a(19) =1797731496a(20) =6506661798a(21) =23583173328a(22) =85847830965a(23) =313063862436