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

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =24a(6) =64a(7) =168a(8) =464a(9) =1280a(10) =3624a(11) =10304a(12) =29728a(13) =86240a(14) =252480a(15) =743040a(16) =2200640a(17) =6547200a(18) =19571200a(19) =58727680a(20) =176883200a(21) =534476800a(22) =1619912320a(23) =4923070464a(24) =14999764480a(25) =45807916544a(26) =140196076544a(27) =429931051008