a(n) = Sum_{i=1..floor((n+2)/4)} a(2i-1)*a(n-2i+1), with a(1)=a(2)=1 and a(3)=2.

Terms

a(0) =1a(1) =1a(2) =2a(3) =2a(4) =2a(5) =6a(6) =10a(7) =14a(8) =26a(9) =50a(10) =90a(11) =162a(12) =290a(13) =622a(14) =1186a(15) =2206a(16) =4274a(17) =8802a(18) =17378a(19) =33538a(20) =65986a(21) =136790a(22) =274282a(23) =540686a(24) =1082698a(25) =2238978a(26) =4537082a(27) =9091330a(28) =18322466a(29) =38230558