a(n) = Sum_{i=1..floor((n+1)/4)} a(2*i-1) * a(n-2*i+1), with a(1)=a(2)=1 and a(3)=5.

Terms

a(0) =1a(1) =1a(2) =5a(3) =5a(4) =5a(5) =5a(6) =30a(7) =55a(8) =80a(9) =230a(10) =530a(11) =1080a(12) =2505a(13) =5555a(14) =13755a(15) =31330a(16) =71405a(17) =168605a(18) =403830a(19) =947180a(20) =2199905a(21) =5189130a(22) =12724755a(23) =30313630a(24) =71503880a(25) =171244630a(26) =421182405a(27) =1012930055a(28) =2420011955a(29) =5834077855