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)=5.

Terms

a(0) =1a(1) =1a(2) =5a(3) =5a(4) =5a(5) =30a(6) =55a(7) =80a(8) =230a(9) =530a(10) =1080a(11) =2505a(12) =5555a(13) =15130a(14) =34705a(15) =80530a(16) =197855a(17) =511455a(18) =1249430a(19) =2966155a(20) =7334380a(21) =18923630a(22) =46926130a(23) =114708755a(24) =287150005a(25) =741017280a(26) =1860971930a(27) =4628582080a(28) =11629943105a(29) =30291303280