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

Terms

a(0) =3a(1) =1a(2) =1a(3) =3a(4) =9a(5) =28a(6) =87a(7) =270a(8) =838a(9) =2682a(10) =8568a(11) =27325a(12) =87087a(13) =284940a(14) =929773a(15) =3026424a(16) =9843471a(17) =32691629a(18) =108290562a(19) =357839667a(20) =1181136576a(21) =3967571310a(22) =13290977466a(23) =44410377199a(24) =148240940865a(25) =501864577470