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)=3.

Terms

a(0) =1a(1) =1a(2) =3a(3) =3a(4) =3a(5) =12a(6) =21a(7) =30a(8) =66a(9) =138a(10) =264a(11) =525a(12) =1029a(13) =2460a(14) =5079a(15) =10344a(16) =22197a(17) =50421a(18) =108966a(19) =229827a(20) =498432a(21) =1136190a(22) =2495118a(23) =5390319a(24) =11879355a(25) =27012990a(26) =60008826a(27) =131877372a(28) =292070991a(29) =670433502