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)=3.

Terms

a(0) =1a(1) =1a(2) =3a(3) =3a(4) =3a(5) =3a(6) =12a(7) =21a(8) =30a(9) =66a(10) =138a(11) =264a(12) =525a(13) =1029a(14) =2271a(15) =4620a(16) =9291a(17) =19335a(18) =41430a(19) =86556a(20) =178689a(21) =373854a(22) =814269a(23) =1726698a(24) =3620550a(25) =7688712a(26) =16786875a(27) =35952933a(28) =76375821a(29) =163325109