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

Terms

a(0) =2a(1) =1a(2) =3a(3) =6a(4) =12a(5) =24a(6) =66a(7) =168a(8) =408a(9) =1014a(10) =2820a(11) =7656a(12) =20370a(13) =54096a(14) =154416a(15) =430710a(16) =1182504a(17) =3258816a(18) =9377922a(19) =26651328a(20) =74941608a(21) =210709446a(22) =612566616a(23) =1760983512a(24) =5017273554a(25) =14308403856a(26) =41833393896a(27) =121321861350a(28) =349444725492