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)=4.

Terms

a(0) =1a(1) =1a(2) =4a(3) =4a(4) =4a(5) =4a(6) =20a(7) =36a(8) =52a(9) =132a(10) =292a(11) =580a(12) =1252a(13) =2628a(14) =6196a(15) =13412a(16) =28884a(17) =64516a(18) =147140a(19) =327684a(20) =722116a(21) =1615236a(22) =3758292a(23) =8500772a(24) =19027572a(25) =43203204a(26) =100782308a(27) =230210884a(28) =522001572a(29) =1193084612