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)=4.

Terms

a(0) =1a(1) =1a(2) =4a(3) =4a(4) =4a(5) =20a(6) =36a(7) =52a(8) =132a(9) =292a(10) =580a(11) =1252a(12) =2628a(13) =6772a(14) =14820a(15) =32404a(16) =75012a(17) =183108a(18) =423428a(19) =953924a(20) =2225028a(21) =5435604a(22) =12770340a(23) =29545076a(24) =69900164a(25) =170528612a(26) =405553988a(27) =954683428a(28) =2267938244a(29) =5585542964