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)=7.

Terms

a(0) =1a(1) =1a(2) =7a(3) =7a(4) =7a(5) =7a(6) =56a(7) =105a(8) =154a(9) =546a(10) =1330a(11) =2800a(12) =7357a(13) =17745a(14) =47047a(15) =116480a(16) =290871a(17) =746179a(18) =1926638a(19) =4908876a(20) =12372409a(21) =31662582a(22) =84227577a(23) =217639730a(24) =556964730a(25) =1448423004a(26) =3868816175a(27) =10079702689a(28) =26120574753a(29) =68425502873