G.f. satisfies: A(x) = 1 + x*A(x)/(1 - x^2*A(x)^2).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =5a(5) =12a(6) =30a(7) =79a(8) =213a(9) =584a(10) =1628a(11) =4600a(12) =13138a(13) =37871a(14) =110043a(15) =321978a(16) =947813a(17) =2805104a(18) =8341608a(19) =24912004a(20) =74686460a(21) =224694128a(22) =678143656a(23) =2052640752a(24) =6229616730a(25) =18952875247a(26) =57792705415a(27) =176596786934a(28) =540679385663