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

Terms

a(0) =1a(1) =0a(2) =1a(3) =1a(4) =4a(5) =7a(6) =22a(7) =49a(8) =143a(9) =359a(10) =1025a(11) =2742a(12) =7812a(13) =21666a(14) =62044a(15) =175927a(16) =507484a(17) =1460297a(18) =4243802a(19) =12340559a(20) =36108354a(21) =105839241a(22) =311551092a(23) =919000678a(24) =2719362502a(25) =8063263402a(26) =23967845874a(27) =71379427920a(28) =213010634136a(29) =636757780808