G.f. A(x) satisfies A(x) = 1 - x/A(x)^2 * (1 - A(x) - A(x)^4).

Terms

a(0) =1a(1) =1a(2) =3a(3) =8a(4) =21a(5) =61a(6) =203a(7) =724a(8) =2600a(9) =9291a(10) =33525a(11) =123537a(12) =463796a(13) =1759184a(14) =6706976a(15) =25696524a(16) =99069838a(17) =384429159a(18) =1499778661a(19) =5875513183a(20) =23099489574a(21) =91123553946a(22) =360649997698