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

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =9a(5) =25a(6) =77a(7) =247a(8) =801a(9) =2657a(10) =8969a(11) =30635a(12) =105785a(13) =368745a(14) =1295493a(15) =4582767a(16) =16309953a(17) =58357313a(18) =209798289a(19) =757461011a(20) =2745281705a(21) =9984464761a(22) =36428252541a(23) =133293594343a(24) =489028250465