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

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =7a(5) =19a(6) =59a(7) =179a(8) =615a(9) =2141a(10) =7853a(11) =30179a(12) =118663a(13) =487581a(14) =2050325a(15) =8890743a(16) =39593811a(17) =180367655a(18) =842738375a(19) =4018170415a(20) =19578229355a(21) =97275623919a(22) =492406520567