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

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =71a(5) =262a(6) =994a(7) =3852a(8) =15183a(9) =60686a(10) =245410a(11) =1002300a(12) =4128448a(13) =17129920a(14) =71529800a(15) =300355184a(16) =1267386163a(17) =5371101382a(18) =22850230642a(19) =97546995260a(20) =417717017392