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

Terms

a(0) =1a(1) =4a(2) =18a(3) =88a(4) =455a(5) =2444a(6) =13486a(7) =75912a(8) =433935a(9) =2511388a(10) =14684422a(11) =86611848a(12) =514704064a(13) =3078845696a(14) =18523994024a(15) =112026315616a(16) =680626958899