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

Terms

a(0) =0a(1) =1a(2) =2a(3) =2a(4) =6a(5) =2a(6) =20a(7) =4a(8) =-10a(9) =334a(10) =-864a(11) =236a(12) =8356a(13) =22252a(14) =-450052a(15) =-347224a(16) =30131822a(17) =-54733650a(18) =-2300158256a(19) =10315132412a(20) =221235097892