G.f. A(x) satisfies A(A(x))=(1-4*x-sqrt(1-8*x))/(8*x).

Terms

a(0) =1a(1) =2a(2) =6a(3) =22a(4) =90a(5) =392a(6) =1772a(7) =8202a(8) =38646a(9) =185076a(10) =900212a(11) =4434356a(12) =22009980a(13) =109780044a(14) =552560376a(15) =2822976810a(16) =14485344790a(17) =72643772868a(18) =361862583908