G.f. satisfies A(x) = 1 + 2*x*A(x) + x^3*A(x)^3.

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =24a(5) =72a(6) =227a(7) =730a(8) =2384a(9) =7916a(10) =26704a(11) =91280a(12) =315319a(13) =1098710a(14) =3856948a(15) =13628441a(16) =48435808a(17) =173030048a(18) =620965396a(19) =2237681720a(20) =8093572960a(21) =29372735368a(22) =106925552672a(23) =390336084256