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

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =32a(5) =112a(6) =396a(7) =1416a(8) =5184a(9) =19424a(10) =73984a(11) =285056a(12) =1108848a(13) =4350816a(14) =17203008a(15) =68473504a(16) =274122752a(17) =1103014912a(18) =4458611968a(19) =18096793088a(20) =73724852224a(21) =301360575488