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

Terms

a(0) =1a(1) =1a(2) =6a(3) =42a(4) =326a(5) =2712a(6) =23676a(7) =214068a(8) =1987488a(9) =18838464a(10) =181548960a(11) =1773566208a(12) =17523740592a(13) =174814263088