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

Terms

a(0) =1a(1) =3a(2) =7a(3) =23a(4) =88a(5) =363a(6) =1576a(7) =7091a(8) =32768a(9) =154588a(10) =741442a(11) =3604495a(12) =17721394a(13) =87960004a(14) =440165522a(15) =2218289051a(16) =11248850578a(17) =57354875692a(18) =293860786178