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

Terms

a(0) =1a(1) =3a(2) =6a(3) =16a(4) =52a(5) =184a(6) =688a(7) =2672a(8) =10672a(9) =43552a(10) =180800a(11) =761088a(12) =3241088a(13) =13937408a(14) =60435968a(15) =263962880a(16) =1160188672a(17) =5127762432a(18) =22775636992a(19) =101608357888a(20) =455105255424