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

Terms

a(0) =1a(1) =4a(2) =11a(3) =39a(4) =166a(5) =765a(6) =3716a(7) =18725a(8) =96956a(9) =512690a(10) =2756806a(11) =15027651a(12) =82853678a(13) =461215414a(14) =2588619402a(15) =14632777719a(16) =83232244238a(17) =476040155118