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

Terms

a(0) =1a(1) =2a(2) =5a(3) =18a(4) =72a(5) =310a(6) =1399a(7) =6532a(8) =31287a(9) =152876a(10) =759034a(11) =3818410a(12) =19420713a(13) =99697784a(14) =515909606a(15) =2688267462a(16) =14093211259a(17) =74281217492a(18) =393389969722