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

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =15a(5) =49a(6) =168a(7) =595a(8) =2160a(9) =7997a(10) =30083a(11) =114660a(12) =441840a(13) =1718531a(14) =6737820a(15) =26600784a(16) =105659970a(17) =421949492a(18) =1693120779a(19) =6823018035a(20) =27602090087a(21) =112053680381a(22) =456343848121