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

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =21a(5) =80a(6) =322a(7) =1347a(8) =5798a(9) =25512a(10) =114236a(11) =518848a(12) =2384538a(13) =11068567a(14) =51817118a(15) =244370806a(16) =1159883685a(17) =5536508864a(18) =26560581688a(19) =127993221140a(20) =619280193640