G.f. satisfies A(x) = 1 + x*A(x) / (1 - x^2*A(x)^4).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =7a(5) =23a(6) =72a(7) =238a(8) =831a(9) =2959a(10) =10645a(11) =38824a(12) =143492a(13) =535700a(14) =2016020a(15) =7641574a(16) =29152015a(17) =111841263a(18) =431209723a(19) =1669945778a(20) =6493144143a(21) =25338440143a(22) =99204579648a(23) =389570145288