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

Terms

a(0) =1a(1) =1a(2) =2a(3) =8a(4) =26a(5) =93a(6) =342a(7) =1294a(8) =5010a(9) =19751a(10) =79036a(11) =320215a(12) =1310986a(13) =5415416a(14) =22543108a(15) =94474620a(16) =398275738a(17) =1687825205a(18) =7186221246a(19) =30725239502a(20) =131866322768a(21) =567889032437