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

Terms

a(0) =1a(1) =1a(2) =2a(3) =7a(4) =26a(5) =102a(6) =420a(7) =1787a(8) =7794a(9) =34666a(10) =156636a(11) =716982a(12) =3317700a(13) =15494156a(14) =72935624a(15) =345701843a(16) =1648489762a(17) =7902956738a(18) =38067806892a(19) =184152092450a(20) =894259126540