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

Terms

a(0) =1a(1) =2a(2) =2a(3) =10a(4) =18a(5) =90a(6) =210a(7) =1010a(8) =2754a(9) =12754a(10) =38626a(11) =173434a(12) =566034a(13) =2481578a(14) =8556722a(15) =36848994a(16) =132406274a(17) =562781730a(18) =2086628034a(19) =8786281322a(20) =33372861074a(21) =139609461370a(22) =540319600530