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

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =30a(5) =98a(6) =336a(7) =1194a(8) =4360a(9) =16258a(10) =61644a(11) =236938a(12) =921102a(13) =3615330a(14) =14307312a(15) =57024426a(16) =228701646a(17) =922283522a(18) =3737497980a(19) =15212318730a(20) =62160993642a(21) =254909413218