G.f.: 2*x*(1-x*sqrt(1-4*x))/((1+2*x^2+sqrt(1-4*x))*sqrt(1-4*x)).

Terms

a(0) =0a(1) =1a(2) =2a(3) =8a(4) =30a(5) =109a(6) =401a(7) =1495a(8) =5623a(9) =21289a(10) =81034a(11) =309817a(12) =1188932a(13) =4576980a(14) =17667647a(15) =68359881a(16) =265045494a(17) =1029512644a(18) =4005417845a(19) =15606129991a(20) =60885118375a(21) =237816401610a(22) =929909358659