a(n) = T(2n,n), T given by A026780.

Terms

a(0) =1a(1) =3a(2) =12a(3) =53a(4) =246a(5) =1178a(6) =5768a(7) =28731a(8) =145108a(9) =741392a(10) =3825418a(11) =19907156a(12) =104370554a(13) =550816506a(14) =2924018194a(15) =15603778253a(16) =83661779470a(17) =450479003038