a(n) = T(2n,n), T given by A026769.

Terms

a(0) =1a(1) =2a(2) =7a(3) =28a(4) =120a(5) =538a(6) =2493a(7) =11854a(8) =57558a(9) =284392a(10) =1426038a(11) =7241356a(12) =37173304a(13) =192638992a(14) =1006564439a(15) =5297715628a(16) =28061959428a(17) =149491856978a(18) =800425486692