a(n) = T(2n, n-1), T given by A026758.

Terms

a(0) =1a(1) =5a(2) =23a(3) =104a(4) =469a(5) =2119a(6) =9607a(7) =43727a(8) =199819a(9) =916631a(10) =4220267a(11) =19497608a(12) =90370622a(13) =420136173a(14) =1958787580a(15) =9156770130a(16) =42912496696a(17) =201579245739a(18) =949002525067