a(n) = T(2n-1, n-1), T given by A026536.

Terms

a(0) =1a(1) =1a(2) =6a(3) =19a(4) =79a(5) =306a(6) =1247a(7) =5069a(8) =20889a(9) =86479a(10) =360205a(11) =1506462a(12) =6324176a(13) =26630423a(14) =112439094a(15) =475838291a(16) =2017827545a(17) =8572102713a(18) =36474080228a(19) =155418445421a(20) =663102388605