a(n) = T(2n-1,n-1), T given by A026747.

Terms

a(0) =1a(1) =4a(2) =17a(3) =74a(4) =327a(5) =1461a(6) =6584a(7) =29879a(8) =136391a(9) =625731a(10) =2883357a(11) =13338421a(12) =61920497a(13) =288368511a(14) =1346873365a(15) =6307694990a(16) =29613690966a(17) =139352892908a(18) =657163401162