a(n) = T(n,2n-4), T given by A027023.

Terms

a(0) =1a(1) =1a(2) =5a(3) =17a(4) =53a(5) =161a(6) =477a(7) =1393a(8) =4033a(9) =11617a(10) =33365a(11) =95681a(12) =274209a(13) =785793a(14) =2252509a(15) =6460433a(16) =18542169a(17) =53260481a(18) =153115765a(19) =440572993a(20) =1268830877a(21) =3657435745a(22) =10551936125a(23) =30469329025a(24) =88056216233a(25) =254690980449