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

Terms

a(0) =1a(1) =5a(2) =13a(3) =27a(4) =53a(5) =101a(6) =189a(7) =351a(8) =649a(9) =1197a(10) =2205a(11) =4059a(12) =7469a(13) =13741a(14) =25277a(15) =46495a(16) =85521a(17) =157301a(18) =289325a(19) =532155a(20) =978789a(21) =1800277a(22) =3311229a(23) =6090303a(24) =11201817a(25) =20603357a(26) =37895485a(27) =69700667a(28) =128199517a(29) =235795677