a(n) = T(n,n+3), T given by A027023.

Terms

a(0) =1a(1) =11a(2) =33a(3) =77a(4) =161a(5) =319a(6) =613a(7) =1157a(8) =2161a(9) =4011a(10) =7417a(11) =13685a(12) =25217a(13) =46431a(14) =85453a(15) =157229a(16) =289249a(17) =532075a(18) =978705a(19) =1800189a(20) =3311137a(21) =6090207a(22) =11201717a(23) =20603253a(24) =37895377a(25) =69700555a(26) =128199401a(27) =235795557a(28) =433695745a(29) =797690943