Powers of sqrt(21) rounded up.

Terms

a(0) =1a(1) =5a(2) =21a(3) =97a(4) =441a(5) =2021a(6) =9261a(7) =42440a(8) =194481a(9) =891224a(10) =4084101a(11) =18715702a(12) =85766121a(13) =393029742a(14) =1801088541a(15) =8253624573a(16) =37822859361a(17) =173326116022a(18) =794280046581