Powers of sqrt(21) rounded down.

Terms

a(0) =1a(1) =4a(2) =21a(3) =96a(4) =441a(5) =2020a(6) =9261a(7) =42439a(8) =194481a(9) =891223a(10) =4084101a(11) =18715701a(12) =85766121a(13) =393029741a(14) =1801088541a(15) =8253624572a(16) =37822859361a(17) =173326116021a(18) =794280046581