Powers of sqrt(20) rounded down.

Terms

a(0) =1a(1) =4a(2) =20a(3) =89a(4) =400a(5) =1788a(6) =8000a(7) =35777a(8) =160000a(9) =715541a(10) =3200000a(11) =14310835a(12) =64000000a(13) =286216701a(14) =1280000000a(15) =5724334022a(16) =25600000000a(17) =114486680447a(18) =512000000000