Powers of sqrt(20) rounded up.

Terms

a(0) =1a(1) =5a(2) =20a(3) =90a(4) =400a(5) =1789a(6) =8000a(7) =35778a(8) =160000a(9) =715542a(10) =3200000a(11) =14310836a(12) =64000000a(13) =286216702a(14) =1280000000a(15) =5724334023a(16) =25600000000a(17) =114486680448a(18) =512000000000