Squares that remain squares when prefixed with a 4.

Terms

a(0) =9a(1) =225a(2) =900a(3) =2025a(4) =22500a(5) =90000a(6) =202500a(7) =950625a(8) =2250000a(9) =9000000a(10) =20250000a(11) =85100625a(12) =95062500a(13) =225000000a(14) =900000000a(15) =2025000000a(16) =2487515625a(17) =8510062500a(18) =9506250000a(19) =18123890625a(20) =22500000000a(21) =90000000000a(22) =202500000000a(23) =248751562500a(24) =851006250000