a(n) = (10^k - n)(10^k + n), where k is the number of digits in n.

Terms

a(0) =99a(1) =96a(2) =91a(3) =84a(4) =75a(5) =64a(6) =51a(7) =36a(8) =19a(9) =9900a(10) =9879a(11) =9856a(12) =9831a(13) =9804a(14) =9775a(15) =9744a(16) =9711a(17) =9676a(18) =9639a(19) =9600a(20) =9559a(21) =9516a(22) =9471a(23) =9424a(24) =9375a(25) =9324a(26) =9271a(27) =9216a(28) =9159a(29) =9100