Consider an -digit
number .
Square it and add the right digits to the left or digits. If the resultant sum is , then is called a Kaprekar number. For example, 9 is a Kaprekar
number since
and 297 is a Kaprekar number since
The first few are 1, 9, 45, 55, 99, 297, 703, ... (OEIS A006886).