Kaprekar Number

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).

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