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. The first few are 1, 9, 45, 55, 99, 297, 703, ... (Sloane's A006886).
See also Digital Root, Digitaddition, Happy Number, Kaprekar Routine, Narcissistic Number, Recurring Digital Invariant
References
Sloane, N. J. A. Sequence
A006886/M4625
in ``An On-Line Version of the Encyclopedia of Integer Sequences.''
http://www.research.att.com/~njas/sequences/eisonline.html and Sloane, N. J. A. and Plouffe, S.
The Encyclopedia of Integer Sequences. San Diego: Academic Press, 1995.