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).

REFERENCES:

Iannucci, D. E. "The Kaprekar Numbers." J. Integer Sequences 3, No. 00.1.2, 2000. https://www.math.uwaterloo.ca/JIS/VOL3/VOL3/iann2a.

Kaprekar, D. "On Kaprekar Numbers." J. Rec. Math. 13, 81-82, 1980-1981.

Sloane, N. J. A. Sequence A006886/M4625 in "The On-Line Encyclopedia of Integer Sequences."

Wells, D. The Penguin Dictionary of Curious and Interesting Numbers. Middlesex, England: Penguin Books, p. 73, 1986.