On 2-digit and 3-digit Kaprekar's Routine

Abstract: Kaprekar's Routine is an iteration process that, with each iteration, sorts the digits of a number in descending and ascending orders and uses their difference for the next iteration. In this paper, we successfully address the structure of the fixed periodic sets and the maximum step of iteration to enter a fixed set in the three-digit case and two-digit case, with a focus on the latter. With three-digit, there exists a unique fixed set of the cardinality of 1 or 2, depending on the parity of m, and the maximum step is determined by m. With two-digit, we offer some powerful properties of the fixed sets and a mechanical method to calculate the fixed sets. We also prove that the number of 2-factors in m+1 determines the maximum step (but has completely no influence on the structure of the fixed sets). This continues Atsushi Yamagami's previous studies in 2018 (published in Journal of Number Theory) and completes it with a general solution.