Take a k-digit number, n, and rearrange its digits in descending order. Call this number B. Now arrange the digits in ascending order and call that number b. Compute and repeat the operation. This algorithm, known as the Kaprekar routine, always becomes cyclic. It was discovered in 1949 by D. R. Kaprekar for 4-digit numbers.
Wolfram Demonstrations Project
Published: March 7 2011