Guess the Mathematician

Take a 4 digit number where all 4 digits are different, order the digits from greatest to smallest. Now order the digits from smallest to greatest and subtract it from the 4 digit number that you ordered from greatest to smallest. Repeat the process a few times. For example: take 4921 and reorder it to 9421 and subtract 1249 from it - repeat the process. What happens? Find out which mathematician made this discovery.