6174 is known as Kaprekar's constant or Kaprekar's operation[1][2][3] after the Indian mathematician D. R. Kaprekar. This number is notable for the following property:

   1. Take any four-digit number with at least two digits different. (Leading zeros are allowed.)
   2. Arrange the digits in ascending and then in descending order to get two four-digit numbers, adding leading zeros if necessary.
   3. Subtract the smaller number from the bigger number.
   4. Go back to step 2.