6174

6174 is known as Kaprekar's constant after the Indian mathematician D. R. Kaprekar.  This number is notable for the following property:

1. Take any four-digit number, using at least two different digits. (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.

The above process, known as Kaprekar's routine, will always reach its fixed point, 6174, in at most 7 iterations.  Once 6174 is reached, the process will continue yielding 7641 – 1467 = 6174.  For example, choose 3524:

5432 – 2345 = 3087

8730 – 0378 = 8352

8532 – 2358 = 6174

The only four-digit numbers for which Kaprekar's routine does not reach 6174 are repdigits such as 1111, which give the result 0 after a single iteration.  All other four-digit numbers eventually reach 6174 if leading zeros are used to keep the number of digits at 4.

495 is the equivalent constant for three-digit numbers.  For five-digit numbers and above, there is no single equivalent constant.