Kaprekar constant

Kaprekar’s constant
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The natural integer 6174 is known as Kaprekar’s constant, after the Indian mathematician D. R. Kaprekar. 
This number is notable for the following curious behavior:

Select any four-digit number that has at least two different digits (leading zeros are allowed),
Create two new four-digit numbers by arranging the original digits in a.) ascending and b.) descending order (adding leading zeros if necessary).
Subtract the smaller number from the bigger number.
If the result is not 6174, return to step 2 and repeat.
This process, known as Kaprekar's routine, is guaranteed to reach a fixed point at the value 6174 in no more than 7 iterations, at which
point it will continue yielding that value (7641 - 1467 = 6174).

The only four-digit numbers for which Kaprekar’s routine does not reach 6174 are repdigits such as 1111, which give the result 0000 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. For numbers with three 
identical digits and a fourth digit that is one higher or lower (such as 2111), it is essential to treat 3-digit numbers with a leading zero; for 
example: 2111 – 1112 = 0999; 9990 – 999 = 8991; 9981 – 1899 = 8082; 8820 – 288 = 8532; 8532 – 2358 = 6174.

https://en.wikipedia.org/wiki/6174
https://en.wikipedia.org/wiki/Kaprekar%27s_routine