the Kaprekar process

example:
start with 1024
then:
4210 – 0124 = 4086
8640 – 0468 = 8172
8721 – 1278 = 7443
7443 – 3447 = 3996
9963 – 3699 = 6264
6642 – 2466 = 4176
7641 – 1467 = 6174
Here is the Wikipedia article on this topic.
keywords: Recreational Mathematics, Kaprekar’s constant