- Suppose 4-figure decimal A.
- Arrange the 4 numbers in descending order, then
let the arranged number
*L*as follows.where

*a*,*b*,*c*,*d*are one-figure numbers and - As well, arrange the 4 numbers in ascending order,
then let the ascending number
*S*as follows. - Let the difference
*L*-*S*as*A*=*L*-*S*. - Repeat 2-4.

The above algorithm gives the trivial number 0 or the non-trivial number 7641 within 8 steps.

In short,

If you repeat i and ii, you will finally get a fixed
number *A*=7641.

e.g. (Suppose 3982) A=3982 ---> L=9832, S=2389 L-S = 7443 A=7443 ---> L=7443, S=3447 L-S = 3996 A=3996 ---> L=9963, S=3699 L-S = 6264 A=6264 ---> L=6642, S=2466 L-S = 4176 A=4176 ---> L=7641, S=1467 L-S = 6174 A=6174