Every 4-digit number reaches 6174 in at most 7 steps
Explore a number
—
steps to 6174
6174
fixed point
Kaprekar's process:
1. Take any 4-digit number (not all digits same)
2. Arrange digits ascending → min
3. Arrange digits descending → max
4. Compute max − min
5. Repeat. Always reaches 6174.
6174 is a fixed point: 7641 − 1467 = 6174
D.R. Kaprekar discovered this in 1949. The 3-digit version reaches 495.