No matter which number you start with, you eventually reach 6174
Choose any four-digit number whose digits are not all the same (1111, for example, is excluded). Repeatedly subtract the number formed by arranging the digits in ascending order from the number formed in descending order, and surprisingly, you will always reach 6174 within at most 7 steps.
Rules: ① Arrange the digits of the four-digit number in descending order (large→small) and ascending order (small→large). ② Subtract the smaller number from the larger number. ③ If the result is not 6174, go back to ①.
💡 Numbers with all identical digits (1111, 9999, ...) always give 0 when the larger arrangement minus the smaller arrangement is calculated, so this process does not work. Every other four-digit number reaches 6174 without exception — this was discovered in 1949 by Indian mathematician D. R. Kaprekar.