Dattatreya Ramachandra Kaprekar discovered 6174 as the magical constant for 4digit numbers. If we construct the largest 4digit number using only the digits of any chosen 4digit number, and then construct the smallest using the same digit, the difference should yield 6174. Otherwise we repeat the process with the new number (difference). After a maximum of 7 attempts, we will arrive at 6174 where the process will constantly yield 6174. Read more in books and encyclopedias.