Kaprekar's Constant 6174

At first glance, the number 6174 looks like any other number plucked from thin air. Perhaps this is what makes this unassuming number so remarkable. In 1949, Indian mathematician D. R. Kaprekar, discovered the mysterious beauty of 6174 after devising a process that we now know as Kaprekar's operation.

The operation:

• start with any four-digit number that is made up of least two different digits, including zero.
• re-arrange the digits in ascending order for one four-digit number and in descending order for a second four-digit number (don't forget those zeros) 
• subtract the smaller number from the larger number
• repeat until you get the same number for every iteration
• and what number is that? It's 6174 -- always

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