Which of the number is known as Kaprekar Constant named after the Indian Mathematician D R Kaprekar?

Question

Which was a mathematician and author on the faculty of the Department of Mathematics at the University of Illinois from 1955-1990, and of Eastern Michigan University from 1990-1999, most famous for writing "The Elements of Real Analysis" and "The Elements of Integration"?

Answer 1

6174 is known as Kaprekar's constant after the Indian mathematician D. R. Kaprekar. This number is notable for the following properties :

• Take any four-digit number, using at least two different digits. (Leading zeros are allowed.)
• Arrange the digits in descending and then in ascending order to get two four-digit numbers, adding leading zeros if necessary.
• Subtract the smaller number from the bigger number.
• Go back to step 2 and repeat.

You will get the 6174....