Programming Praxis

For today’s exercise we return to the world of recreational mathematics with two exercises due to the Indian mathematician Dattaraya Ramchandra Kaprekar. First we compute Kaprekar chains:

1. Choose any four-digit number, with at least two different digits. Leading zeros are permitted.

2. Arrange the digits into two numbers, one with the digits sorted into ascending order, the other with the digits sorted into descending order.

3. Subtract the smaller number from the larger number.

4. Repeat until the number is 6174. At that point, the process will cycle with 7641 − 1467 = 6174.

For instance, starting with 2011, the chain is 2110 − 112 = 1998, 9981 − 1899 = 8082, 8820 − 288 = 8532, and 8532 − 2358 = 6174.

The second exercise determines if a number is a Kaprekar number, defined as an n-digit number such that, when it is squared, the sum of the first n or n−1 digits and the last n digits is the original number. For instance, 703 is a Kaprekar number because 703² = 494209 and 494 + 209 = 703. Sloane gives the list of Kaprekar numbers at A053816.

Your task is twofold: first, write a program that computes the Kaprekar chain for a given starting number, and compute the longest possible Kaprekar chain; second, write a program to determine if a particular number is a Kaprekar number, and compute the list of all the Kaprekar numbers less than a thousand. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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