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Let's omit the first stage of Kaprekar's algorithm and just look at what happens when the only rearrangement is reversing the digits in three of the numbers he looks at and doing a repeated operation.
Find the absolute difference between a number and its reverse, and then the difference between that and its reverse, and so on.
2005, 2997, 4995, 999
1789, 8082, 5274, 549, 396, 297, 495, 99
6174, 1458, 7083, 3276, 3447, 3996, 2997, 4995, 999
The absolute difference between the reverses of these repnines gives us our kernel, which is zero. That's true of all palindromes of course, though the operation in the algorithm producing a palindrome won't necessarily lead to a kernel. For example, consider summing them instead
2005, 7007
1789, 11660, 18271, 35552, 61105, 111221, 233332
6174, 10890, 20691, 40293, 79497
Lastly standard number line subtraction:
2005 ... (17 steps!) ... 8939779398
1789, 8082, 10890, 20691, 40293, 79497
6174, 1458, 7083, 10890, 20691, 40293, 79497 (A negative number can be regarded as a palindrome, eg 121 = 01210)
Look at 6174 still behaving mysteriously, what's more in cahoots with that other notorious number 1089.
2005, 7007