20 lines
1.3 KiB
Markdown
20 lines
1.3 KiB
Markdown
If we take 47, reverse and add, `47 + 74 = 121`, which is palindromic.
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Not all numbers produce palindromes so quickly. For example,
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~~~
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349 + 943 = 1292,
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1292 + 2921 = 4213
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4213 + 3124 = 7337
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~~~
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That is, 349 took three iterations to arrive at a palindrome.
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Although no one has proved it yet, it is thought that some numbers, like `196`, never produce a palindrome. A number that never forms a palindrome through the reverse and add process is called a Lychrel number. Due to the theoretical nature of these numbers, and for the purpose of this problem, we shall assume that a number is Lychrel until proven otherwise. In addition you are given that for every number below ten-thousand, it will either (i) become a palindrome in less than fifty iterations, or, (ii) no one, with all the computing power that exists, has managed so far to map it to a palindrome. In fact, `10677` is the first number to be shown to require over fifty iterations before producing a palindrome: `4668731596684224866951378664` (53 iterations, 28-digits).
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Surprisingly, there are palindromic numbers that are themselves Lychrel numbers; the first example is `4994`.
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How many Lychrel numbers are there below ten-thousand?
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> NOTE: Wording was modified slightly on 24 April 2007 to emphasise the theoretical nature of Lychrel numbers.
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