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Ruby
[QUIZ] Happy Numbers (#93)
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[QUOTE="Sander Land, post: 4525295"] Something which reduces to HP is not equivalent to HP, only if you can reduce HP to this problem is it equivalent to HP/undecidable. Basic math does. number of length 100 : 99999...., next step = 100*9^2 = 8100, i.e. large numbers always go down _a lot_, and therefore cannot be part of a cycle. Proving this for n >= 243 (or even smaller n) is left as an exercise for the reader ;) [/QUOTE]
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[QUIZ] Happy Numbers (#93)
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