For every non-negative integer , let be the sum of the digits in the decimal expansion of . Is the sequence eventually increasing?
Solution
Suppose, if possible, that the sequence is eventually increasing, say from some rank on. Fix a non-negative integer such that to write
and deduce thereby that , so
On the other hand, the number of non-vanishing digits in the decimal expansion of does not exceed , so , contradicting (*) for large enough. The conclusion follows.
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