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This came up in our office today. Let

$$f(x) = \begin{cases} x & \mbox{if } x\leq 1 \cr x\cdot f(\ln(x)) & \mbox{otherwise}\end{cases}$$

Does this series converge?

$$ \sum_{n=1}^\infty \frac{1}{f(n)} $$

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This problem came up on a recent Putnam. The answer was that it diverges at a similar rate to the function $log_*$. – zeb Nov 2 '12 at 3:12
2008 A4, except it had $f(x)=x$ for $x\le e$. Solution available at – Gerry Myerson Nov 2 '12 at 5:01

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