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OPEN - $250
Schoenberg proved that for every $c\in [0,1]$ the density of \[\{ n\in \mathbb{N} : \phi(n)<cn\}\] exists. Let this density be denoted by $f(c)$. Is it true that there are no $x$ such that $f'(x)$ exists and is positive?
#50
:
[Er95]
number theory
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