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Old 2021-02-13, 21:46   #4
R. Gerbicz
 
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"Robert Gerbicz"
Oct 2005
Hungary

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If x is composite then we know:
c*x/log(log(x))<phi(x)<=x-sqrt(x), where c>0 is a constant [c=0.25 is good for all x>6].

ok, not very elegant to use these, though this is still elementary.
With this you can easily solve the problem, the remaining x=m^n+n prime case is very easy.

Last fiddled with by R. Gerbicz on 2021-02-13 at 21:47
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