Filled Julia set - check my solution if it is right.

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SUMMARY

The discussion centers on determining the value of R for the function f(z) = z^3 - 27z + 15, such that |f(z)| > |z| for |z| > R. The solution provided identifies R as 5, derived from the roots of the equation z^3 - 28z + 15 = 0, which are 5, (-5+√17)/2, and (-5-√17)/2. The participant expresses uncertainty about the solution, particularly regarding the behavior of f(z) near the critical point z = 5, suggesting further analysis is needed to confirm the inequality |f(z)| - |z| > 0.

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Homework Statement


Suppose f(z) = z^3 -27z + 15
Find R such that |f(z)|>|z| whenever |z|>R.

Homework Equations





The Attempt at a Solution


Let f(z)=z, then I have z^3 -28z + 15 = 0
then,

z=5, (-5+√(17))/2, (-5-√(17))/2.

since 5 is the most further point, R=5.

check my solution if it is right. I did it, but I am not so sure.
 
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so at z=5, f(5) = 0 and at z=5+e, for some small e>0, f will still be pretty small, so I don't think you've nailed it

you need to find where
|f(z)|-|z|>0
 

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