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An archived instance of discourse for discussion in undergraduate Complex Dynamics.

A simple neutral fixed point problem

mark

Let f(z)=z+z4. Use the the Leau-Fatou flower theorem to find (weakly) attracting and repulsive directions for the neutral fixed point at the origin.

RedCrayon

Note n=3 and a=1. Suppose navn=3v3=1 Then v=(1/3)1/3=31/3(e2kπi)1/3=31/3e2kπi/3,k=0,1,2,3, but there exist unique values of v only for k=0,1,2. To get the repulsive directions, substitute our values of k into the exponential term and simplify. f is repulsive in the directions 1, 1/2+i3/2, and 1/2i3/2.

Suppose instead that navn=3v3=1 Then v=(1/3)1/3=31/3(e(2k+1)πi)1/3=31/3e(2k+1)πi/3,k=0,1,2,3, but there exist unique values of v only for k=0,1,2. To get the attractive directions, substitute our values of k into the exponential term and simplify. f is attractive in the directions 1/2+i3/2, 1, and 1/2i3/2.

mark

This looks good. Here's an image of the Julia set with the weakly attractive and repulsive vectors shown:

Yousername

This all makes sense to me, but can you explain exactly how you are plugging in the values of k into the exponential term and simplifying?

mark

@Yousername

RedCrayon is using the fact that
eiθ=cos(θ)+isin(θ).
Thus, for example,
eiπ/3=cos(π/3)+isin(π/3)=12+i32.


Keep in mind that, for an equation like vn=1, we expect n solutions equally spaced about the unit circle.