Newton basin fractals are visualizations of the Julia sets of functions of the form:
$$f_p(z) = z - p(z)/p'(z)$$
where $p$ is a complex polynomial. My question is:
When is the Julia set, $J(f_p)$, continuously determined by the roots of $p$, in the sense of the Hausdorff metric?
I have convinced myself that it is a necessary condition that the roots of $p$ be simple, and I'm happy to elaborate on my reasoning, but I suspect that the statement is either obvious (or obviously false) to experts. (Update: It turns out that it is obviously true.)
B. Krauskopf and H. Kriete proved that:
If a sequence of meromorphic functions $f_n$ converges uniformly on compact sets to a limit function $f$, and $F(f)$ is a union of basins of attracting periodic orbits, and $\infty\in J(f)$, then $J(f_n)$ converges to $J(f)$ in the Hausdorff metric.
And at first glance that theorem looks like a great starting point for showing that the simple roots condition is sufficient. Unfortunately it is not the case that $F(f_p)$ is a union of attracting basins whenever $p$ has simple roots.
Is anything more known about this? Perhaps there is some other condition (not merely simple roots) that makes $F(f_p)$ a union of attracting basins.
I expected for this to be a well studied problem. An answer to the related question of "When is convergence of Newton-Raphson iteration not sensitive to perturbation of roots?" seems like it would have useful applications. But I have not found much beyond results like Krauskopf and Kriete's theorem, above, which don't quite apply.
Edit: To give the question a little more context and motivation: Here is a program to demonstrate that in some sense, perhaps not exactly the way I've phrased it, a Newton basin fractal may transform continuously under changes to the roots of its polynomial. In the demonstration (which requires a WebGL enabled browser) the Newton basin fractal for the polynomial $z\to (z^2 - 1)(z - \lambda)$ is rendered in a 10x10 box centered at the origin. Clicking and dragging updates $\lambda$ and the resulting basins.
Edit 2: I've updated the demonstration in an attempt to get more information onto the screen. Now the usual Newton basin rendering is blended with a rendering of the parameters $\lambda$ such that the sequence of iterates starting at the critical point converges very slowly. These are the parameters for which the Fatou set might include components that are not basins of attracting periodic orbits, preventing application of Krauskopf and Kriete's theorem.
The roots are highlighted in yellow and the critical point is highlighted in cyan. It's easy to see that the Julia set experiences a discontinuous transformation when any two roots overlap, and that when $\lambda$ is in the "problem area" the critical point is sort of pressed towards the Julia set (which makes sense given how the problematic parameters are determined in the first place). But even as I move $\lambda$ through the problematic region I don't see the Julia set transforming in a discontinuous way, certainly nothing as obviously discontinuous as the transformations that occur when two roots overlap. I wonder if the Julia set actually is continuously determined by $\lambda$ in this region, even if the theorem above isn't quite enough to prove it.
