Julia Sets of Positive Measure V : The bouncing
Fix θ0 = [a1, a2, ...] (continued fraction) of bounded type with all coefficients ai greater
than the Shishikura-Inou constant S. Choosing n and N great enough (in this order),
take θ = [a1, ..., an, N, S, S, S, ...]. Set fθ(z) = e
2iπθ
z + z
2
, denote by Kθ and ∆θ its filled
Julia set and its Siegel disc; define in the same way f0, K0 and ∆0.
Choose δ very small and define the annulus W = {z ∈ C|2δ < d(z, ∆0) < 10δ} , so
that a point in ∆0 cannot escape to infinity under fθ without stepping in W . By JSPM
II, we can suppose that (∀z ∈ ∆θ)d(z, ∆0) <
1
2
δ. Denote by Xp the set of points in ∆0
which bounce at least p times between ∆0 and W under fθ, and set X∗
p = Xp − Xp+1.
Using Mc Mullen’s density theorem, Koebe distortion estimates and JSPM III, we get
estimates on the measure of Xp and X∗
p
, and finally show that m(∆0 \Kθ) =
Pm(X∗
2p+1)
can be made arbitrarily small. From this the construction of a τ such that Kτ has empty
interior but positive measure follows easily.