Discrete Holomorphic Local Dynamical Systems

Discrete Holomorphic Local Dynamical Systems 29
So η : Δ → Δ2 is a bounded holomorphic function not identically zero; Fatou’s
theorem on radial limits of bounded holomorphic functions then implies that
ρ(λ0) := limsup
r→1− |η(rλ0)| > 0
for almost every λ0 ∈ S 1 . This means that we can find 0 < ρ0 < ρ(λ0) and a sequence
{λ j}⊂Δ such that λ j → λ0 and |η(λ j)| > ρ0. This means that ϕ −1
λ is defined in Δρ0
j
for all j ≥ 1; up to a subsequence, we can assume that ϕ −1
λ j → ψ : Δρ0 → Δ2. But
then we have ψ ′ (0)=1and
� �
fλ0 ψ(z) = ψ(λ0z)
in Δρ0 , and thus the origin is a Siegel point for fλ0 . ⊓⊔
The third result we would like to present is the implication (i) =⇒ (ii) in
Theorem 4.10. The proof depends on the following result of Douady and Hubbard,
obtained using the theory of quasiconformal maps:
Theorem 4.16 (Douady-Hubbard, 1985 [DH]). Given λ ∈ C ∗ ,let f λ (z)=λ z+z 2
be a quadratic polynomial. Then there exists a universal constant C > 0 such that for
every holomorphic function ψ : Δ 3|λ |/2 → C with ψ(0)=ψ ′ (0)=0 and |ψ(z)| ≤
C|λ | for all z ∈ Δ 3|λ |/2 the function f = f λ + ψ is topologically conjugated to f λ
in Δ |λ |.
Then
Theorem 4.17 (Yoccoz, 1988 [Y2]). Let λ ∈ S 1 be such that the origin is a Siegel
point for f λ (z)=λ z + z 2 . Then the origin is a Siegel point for every f ∈ End(C,0)
with multiplier λ .
Sketch of proof . Write
and let
f (z)=λz + a2z 2 + ∑ akz
k≥3
k ,
f a (z)=λz + az 2 + ∑ akz
k≥3
k ,
so that f = f a2. If|a| is large enough then the germ
g a (z)=af a (z/a)=λ z + z 2 + a ∑ ak(z/a)
k≥3
k = fλ (z)+ψ a (z)
is defined on Δ 3/2 and |ψ a (z)| < C for all z ∈ Δ 3/2,whereC is the constant given by
Theorem 4.16. It follows that g a is topologically conjugated to f λ . By assumption,
f λ is topologically linearizable; hence g a is too. Proposition 4.3 then implies that g a
is holomorphically linearizable, and hence f a is too. Furthermore, it is also possible