Koenigs function

In mathematics, the Koenigs function is a function arising in complex analysis and dynamical systems. Introduced in 1884 by the French mathematician Gabriel Koenigs, it gives a canonical representation as dilations of a univalent holomorphic mapping, or a semigroup of mappings, of the unit disk in the complex numbers into itself.

Existence and uniqueness of Koenigs function

Let D be the unit disk in the complex numbers. Let f be a holomorphic function mapping D into itself, fixing the point 0, with f not identically 0 and f not an automorphism of D, i.e. a Möbius transformation defined by a matrix in SU(1,1).

By the Denjoy-Wolff theorem, f leaves invariant each disk |z | < r and the iterates of f converge uniformly on compacta to 0: in fact for 0 < r < 1,

for |z | ≤ r with M(r ) < 1. Moreover f '(0) = λ with 0 < |λ| < 1.

Koenigs (1884) proved that there is a unique holomorphic function h defined on D, called the Koenigs function, such that h(0) = 0, h '(0) = 1 and Schröder's equation is satisfied,

The function h is the uniform limit on compacta of the normalized iterates, .

Moreover, if f is univalent, so is h.[1][2]

As a consequence, when f (and hence h) are univalent, D can be identified with the open domain U = h(D). Under this conformal identification, the mapping   f becomes multiplication by λ, a dilation on U.

Proof

near 0. Thus H(0) =0, H'(0)=1 and, for |z | small,
Substituting into the power series for H, it follows that H(z) = z near 0. Hence h = k near 0.
On the other hand,
Hence gn converges uniformly for |z| ≤ r by the Weierstrass M-test since

Koenigs function of a semigroup

Let ft (z) be a semigroup of holomorphic univalent mappings of D into itself fixing 0 defined for t ∈ [0, ∞) such that

Each fs with s > 0 has the same Koenigs function, cf. iterated function. In fact, if h is the Koenigs function of f = f1, then h(fs(z)) satisfies Schroeder's equation and hence is proportion to h.

Taking derivatives gives

Hence h is the Koenigs function of fs.

Structure of univalent semigroups

On the domain U = h(D), the maps fs become multiplication by , a continuous semigroup. So where μ is a uniquely determined solution of e μ = λ with Reμ < 0. It follows that the semigroup is differentiable at 0. Let

a holomorphic function on D with v(0) = 0 and v'(0) = μ.

Then

so that

and

the flow equation for a vector field.

Restricting to the case with 0 < λ < 1, the h(D) must be starlike so that

Since the same result holds for the reciprocal,

so that v(z) satisfies the conditions of Berkson & Porta (1978)

Conversely, reversing the above steps, any holomorphic vector field v(z) satisfying these conditions is associated to a semigroup ft, with

Notes

  1. Carleson & Gamelin 1993, pp. 28–32
  2. Shapiro 1993, pp. 90–93

References

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