Constant Coefficients in the Radial KomatuLoewner Equation for Multiple Slits
Abstract
The radial KomatuLoewner equation is a differential equation for certain normalized conformal mappings that can be used to describe the growth of slits within multiply connected domains. We show that it is possible to choose constant coefficients in this equation in order to generate given disjoint slits and that those coefficients are uniquely determined under a suitable normalization of the differential equation.
1 Introduction and results
In 1923, C. Loewner derived a differential equation for a certain family of conformal mappings to
attack the Bieberbach conjecture,
see [8]. Loewner’s method has been extended and turned out to be a useful tool
within complex analysis. In particular, the Loewner differential equations provide a powerful
tool for the description of the growth of slits in a given planar domain.
After O. Schramm discovered Stochastic Loewner Evolution (or Schramm Loewner Evolution, SLE)
in [11], it became clear that those models have many applications in different
mathematical and physical disciplines, especially in statistical physics.
In the classical setting, Loewner theory describes the evolution of a family of simply connected,
proper subsets of the complex plane which are all conformally equivalent to the
unit disc according to the Riemann Mapping Theorem.
In 1943, Y. Komatu showed that Loewner’s ideas are not confined to the simply connected case:
He derived a Loewner equation for the growth of a slit within a doubly connected domain,
see [6]. In [7], he considered a generalization of Loewner’s
differential equation to a more general finitely connected domain. Komatu’s ideas have been applied and
extended by several authors. Recently, R. Bauer and R. Friedrich derived a radial and a
chordal KomatuLoewner equation to deal with the growth of a (stochastic) slit in a multiply
connected domain, see [1] and [2]. In the radial
setting, the slit grows within a circular slit disk , i.e.
, where each
is a circular arc centered at such that
whenever Note that every connected domain can
be mapped onto such a circular slit disk by a conformal map
This mapping is unique if we require the normalization for some
see [4], Chapter 15.6.
In [3], W. Lauf and the first author generalized the radial KomatuLoewner
equation for the growth of several slits:
Let be an arbitrary circular slit disk and let
be parametrizations of pairwise disjoint simple
curves such that and for all
Furthermore, if is a circular slit disk and , we denote by
the unique conformal mapping from onto the right halfplane minus
slits parallel to the imaginary axis with and
We summarize one of the main results of [3] in the following theorem:
Theorem A (Corollary 5 in [3]).
Let and denote by the unique
conformal mapping where is a circular slit disk
and
Then there exists a Lebesgue measure zero set such that for every
the function is differentiable on
with
(1.1) 
where the continuous function is the image of under the map and the coefficient functions are measurable with for every .
Remark 1.
The continuous functions
are usually called driving functions.
Informally, the coefficient function corresponds to the speed
of growth of the slit parametrized by From the normalization it follows that
Note that there are no further assumptions on the parametrizations of the slits
in Theorem A.
Now suppose we are given only the circular slit disk and the slits
without parametrization. Roughly speaking, we address the question if it is possible to find a simple form of equation (1.1) such that it has still enough parameters to generate the slits , but, on the
other hand, the choice of those parameters is unique. We will see that this is possible and,
moreover, it will turn out that equation (1.1) is satisfied for all
in this case.
The latter can be interpreted as a generalization of Loewner’s original idea of finding a parametrization of an arbitrary curve such that the family of certain associated conformal mappings is differentiable. In some sense, this problem is related to Hilbert’s fifth problem of finding differentiable structures for continuous groups, see [5].
In a first step, we can choose parametrizations such that the mappings satisfy i.e. However, there are many of such parametrizations when .
In this work we will show that there exist unique parametrizations of the slits , such that the corresponding KomatuLoewner equation is satisfied for all where all coefficient functions are constant and sum up to 1.
We need one further notation: Let be a circular slit disc and let be the unique conformal mapping onto a circular slit disk with and , then and the logarithmic mapping radius of is defined to be the real number . The inequality is an immediate consequence of Lemma 3 b).
Theorem 1.
Let be the logarithmic mapping radius of . There exist unique continuous parametrizations
and unique with such that
the following holds:
Let and denote by the unique
conformal mapping where is a circular slit disk and
Then and for every the function is
differentiable on with
(1.2) 
where is the image of under the map . Moreover, the driving functions are continuous.
In the simply connected case, we have for all and equation (1.2) can be written down explicitly as
Furthermore, coefficients and continuous driving
functions determine the unique solution to
equation (1.2) in the simply connected case.
If the solution generates slit mappings, then the parametrization of these slits are
uniquely determined by the driving functions and the coefficients.
Thus we can formulate the simply connected case of Theorem 1 as follows.
Corollary 2.
Let be disjoint slits in and let be the logarithmic mapping radius of . Then there exist unique and unique continuous driving functions such that the solution of the Loewner equation
generates the slits i.e. maps conformally onto .
Remark 2.
In [9], D. Prokhorov has proven the existence and uniqueness of constant coefficients for several slits in the simply connected case under the assumption that all slits are piecewise analytic. This theorem forms the basis for Prokhorov’s study of extremal problems for univalent functions in [9] by using controltheoretic methods. Our proof shows that one can drop any assumption on the regularity of the slits in order to generate them with constant coefficients.
We shall give the details of the proof only in the case , i.e. for two slits,
in order to allow a simple notation. The general case of slits can be proved
inductively in exactly the same way.
Our proof combines some technical tools from [3] that were used to prove
Theorem A and an idea from [10], where a similar result was
proven for the so called chordal Loewner equation in the simply connected case.
The rest of this paper is organized as follows.
In Section 2 we describe the setting for the proof and cite some technical results
from [3].
The proof of Theorem 1 is divided into two parts: In Section 3 we prove
the existence statement and in Section 4 we give the proof of the uniqueness statement
of Theorem 1.
2 The setting for the proof
Suppose is a circular slit disk and , are disjoint slits,
i.e. there are continuous, onetoone functions
with such that ,
and for
Let the function be defined as in Theorem A.
As is monotonically increasing (see Lemma 3) and ,
we can assume without loss of generality that
Otherwise, we can simultaneously reparameterize the two slits.
Consequently we have , where denotes the logarithmic mapping radius of
.
Furthermore, we assume that in order to simplify some of the notations.
For every we let be the unique conformal mapping from onto a circular slit disk with and and we define
Now, in order to prove Theorem 1, we have to show that there exist

two uniquely determined increasing homeomorphisms and

a uniquely determined
such that the KomatuLoewner equation for the slits and
is satisfied for all with
and and for all
First, we summarize some basic properties of lmr in the following lemma.
Lemma 3.

The function is continuous in .

The function is strictly increasing with respect to and respectively.

For every there exists a so that for all and with and the following holds:
Proof.
See Proposition 6, 8 and 15 in [3]. ∎
Furthermore, we will use a dynamic interpretation of the coefficient functions from Theorem A. Let be arbitrary strictly increasing homeomorphisms and let be a partition of the interval for a fixed , i.e. We will denote by the norm of Now we define the two sums
The following proposition relates the limit of for to the coefficient functions of Theorem A.
Proposition 4.
Let be two increasing selfhomeomorphisms and let denote the conformal mapping for the slits and from Theorem A. Assume that , i.e. for all . Then the limits
exist and form two increasing and Lipschitz continuous functions with and . Furthermore, if is differentiable in for and then the differential equation (1.1) holds for with
In this case, is equal to the first derivative of the function in and is equal to the first derivative of the function in .
Proof.
Beside and we define for a partition of the interval
If holds for every , then it is easy to see that and . By Proposition 4 we see
3 Existence
Now we are able to prove the existence part of Theorem 1. Recall that we have to show the existence of two strictly increasing homeomorphisms and a such that the KomatuLoewner equation for the slits and is satisfied for all with and and for all
The proceeding of this proof is as follows.

First of all we will use a BangBang method introduced in [10] to construct two sequences and of increasing selfhomeomorphisms of .

By using a diagonal argument on and we will find two subsequences and which converge pointwise on a dense set to increasing functions and respectively. The functions and can be extended to continuous functions defined on , with . Furthermore, we will get by the construction of and .

Next we will derive a connection between the sum and the sum for a given partition of the interval .

Moreover, we will find a connection between and .

By combining these results we will find if . Furthermore, as a consequence of this, we will find .

Next will show that and are strictly increasing, i.e. both functions are increasing selfhomeomorphisms of

Finally we will obtain the KomatuLoewnerEquation with constant coefficients and for the parametrizations and .
Proof of Theorem 1 (Existence).

To construct and , we first extend both and to an interval such that and are still disjoint slits and Let and . We let and for we define and recursively as the unique values with
Since is strictly increasing in both variables, see Lemma 3 b), we get
Consequently .
Furthermore, note that the values and depend continuously on : This follows easily by induction and the continuity and strict monotonicity of the function , see Lemma 3. Consequently, for every , we can find a value with . Now we define a sequence of functions and . Definefor all . The values of and between the supporting points are defined by linear interpolation. An immediate consequence of this construction is
(3.1) 
Since is bounded, we find a subsequence such that is convergent with the limit . Next we set
is a dense and countable subset of . Denote by a bijective mapping.
Since the sequences and are bounded (by ), we find a subsequence of , so that and are convergent.Inductively, we define , , to be a subsequence of such that and are convergent.
Consequently we can define sequences and which are (pointwise) convergent in . We denote by and the limit function, i.e.
for all . Moreover we set and . By using equation (3.1) we get for if is big enough. Consequently we find by using Lemma 3 a)
(3.2) for all . Furthermore, since and are strictly increasing, the functions and are increasing too. Moreover and can be extended in a continuous and unique way to [0,1]. To see this, let and define
Thus we find by Lemma 3 a) and equation (3.2)
Since is strictly increasing in both variables and and , we find and . If we can argue in the same way, so and are continuous in . Summarizing, and are continuous and increasing in [0,1] with and . For later use we define and .

Next we show that for every fixed , fixed and a fixed partition of the interval , there exists an so that
holds for all , where .
Fix . As the function is (uniformly) continuous in by Lemma 3 a), there exists such that
Since , we find an so that , holds for all and . Consequently we find

For now we fix . We show that for all we find a so that for all partitions of with there exists an so that for all we have
Let . Then there exists such that the inequality from Lemma 3 c) holds. Since the functions and are (uniformly) continuous we get
Denote by a partition of with and . Then we find an with , and
for all and all . As a consequence we get
for all and all . In an analog way we get . Next, we set . is a partition of the interval and we write .
where if with and . Since , we have for all and all . Thus we get
Since and for all and all , we have by Lemma 3 c)
for all . The last inequality can be proven by using the monotonicity of as follows