[
Abstract
We show wellposedness for an evolution problem associated with the DirichlettoRobin operator for certain Robin boundary data. Moreover, it turns out that the semigroup generated by the DirichlettoRobin operator is closely related to a weighted semigroup of composition operators on an appropriate Banach space of analytic functions.
DirichlettoRobin Operators]DirichlettoRobin Operators via Composition Semigroups
L. Perlich]Lars Perlich
The author is supported by Sächsisches Landesstipendium..\par\@mkboth\shortauthors\shorttitle \par [\par
Mathematics Subject Classification (2010). 47B38, 47B33, 47D06.
Keywords. Composition operators, Spaces of holomorphic functions, DirichlettoNeumann, DirichlettoRobin..
In recent years, the DirichlettoNeumann operator has been studied
intensively. In the beginning of the 20th century, these operators
were dealt with theoretically, while in the 1980s and 1990s they were
used to analyze inverse problems to determine coefficients of a differential
operator. These problems apply, e.g., to image techniques in medicine
and also to find defects in materials.
According to Arendt and ter Elst, the DirichlettoNeumann operator
can be obtained as an example of an operator associated with sectorial
forms, see [3]. Using methods from function theory, our
purpose is to give an alternative approach to PoincaréSteklov operators
and the related semigroups on boundary spaces of Banach spaces of
analytic functions. It turns out, as pointed out by Lax [14],
that there is a surprising connection between semigroups of composition
operators on spaces of harmonic functions on the unit disk referring
to a specific semiflow and the DirichlettoNeumann operator. In fact,
we can extend this observation to the Laplace equation with Robin
boundary conditions on Jordan domains in .
More precisely, we study the evolution problem
\par
(1.1) 
where is a Jordan domain and and are boundary values of appropriate holomorphic functions on . We prove wellposedness of (\@setrefeq:DTR) in various spaces of distributions on including the scale of spaces. As mentioned above, our approach does not use form methods but the theory of (weighted) composition operators on spaces of holomorphic and harmonic functions (for the moment only) on planar domains. Our method appears to be restricted to problems involving the Laplace operator, while the variational approach to DirichlettoNeumann and DirichlettoRobin operators using the theory of forms is quite flexible with respect the choice of elliptic operators in the domain . However, there it seems difficult to handle coefficients in front of the associated Neumann derivative (at least, we do not see how to handle them). Here, we can allow a large class of coefficient functions and . In particular, it may happen that degenerates at one point on the boundary. Moreover, using our method, we can define DirichlettoNeumann and DirichlettoRobin operators on several spaces of distributions. \parThis article is organized as follows. In Section 2 we introduce the notion of admissible spaces which is eventually our tool to solve the above posed evolution problem. We discuss some examples of admissible spaces, and we investigate corresponding boundary spaces. Then, in Section 3, we examine the connection between certain PoincaréStecklov operators, namely DirichlettoNeumann and DirichlettoRobin operators, and weighted semigroups of composition operators, and prove our main theorem. \par\par\@xsect \parInitiated by the famous paper by Berkson and Porta [6], semigroups of composition operators were studied intensively by many authors on various spaces of holomorphic functions defined on the unit disk, see, for example, [2, 5, 13, 18, 17]. In our approach, we consider (weighted) semigroups of composition operators on spaces of harmonic and holomorphic functions which are defined on a simply connected domain bounded by a Jordan curve. To give the definition of such a semigroup, we need the notion of a semiflow of holomorphic functions. \par\par\@xsect
Definition 2.1.
Let be simply connected. Let be holomorphic (we write ) such that for every the fractional iterates are holomorphic selfmaps in . A family is called a semiflow of holomorphic functions if it satisfies the following properties:

for all ,

for all and ,

as for all .
Given a semiflow we define its generator by
for every .
\parSince is simply connected, by the Riemann mapping theorem
there exists a conformal map , and thus every semiflow
on can be written in terms of a semiflow on the unit disk.
Let be a semiflow on . As a consequence
of the chain rule, the generator of
can be written in terms of the generator of .
For all holomorphic selfmaps in the unit disk which are
not automorphisms, the embeddability into a semiflow can be characterized
in terms of the DenjoyWolff point of , see for instance
[10]. The DenjoyWolff point is defined as the unique fixed
point of a holomorphic selfmap in the unit disk which is not an automorphism
in the unit disk. Such a point can be found in the interior of the
unit disk as well as on the boundary. Thus we can use appropriate
Möbius transforms to shift an interior DenjoyWolff point to zero
and a DenjoyWolff point on the boundary to 1. In our case, the representation
of on in terms of a semiflow on the unit
disk gives also the unique fixed point of every as
where is the DenjoyWolff point of .
From the theory of differential equations, we obtain that
is univalent for every , hence the same is true for .
Let be the DenjoyWolff point of a semiflow
in . Then, by [6], the generator of
is given by the Berkson and Porta formula
(2.1) 
where is holomorphic and . It is also well known that is holomorphic in and that . In fact, if a holomorphic function extends continuously to and for every , then is the generator of a semiflow in , see [1, Thm 1]. Conversely, a generator of a semiflow need not extend continuously to the closure of On the other hand, note that, by Fatou’s theorem, a generator has radial limits almost everywhere since the function is the composition of a bounded holomorphic function and a Möbius transform. The angle condition at the boundary still holds.
Lemma 2.2.
Let be a semiflow in the unit disk and its generator. Then .
Proof.
Let be the DenjoyWolff point of . Then, by [6], the generator is given by (\@setrefeq:BP) and radial limits exist almost everywhere. For we have \par
∎ The same result holds true for generators of semiflows on Jordan domains.
Lemma 2.3.
Let be a Jordan domain. Let be a semiflow in and its generator. Then , where is the normal vector at
Proof.
Let be conformal. Therefore
is a semiflow in the unit disk. Let be the generator
of . Then .
For , we have
(2.2) 
The function extends continuously to (see [16, Thm. 2.6]) and has nonvanishing angular derivative a.e. (see [16, Thm. 6.8]). Furthermore, for , we have . For every there exists a unique such that , so \par
∎
Next, we transfer the characterization of generators of semiflows in the unit disk given above to Jordan domains. \par
Proposition 2.4.
Let be holomorphic, where
is simply connected.
(I) If is Dinismooth and extends continuously
to and for a.e. ,
then is the generator of a semiflow in .
(II) If for every conformal map there exists
and a holomorphic function with positive real part such
that
(2.3) 
is the generator of a semiflow in . In this case, we say that admits a conformal Berkson and Porta representation.
Proof.
(I) Let conformal. Define for . Then is a holomorphic function which admits a uniformly continuous extension to , by [16, Thm 3.5]. Moreover, for ,
So we can apply [1, Thm. 1] which shows that
is the generator of a semiflow in , and by (\@setrefeq:konfGen)
is the generator of the semiflow
.
(II) The function is given by the
Berkson and Porta formula, hence it is the generator of a semiflow
in with DenjoyWolff point . The assertion follows
again by (\@setrefeq:konfGen).
∎
\par\par\@xsect
\parSemiflows of holomorphic mappings lead to semigroups of composition
operators on spaces of holomorphic functions. Let
be simply connected, and consider the Frechét space
equipped with the topology of uniform convergence
on compact subset of . Let be an increasing
sequence of compact subsets of such that .
We define a sequence of seminorms on as follows
and a metric induced by these seminorms by
For a given semiflow , we define a family of composition operators acting on as follows
(2.4)  
By the definiton of semiflows, this family is an operator semigroup which is, in particular, strongly continuous since for all , we have
This defintion makes also sense when the space of harmonic functions is under consideration. Since, by the CauchyRiemann equations, for every function , we have .
Definition 2.5.
Let be a Banach space and a semiflow of holomorphic functions in generated by . The space is called admissible if the family of operators defined by (\@setrefeq:CO) satisfies the following two conditions:

is invariant under i.e., for all .

is strongly continuous on .
Given a semigroup of composition operators on a admissible Banach space , the generator admits a special form:
Note that is a directional derivative. This is true for holomorphic functions and harmonic functions as well, but for convenience we write instead of for harmonic functions to distinguish products of complex numbers from inner products. \par\@xsect \parTypical choices for the space are the Bergman spaces
where denotes the normalized Lebesgue measure on , and the Hardy spaces
The invariance is a consequence of Littlewood’s subordination principle, and the strong continuity follows from the density of the polynomials and the dominated convergence theorem, see [17], which is also a comprehensive survey on semigroups of composition operators. \parIndeed, this result carries over to Bergman and Hardy spaces on simply connected domains. The Bergman spaces can be defined analogously to the Bergman spaces for functions in the unit disk. For the Hardy space, we can give at least two definitions for simply connected domains, see [9], either using harmonic majorants or via approximating the boundary of by rectifiable curves. Both definitions are equivalent when analytic Jordan domains are considered. We use the definition in terms of harmonic majorants.
Definition 2.6.
Let be simply connected. For , the Hardy space consists of those functions such that the subharmonic functions is dominated by a harmonic function .
Equipped with the norm where is some fixed point and is the least harmonic majorant for , the Hardy space over is a Banach space. As in the unit disk, functions in admit nontangential limits a.e. on and the boundary function is in . For more details about Hardy spaces over general domains, we refer to [9, Ch. 10]. \par\par
Proposition 2.7.
Let be simply connected. Let be a semiflow of holomorphic functions in generated by . The Hardy space () is admissible.
Proof.
Remark 2.8.
If we were using the definition of Hardy spaces by approximating level curves (sometimes called HardySmirnov spaces), the last proof would involve boundary values of conformal maps. This would have forced us to prescribe conditions concerning the boundary of . Therefore it seems more appropriate to define Hardy spaces via harmonic majorants.
Proposition 2.9.
Let be a Jordan domain. Let be a semiflow of holomorphic functions in generated by . The Bergman space () is admissible.
Proof.
Let be conformal. Then there exists a semiflow in such that . Thus, for ,
The derivative of is nonvanishing in , see [16, Thm. 6.8]. \parFor invariance, we only need to show that . Indeed,
Now Littlewood’s subordination principle yields invariance. \parBy the same calculation, we obtain strong continuity of on from strong continuity on . ∎
Further examples of holomorphic function spaces on the unit disk which appear in the literature concerning semigroups of composition operators are the Bloch space and the space BMOA as well as their subspaces and VMOA. On these spaces the question of strong continuity is much more delicate, and in fact there is no nontrivial strongly continuous semigroup on and BMOA. So in these cases, one is studying socalled maximal subspaces of strong continuity denoted by and such that a given semiflow defines a strongly continuous semigroup of composition operators on resp. . In [5] it has been shown that , and in the recent paper [2] the analogous result for BMOA has been obtained, that is, \parIt is also natural to consider weighted semigroups of composition operators. Let be simply connected. Let be holomorphic. For we define a weight as follows
(2.5) 
For a family of composition operators on with semiflow , we define a family of weighted composition operators as follows \par
(2.6) 
This is again an operator semigroup on and also on but the question of strong continuity is more difficult since it depends heavily on the choice of . \parSpecial weights we are interested in are socalled cocycles.
Definition 2.10.
Let be a semiflow in . A family of holomorphic functions is called cocycle if

,

for all and ,

is continuous for every .
If there exists a holomorphic function such that then the family is called a coboundary of . \parIt is easy to see that a family of cocycle weighted composition operators is also an operator semigroup on . Moreover, given an arbitrary holomorphic function , we can easily construct a cocycle to a semiflow : for , \par
(2.7) 
is a cocycle.
Definition 2.11.
Let be a weighted semigroup of composition operators on , cf. (\@setrefeq:WSG), with semiflow generated by the holomorphic function and cocycle weight in terms of a holomorphic function , see (\@setrefeq:cocy) . A Banach space is called admissible if it satisfies the following two conditions:

is invariant under i.e., for all .

is strongly continuous on .
Let be admissible. Then the generator of is given by
In [13, Theorem 2] it has been shown that for certain holomorphic functions and their associated cocycles as in (\@setrefeq:cocy), and a semiflow generated by ,the Hardy space is admissible in the sense of Definition \@setrefdef:admsibble. By a slight adjustment of the arguments in Proposition \@setrefprop:SGHardy, we obtain the result for Hardy spaces over simply connected sets.
Lemma 2.12.
Let be simply connected. Let be a holomorphic function such that , and let be a semiflow in with generator . Then is admissible.
Proof.
Invariance follows by boundedness of and Proposition \@setrefprop:SGHardy. To show strong continuity, we use the same technique as in Proposition \@setrefprop:SGHardy, too. Since the real part of is bounded as well, we obtain the assertion from [18, Theorem 1]. ∎ Indeed, the proof of [18, Theorem 1] works as well for a family of weighted composition operators on the Bergman space , where is a Jordan domain. \par
Lemma 2.13.
Let be a Jordan domain. Let be a holomorphic function such that , and let be a semiflow in with generator . Then is admissible.
Proof.
It suffices to prove the statement for and then
apply the same technique as in Proposition \@setrefprop:SGBergman.
Due to Siskakis [18, Theorem 1], strong continuity for a
weighted SGCO on is achieved if
which is satisfied by our assumptions on , see [13, Lemma 3.1].
To prove the assertion, we can simply follow the steps in the proof
of [18, Theorem 1].
For all we have . This
and the cocycle properties yield that defines a family
of bounded operators on . Let .
By Littlewood’s subordination principle we get that
(2.8) 
thus
for all .
First, we prove strong continuity if . Let
be a sequence such that . Then we
have . Since
is reflexive and by (\@setrefeq:LSP), after passing
to a subsequence again denoted by ,
the sequence is weakly convergent. The weak
limit is because for all .
By lowersemicontinuity of the norm, ,
and thus . This yields the desired strong continuity.
To show strong continuity in the case we
use that ( is dense in . Let .
For every there exists such that . Moreover,
Since , for all there exists a sufficiently small
such that
Thus as .
∎
Remark 2.14.
Several authors are especially interested in semigroups of composition operators weighted by the derivative of the semiflow with respect to the complex varibale, i.e.,
See for example the recent paper [4]. \parIndeed, this weight is a cocycle given by
Finding boundary values of holomorphic functions is a fundamental problem in function theory. Strong results concerning the boundary values of functions in Hardy spaces are Fatou’s theorem and the theorem by F. and M. Riesz. But, in many spaces of holomorphic functions, convergence to boundary values in a nontangential sense is a rather strong condition. Therefore we consider boundary values in a weaker sense, namly in the sense of distributions. \parLet be a Jordan domain. This restriction guarantees existence and nonvanishing of boundary values of derivatives of conformal maps defined on . Up to now, we are not sure if the established theory works for rectifiable boundaries as well. \parIn what follows, we are exploring boundary distributions of functions in Banach spaces . Our first aim is to define the boundary space of consisting of appropriately defined distributional boundary values of elements of .
Definition 2.15.
Let be a Jordan domain. Let be a Banach space. If for every there exists a uniquely defined boundary distribution in the following sense
for every , where , and is any conformal map, then we denote the set consisting of all such boundary values by . If there exists an isomorphism , then is called the boundary space corresponding to . Moreover, we define a norm on by for every .
A first (though artificial) example is the space
where denotes the disk algebra. The restriction to
the boundary is an isometric homomorphism from into
. So is a Banach subalgebra of
which is even maximal due to Wermer’s maximality theorem. Thus the
boundary space can be defined as the space of continuous
functions on which are holomorphically extendable to
.
Let and define as the Hardy space .
Then it is well known that every function in has nontangential
limits a.e. and the boundary function is in .
For a comprehensive overview, we refer especially to [9, Chapter 3].
These boundary functions form a closed subspace of )
which consists of those function in with vanishing
negative Fourier coefficients. Note that this theory is almost applicable
when the analogously defined Hardy space of harmonic functions
is considered. However, the case appears to be different. The
boundary space on consists of finite Borel measures on the
unit circle.
\parIn both examples, the boundary space inherits some properties of the
underlying space of holomorphic functions. Moreover, by the LuzinPrivalov
theorem, a holomorphic function is in either case identically zero
if the boundary function vanishes on a set of positive measure. Given
a function in one of the two boundary spaces from the examples above,
we can recover the holomorphic function in via Cauchy’s integral
formula and the Poisson integral as well which acts as an isometric
isomorphism between and .
\par\par\@xsect
\parThe theory of boundary values for functions in Hardy spaces on the
unit disc is well established. The question of boundary functions
is much more complicated if one wishes to work on Bergman spaces.
In fact, the Bergman spaces contain functions which do not admit nontangential
or radial limits almost everywhere, such as the Lacunary series. So
it seems more appropriate to define boundary values in the sense of
distributions. To establish such distributional boundary values, we
emphasize a connection between Hardy and Bergman spaces. For simplicity
we use the notation and .
The following theorem can be found in [8, Lem. 4].
Theorem 2.16.
If and is an antiderivative of , then .
For , Theorem \@setrefthm:.p=00003D00003D1 can be generalized to in the following way.
Theorem 2.17.
Let and an antiderivative of . Then .
Proof.
Let . Then we have
To estimate , we examine the following two integrals
For the first term we have
Without loss of generality, we assume . Thus we obtain for the second integral
Combining these results, we have
Letting , the righthand side is still finite since can be chosen arbitrarily in . ∎
This theorem remains true if we replace by a Jordan domain .
Corollary 2.18.
Theorem \@setrefthm:anti remains true if is replaced by a Jordan domain . \par
Proof.
By [9, Cor. to Thm. 10.1], it is enough to show that ) for some conformal mapping . Therefore, one can mostly copy the proof of Theorem \@setrefthm:anti, noting that for one has . \parThe derivative of does not vanish in , and so we have
It remains to show that :
∎ Now we can define distributional boundary values for Bergman functions.
Theorem 2.19.
Let . Every function admits a distributional boundary value in , the dual space of ), where is the usual conjugate exponent of \par
Proof.
Let . We denote by the antiderivative of , so we obtain
This limit exists by using Theorem \@setrefthm:anti, Hölder’s inequality, and the dominated convergence theorem. ∎
Corollary 2.20.
Let be Jordan domain. Then every function admits a distributional boundary value in .
Proof.
Let , and let be conformal. For we define as usual Thus as . Then
(2.9) 
It is easy to show that and . Since is a conformal, we also have . So, by [16, Thm 6.8], we obtain convergence of the integral (\@setrefeq:dbvo) as . ∎ Distributional boundary values of harmonic and holomorphic functions defined on a simply connected domain with smooth boundary have been studied in [19]. There it has been shown that a holomorphic function admits a distributional boundary value if and only if it lies in the Sobolev space for some , see [19, Thm. 1.3]. Moreover, by [19, Cor. 1.7], for all the map defined by
where is the Poisson kernel for , is an isomorphism. The inverse is given by assigning the distributional boundary value to a given function. Thus, functions in are uniquely determined by their boundary distributions. Therefore, restricting the map to the boundary space for some , we can recover each function in using the Poisson operator. \par\par\@xsect \parIn this section we work out our main result, the connection between partial differential equations on the boundary associated with PoincaréSteklov operators and semigroups of composition operators on Banach spaces of holomorphic functions. \par\par\@xsect \parLet be a ’nice’ function and consider the following elliptic equation \par
(3.1) 
The DirichlettoNeumann operator maps the function to the Neumann derivative of the solution of (\@setrefeq:DP.D) provided that a solution exists and is sufficiently regular. As it is shown by Lax [14], if or in