Discrete Dynamics in Nature and Society

VolumeÂ 2009Â (2009), Article IDÂ 742019, 11 pages

http://dx.doi.org/10.1155/2009/742019

## Composition Operators from the Weighted Bergman Space to the th Weighted Spaces on the Unit Disc

Mathematical Institute of the Serbian Academy of Sciences, Knez Mihailova 36/III, 11000 Beograd, Serbia

Received 23 May 2009; Revised 27 August 2009; Accepted 4 September 2009

Academic Editor: LeonidÂ Berezansky

Copyright Â© 2009 Stevo SteviÄ‡. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The boundedness of the composition operator from the weighted Bergman space to the recently introduced by the author, the th weighted space on the unit disc, is characterized. Moreover, the norm of the operator in terms of the inducing function and weights is estimated.

#### 1. Introduction

Let be the open unit disc in the complex plane , the Lebesgue area measure on , , , and the space of all analytic functions on the unit disc.

The weighted Bergman space , where and , consists of all such that With this norm, is a Banach space when , while for it is a FrĂ©chet space with the translation invariant metric

Let be a positive continuous function on a set (*weight*) and be fixed. The *th weighted-type space* on , denoted by consists of all such that

For the space becomes the weighted-type space , for the Bloch-type space , and for the Zygmund-type space .

For , the quantity is a seminorm on the th weighted-type space and a norm on , where is the set of all polynomials whose degrees are less than or equal to . A natural norm on the th weighted-type space can be introduced as follows: where is an element in . With this norm, the th weighted-type space becomes a Banach space.

For is obtained the space , on which a norm is introduced as follows: Some information on Zygmund-type spaces on the unit disc and some operators on them can be found, for example, in [1â€“6], for the case of the upper half-plane, see [7, 8], while some information in the setting of the unit ball can be found, for example, in [9â€“13]. This considerable interest in Zygmund-type spaces motivated us to introduce the th weighted-type space (see [8]).

Assume is a holomorphic self-map of . The composition operator induced by is defined on by

A typical problem is to provide function theoretic characterizations when induces bounded or compact composition operators between two given spaces of holomorphic functions. Some classical results on composition and weighted composition operators can be found, for example, in [14], while some recent results can be found in [1, 5, 7, 15â€“34] (see also related references therein).

Here we characterize the boundedness of the composition operator from the weighted Bergman space to the th weighted space on the unit disc when . The case was previously treated in [16, 22, 24, 31, 35]. Hence we will not consider this case here. See also [36] for some good results on weighted composition operators between weighted-type spaces. The case was treated, for example, in [26, 32]. For some other results on weighted composition operators which map a space into a weighted or a Bloch-type space, see, for example, [15, 17â€“21, 23, 25, 33, 34].

Let and be topological vector spaces whose topologies are given by translation-invariant metrics and , respectively, and be a linear operator. It is said that is *metrically bounded* if there exists a positive constant such that
for all . When and are Banach spaces, the metrically boundedness coincides with the usual definition of bounded operators between Banach spaces.

If is a Banach space, then the quantity is defined as follows: It is easy to see that this quantity is finite if and only if the operator is metrically bounded. For the case this is the standard definition of the norm of the operator , between two Banach spaces. If we say that an operator is bounded, it means that it is metrically bounded.

Throughout this paper, constants are denoted by , they are positive and may differ from one occurrence to the other. The notation means that there is a positive constant such that . Moreover, if both and hold, then one says that .

#### 2. Auxiliary Results

Here, we quote several auxiliary results. The first lemma is a direct consequence of a well-known estimate in [37, Propositionâ€‰â€‰1.4.10]. Hence, we omit its proof.

Lemma 2.1. *Assume , , and . Then the function
**
belongs to . Moreover, .*

The next lemma is folklore and was essentially proved in [38]. We will sketch a proof of it for the completeness and the benefit of the reader.

Lemma 2.2. *Assume , , and . Then there is a positive constant independent of such that
*

*Proof. *By the subharmonicity of the function , , applied on the disk:
and since
we have that

From (2.5) and in light of the following well-known asymptotic relation [38]:
the lemma easily follows.

Lemma 2.3. *Assume and
**
Then .*

*Proof. *By using elementary transformations, we have
from which it follows that
which along with the fact implies the lemma.

We will also need the classical FaĂ di Bruno's formula where and the sum is over all nonnegative integers satisfying . For a nice exposition related to this formula see, for example, [39].

By using Bell polynomials , (2.10) can be written in the following form:

*Remark 2.4. *Since the summation in (2.11) is from 1 to . Moreover, since and , (2.11) can be written in the following form:

#### 3. Main Result

Here, we formulate and prove our main result.

Theorem 3.1. *Assume , , , is a weight on and is a holomorphic self-map of . Then is bounded if and only if
**
where for each fixed , the sum is over all nonnegative integers such that and .**Moreover, if the operator is bounded, then
*

*Remark 3.2. *Note that by (2.11) we see that the conditions in (3.1) can be written in the following form:

*Proof. *First assume that conditions in (3.1) hold. By formula (2.10) and Lemma 2.2 we have

From this, (2.2) with , and by conditions in (3.1), it follows that the operator is bounded. Moreover, if we consider the space , we have that

Now assume that the operator is bounded. For a fixed , and constants , set

Applying Lemma 2.1 we see that for every . Moreover, we have that

Now we show that for each , there are constants , such that
Indeed, by differentiating function , for each , the system in (3.8) becomes

By using Lemma 2.3 with , we obtain that the determinant of system (3.9) is different from zero from which the claim follows.

Now for each , we choose the corresponding family of functions which satisfy (3.8) and denote it by .

For each , the boundedness of the operator along with (2.10) and (3.7) implies that for each :
where (for each fixed ) the sum is over all nonnegative integers such that and .

From (3.10), it follows that for each ,

Now we use consecutively the test functions
in order to deal with the case . Note that

By applying (2.11) to the function we get
which along with the boundedness of the operator and (3.13) implies that
or equivalently (see Remark 2.4).

Further, by applying formula (2.11) to the function we get
From the boundedness of and (3.13), we get

From (3.16) and (3.17), and by using the triangle inequality it follows that

Using the fact and applying inequality (3.15) in (3.18) we get

Assume that we have proved the following inequalities:
for and a .

Applying formula (2.11) to the function , , we have that
From this, by using the boundedness of the operator , the boundedness of function , the triangle inequality, noticing that the coefficient at is independent of (it is equal ), and finally using hypothesis (3.20), we easily obtain
Hence, by induction, we get that (3.22) holds for each .

From (3.22) and bearing in mind Remark 2.4, for each fixed , we have that
where as usual for a fixed the sum is over all nonnegative integers such that and .

Hence from (3.11) and (3.23), we get

From (3.5) and (3.24), we obtain asymptotic relation (3.2).

#### Acknowledgment

The author would like to express his sincere thanks to the referees for numerous comments which improved the presentation of this paper.

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