Research Article | Open Access

Xiangling Zhu, Nanhui Hu, "Weighted Composition Operators from Besov Zygmund-Type Spaces into Zygmund-Type Spaces", *Journal of Function Spaces*, vol. 2020, Article ID 2384971, 7 pages, 2020. https://doi.org/10.1155/2020/2384971

# Weighted Composition Operators from Besov Zygmund-Type Spaces into Zygmund-Type Spaces

**Academic Editor:**John R. Akeroyd

#### Abstract

The boundedness, compactness, and essential norm of weighted composition operators from Besov Zygmund-type spaces into Zygmund-type spaces are investigated in this paper.

#### 1. Introduction

Let denote the open unit disk in the complex plane and the space of all analytic functions in . For an analytic self-map of and , the weighted composition operator is defined as follows:

When , is just the composition operator, denoted by . In the past several decades, composition operators and weighted composition operators have received much attention and appear in various settings in the literature (see, for example, [2–5, 8, 10, 13, 15, 16, 19]).

Let . The Bloch type space consists of those functions for which

is a Banach space under the above norm. It is known that when , is the classical Bloch space.

For , an is said to be in the Zygmund-type space , if

It is easy to check that is a Banach space under the norm . When , is the Zygmund space. When , is just the Bloch type space . In particular, when , is just the Bloch space . Hence, the Zygmund space is the space of all such that with norm

Let be the normalized area measure on . For , the Besov space, denoted by , is the space of all such that

This space is a Banach space with the following norm In particular, is the classical Dirichlet space. Besov spaces are Möbius invariant in the sense that for all and , the set of all Möbius maps of (see [1, 19]).

In [4], Colonna and Tjani introduced a new class type space , called the Besov Zygmund-type space, which consists of all such that . Since the Besov space is contained in the Bloch space, it follows that the Besov Zygmund-type space is a subset of the Zygmund space, and hence, it is contained in the disk algebra.

Colonna and Li studied the boundedness and compactness of the operator and in [2, 3], respectively. Here, is the space of bounded analytic functions. (See [2, 3, 6–14, 16, 17] for more results of composition operators, weighted composition operators, and related operators on the Zygmund space and Zygmund-type spaces.) Colonna and Tjani characterized the boundedness and compactness of in [4].

In this work, we give some characterizations for the boundedness, compactness, and the essential norm of the operator .

Throughout the paper, we denote by a positive constant which may differ from one occurrence to the next. In addition, we say that if there exists a constant such that . The symbol means that .

#### 2. Main Results and Proofs

In this section, we formulate and prove our main results in this paper. For this purpose, we need the following lemmas.

Lemma 1. *Suppose . Then, there exists a positive constant such that
for every .*

*Proof. *For , it is well known that
Then, the inequalities in (6) follow from the definition of the Besov Zygmund-type space. Since the Zygmund space is continuously embedded into , as shown in Lemma 2.1 of [18], we get that . The proof is complete.

Lemma 2 (see [4]). *Let . Every sequence in bounded in norm has a subsequence which converges uniformly in to a function in .*

Lemma 3 (see [4]). *Let be a Banach space that is continuously contained in the disk algebra, and let be any Banach space of analytic functions on . Suppose that
*(i)*the point evaluation functionals on are continuous*(ii)*for every sequence in the unit ball of that exists and a subsequence such that uniformly on *(iii)*the operator is continuous if has the supremum norm and is given the topology of uniform convergence on compact sets**Then, is a compact operator if and only if given a bounded sequence in such that uniformly on , then the sequence as .*

The following result is a direct consequence of Lemmas 2 and 3.

Lemma 4. *Let and . If is bounded, then is compact if and only if as for any sequence in bounded in norm which converge to uniformly in .*

The following estimates are fundamental in operator theory and function spaces on the unit disk (see ([19], Lemma 3.10)).

Lemma 5 (see [19]). *Suppose that is real, and
*(i)*If then as a function of is bounded on *(ii)*If then *(iii)*If then **Now, we are in a position to give the following characterization of bound composition operators from to .*

Theorem 6. *Let , , , and be an analytic self-map of . Then, is bounded if and only if ,
*

*Proof. *First, suppose that , (11) and (12) hold. For arbitrary and , by Lemma 1, we have
Therefore, is bounded.

Conversely, suppose that is bounded. Applying the operator to with , and using the boundedness of , we get that , , and . Hence,
For such that , set
Then,
Thus, for , we have
and by Lemma 5,
So,
Therefore, by the boundedness of , we get
Since , for any such that , we have
which implies that
By (14), we get
From (23) and (24), we see that (11) holds.

For , define
So,
By Lemma 5, we see that and . By the boundedness of , we get After a calculation,
Hence, for any ,
On the one hand, from (28), we obtain
On the other hand, by (15), we get
From (29) and (30), we see that (12) holds. The proof is complete.

Next, we estimate the essential norm of . Recall that the essential norm of is defined as the distance from to the set of compact operators , that is,

Theorem 7. *Let , , , and be an analytic self-map of such that is bounded. Then,
**Here,
*

*Proof. *First we prove that
Let be a sequence in such that as . Define
From the proof of Theorem 6, we see that and belong to . Moreover, and converge to uniformly on as . Hence, for any compact operator , by Lemma 4, we obtain
Here, we used the fact that since and converges to uniformly on as .

Therefore, we get
as desired.

Next, we prove that
Let . Define by
It is clear that is compact on and ; moreover, uniformly on compact subsets of as . Let such that as . Then, for each , is compact. Hence,
Thus, we only need to show that
For any with , from the facts that
we have
where is large enough such that for all . Since as , by Lemma 2, we have
Since and uniformly on compact subsets of as , we have
Now, we estimate . Using Lemma 1 and the fact that , we have
Taking the limit as , we get
Similarly, again by Lemma 1,
Taking the limit as , we get
Hence, by (43), (44), (45), (46), (48), and (50), we get
which with (40) implies the desired result. The proof is complete.

From Theorem 7 and the result that if and only if is compact, we get the following corollary.

Corollary 8. *Let , , , and be an analytic self-map of such that is bounded. Then, is compact if and only if
*

#### Data Availability

No data were used to support this study.

#### Conflicts of Interest

The authors declare that they have no conflicts of interest.

#### References

- S. D. Fisher, J. Arazy, and J. Peetre, “Möbius invariant function spaces,”
*Journal fur die Reine und Angewandte Mathematik*, vol. 1985, no. 363, pp. 110–145, 1985. View at: Publisher Site | Google Scholar - F. Colonna and S. Li, “Weighted composition operators from into the Zygmund spaces,”
*Complex Analysis and Operator Theory*, vol. 7, no. 5, pp. 1495–1512, 2013. View at: Publisher Site | Google Scholar - F. Colonna and S. Li, “Weighted composition operators from the Lipschitz space into the Zygmund space,”
*Mathematical Inequalities & Applications*, vol. 17, no. 3, pp. 963–975, 2014. View at: Publisher Site | Google Scholar - F. Colonna and M. Tjani, “Weighted composition operators from the Besov spaces into the weighted-type space ,”
*Journal of Mathematical Analysis and Applications*, vol. 402, no. 2, pp. 594–611, 2013. View at: Publisher Site | Google Scholar - C. Cowen and B. MacCluer,
*Composition Operators on Spaces of Analytic Functions*, CRC Press, Boca Raton, FL, 1995. - J. Du, S. Li, and Y. Zhang, “Essential norm of generalized composition operators on Zygmund type spaces and Bloch type spaces,”
*Annales Polonici Mathematici*, vol. 119, no. 2, pp. 107–119, 2017. View at: Publisher Site | Google Scholar - J. Du, S. Li, and Y. Zhang, “Essential norm of weighted composition operators on Zygmund-type spaces with normal weight,”
*Mathematical Inequalities & Applications*, vol. 21, no. 3, pp. 701–714, 2018. View at: Publisher Site | Google Scholar - K. Esmaeili and M. Lindström, “Weighted composition operators between Zygmund type spaces and their essential norms,”
*Integral Equations and Operator Theory*, vol. 75, no. 4, pp. 473–490, 2013. View at: Publisher Site | Google Scholar - Q. Hu, S. Li, and Y. Zhang, “Essential norm of weighted composition operators from analytic Besov spaces into Zygmund type spaces,”
*Journal of Contemporary Mathematical Analysis*, vol. 54, no. 3, pp. 129–142, 2019. View at: Google Scholar - H. Li and X. Fu, “A new characterization of generalized weighted composition operators from the Bloch space into the Zygmund space,”
*Journal of Function Spaces and Applications*, vol. 2013, article 925901, 6 pages, 2013. View at: Publisher Site | Google Scholar - S. Li and S. Stević, “Volterra type operators on Zygmund spaces,”
*Journal of Inequalities and Applications*, vol. 2007, Article ID 32124, 11 pages, 2007. View at: Publisher Site | Google Scholar - S. Li and S. Stević, “Generalized composition operators on Zygmund spaces and Bloch type spaces,”
*Journal of Mathematical Analysis and Applications*, vol. 338, no. 2, pp. 1282–1295, 2008. View at: Publisher Site | Google Scholar - S. Li and S. Stević, “Weighted composition operators from Zygmund spaces into Bloch spaces,”
*Applied Mathematics and Computation*, vol. 206, no. 2, pp. 825–831, 2008. View at: Publisher Site | Google Scholar - S. Li and S. Stević, “Products of composition and differentiation operators from Zygmund spaces to Bloch spaces and Bers spaces,”
*Applied Mathematics and Computation*, vol. 217, no. 7, pp. 3144–3154, 2010. View at: Publisher Site | Google Scholar - J. H. Shapiro,
*Composition Operators and Classical Function Theory*, Springer-Verlag, New York, 1993. View at: Publisher Site - S. Stević, “Weighted differentiation Composition Operators from the Mixed-Norm Space to the th Weigthed-Type Space on the Unit Disk,”
*Abstract and Applied Analysis*, vol. 2010, Article ID 246287, 15 pages, 2010. View at: Publisher Site | Google Scholar - S. Stević, “Weighted differentiation composition operators from and Bloch spaces to th weighted-type spaces on the unit disk,”
*Applied Mathematics and Computation*, vol. 216, no. 12, pp. 3634–3641, 2010. View at: Publisher Site | Google Scholar - S. Ye and Q. Hu, “Weighted composition operators on the Zygmund space,”
*Abstract and Applied Analysis*, vol. 2012, Article ID 462482, 18 pages, 2012. View at: Publisher Site | Google Scholar - K. Zhu,
*Operator Theory in Function Spaces*, vol. 138 of*Math. Surveys and Monographs*, American Mathematical Society, Providence, Rhode Island, 2nd edition, 2007. View at: Publisher Site

#### Copyright

Copyright © 2020 Xiangling Zhu and Nanhui Hu. 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.