Weighted Composition Operators from Hardy to Zygmund Type Spaces
Abstract
This paper aims at studying the boundedness and compactness of weighted composition operator between spaces of analytic functions. We characterize boundedness and compactness of the weighted composition operator uCĻ from the Hardy spaces Hp to the Zygmund type spaces and the little Zygmund type spaces šµĪ±,0 in terms of function theoretic properties of the symbols u and Ļ.
1. Introduction
Let D = {z : |z| < 1} be the open unit disk in the complex plane C and T = {z : |z| = 1} its boundary, and H(D) denotes the set of all analytic functions on D. An analytic self-map Ļ : D ā D induces the composition operator CĻ on H(D), defined by CĻ(f) = f(Ļ(z)) for f analytic on D. It is a well-known consequence of Littlewoodās subordination principle that the composition operator CĻ is bounded on the classical Hardy Hpāā(0 < p ⤠ā) spaces, Bergman Apāā(0 < p ⤠ā) spaces, and Bloch spaces (see, e.g., [1ā4]).
Let u be a fixed analytic function on the open unit disk. Define a linear operator uCĻ on the space of analytic functions on D, called a weighted composition operator, by uCĻf = u Ā· (fāĻ), where f is an analytic function on D. We can regard this operator as a generalization of a multiplication operator and a composition operator. In recent years the weighted composition operator has received much attention and appears in various settings in the literature. For example, it is known that isometries of many analytic function spaces are weighted composition operators (see [5], for instance). Their boundedness and compactness have been studied on various Banach spaces of analytic functions, such as Hardy, Bergman, BMOA, Bloch-type, and Zygmund spaces; see, for example, [6ā11]. Also, it has been studied from one Banach space of analytic functions to another; one may see [12ā23].
The purpose of this paper is to consider the weighted composition operators from the Hardy space Hpāā(0 < p < ā) to the Zygmund type spaces šµĪ±. Our main goal is to characterize boundedness and compactness of the operators uCĻ from Hp to šµĪ± in terms of function theoretic properties of the symbols u and Ļ.
Throughout this paper, constants are denoted by C, C(p), they are positive, and C(p) are only depending on p and may differ from one occurrence to the another.
2. Auxiliary Results
In order to prove the main results of this paper. We need some auxiliary results. The first lemma is well known.
Lemma 1 (see [24], p. 65.)For p > 1, there exists a constant C(p) such that
Lemma 2. Suppose that 0 < p < ā, f ā Hp; then
Proof. We use induction on n. The case n = 0 holds because it is Exerciseāā5 in [25, p. 85]. Assume the case n = k holds. Fix 0 < r < 1 and let g(z) = f(k)(rz). Then g(z) is in Hā ā β1, and . It follows that
Lemma 3. For 0 < p < ā, suppose uCĻ : Hp ā šµĪ±,0 is a bounded operator. Then uCĻ : Hp ā šµĪ± is a bounded operator.
This is obvious.
3. Boundedness of uCĻ from Hpāā(0ā < āpā < āā) to šµĪ± and šµĪ±,0
In this section we characterize bounded weighted composition operators from the Hardy space Hpāā(0 < p < ā) to the Zygmund spaces šµĪ±.
Theorem 4. Let α > 0,āā0 < p < ā,āāand u be an analytic function on the unit disc D and Ļ an analytic self-map of D. Then uCĻ is a bounded operator from Hp to the Zygmund spaces šµĪ± if and only if the following are satisfied:
Proof . Suppose uCĻ is bounded from Hp to the Zygmund spaces šµĪ±. Then we can easily obtain the following results by taking f(z) = 1 and f(z) = z in Hp, respectively:
Next, fix a ā D; we take another test functions
Finally, fix a ā D, and, for all z ā D, let
Conversely, suppose that (11), (12), and (13) hold. For f ā Hp, by Lemma 2, we have the following inequality:
Theorem 5. Let α > 0,āā0 < p < ā, and u be an analytic function on the unit disc D and Ļ an analytic self-map of D. Then uCĻ : Hp ā šµĪ±,0 is a bounded operator provided that the following are satisfied:
Proof. Assume that (28), (29), and (30) hold. Then for any ϵ > 0, there is a constant Ī“,āā0 < Ī“ < 1, such that Ī“ < |z| < 1 implies
Conversely, assume that uCĻ is bounded from Hp to the little Zygmund type space šµĪ±,0. Then u = uCĻ1 ā šµĪ±,0. Also uĻ = uCĻz ā šµĪ±,0; thus
Similarly, uCĻz2 ā šµĪ±,0; then
On the other hand, from Lemma 3 and Theorem 4, we obtain that (11), (12), and (13) hold.
4. Compactness of uCĻ
In order to prove the compactness of uCĻ from Hp to the Zygmund spaces šµĪ±, we require the following lemmas.
Lemma 6. Let α > 0,āā0 < p < ā, and u be an analytic function on the unit disc D and Ļ an analytic self-map of D. Suppose that uCĻ is a bounded operator from Hp to šµĪ±. Then uCĻ is compact if and only if, for any bounded sequence {fn} in Hp which converges to 0 uniformly on compact subsets of D, one has as n ā ā.
The proof is similar to that of Propositionāā3.11 in [32]. The details are omitted.
Theorem 7. Let α > 0,0 < p < ā, u be an analytic function on the unit disc D and Ļ an analytic self-map of D. Then uCĻ is a compact operator from Hp to šµĪ± if and only if uCĻ is a bounded operator and the following are satisfied:
Proof. Suppose that uCĻ is compact from Hp to the Zygmund type space šµĪ±. Let {zn} be a sequence in D such that |Ļ(zn)| ā 1 as n ā ā. If such a sequence does not exist, then (37) are automatically satisfied. Without loss of generality we may suppose that |Ļ(zn)| > 1/2 for all n. We take the test functions
Next, let
Finally, let
Conversely, Suppose that (37) hold. Since uCĻ is a bounded operator, from Theorem 4, we have
In order to prove the compactness of uCĻ on the little Zygmund spaces šµĪ±,0, we require the following lemma.
Lemma 8. Let U ā šµĪ±,0. Then U is compact if and only if it is closed, bounded and satisfies
The proof is similar to that of Lemmaāā1 in [1], but we omit it.
Theorem 9. Let α > 0, 0 < p < ā,āu be an analytic function on the unit disc D and Ļ an analytic self-map of D. Then uCĻ is compact from Hp to the little Zygmund type spaces šµĪ±,0 if and only if (28), (29), and (30) hold.
Proof. Assume that (28), (29), and (30) hold. By Theorem 5, we know that uCĻ is bounded from Hp to the little Zygmund type spaces šµĪ±,0. Suppose that f ā Hp with ā„fā„p ⤠1. From Lemmas 1 and 2 we obtain that
Conversely, suppose that uCĻ : Hp ā šµĪ±,0 is compact.
Firstly, it is obvious uCĻ is bounded, from Theorem 5 we have u ā šµĪ±,0, and (31), (32) hold. On the other hand, we have
Next, note that the proof of Theorem 4 and the fact that the functions given in (18) are in Hp and have norms bounded independently of a; we obtain that
Similarly, note that the functions given in (22) and (25) are in Hp and have norms bounded independently of a, we obtain that
Remark 10. From Theorems 5 and 9, we conjecture that uCĻ : Hp ā šµĪ±,0 is compact if and only if uCĻ : Hp ā šµĪ±,0 is bounded.
Taking u(z) = 1 from Theorems 4, 7, and 9, we obtain the following results about the characterization of the boundedness and compactness of the composition operator CĻ : Hp ā šµĪ±(orāāšµĪ±,0).
Corollary 11. Let α > 0,āā0 < p < ā,āāandāāĻ be an analytic self-map of D. Then CĻ : Hp ā šµĪ± is a bounded operator if and only if the following are satisfied:
Corollary 12. Let α > 0,āā0 < p < ā,āāandāāĻ be an analytic self-map of D. Then CĻ : Hp ā šµĪ± is a compact operator if and only if CĻ is bounded and the following are satisfied:
Corollary 13. Let α > 0,āā0 < p < ā,āāandāāĻ be an analytic self-map of D. Then CĻ : Hp ā šµĪ±,0 is a compact operator if and only if
In the formulation of corollary, we use the notation Mu on H(D) defined by Muf = uf for f ā H(D). Taking Ļ(z) = z from Theorems 4, 5, 7, and 9, we obtain the following results about the characterization of the boundedness and compactness of pointwise multiplier Mu : Hp ā šµĪ±(orāāšµĪ±,0).
Corollary 14. Let α > 0,āā0 < p < ā. Then the pointwise multiplier Mu : Hp ā šµĪ± is a bounded operator if and only if
- (i)
u = 0 if α < 2 + 1/p;
- (ii)
u ā Hā if α = 2 + 1/p;
- (iii)
supāzāD(1ā|z|2)āαā2ā1/p | u(z)| < ā if α > 2 + 1/p.
Corollary 15. Let α > 0,āā0 < p < ā. Then the pointwise multiplier Mu : Hp ā šµĪ±,0 is a bounded operator if and only if Mu : Hp ā šµĪ±,0 is a compact operator if and only if Mu : Hp ā šµĪ± is a compact operator if and only if
- (i)
u = 0 if α ⤠2 + 1/p,
- (ii)
if α > 2 + 1/p.
Acknowledgments
The research was supported by Special Fund of Colleges and Universities in Fujian Province (no. JK2012010) and Natural Science Foundation of Fujian Province, China (no. 2009J01004).