Volume 2013, Issue 1 365286
Research Article
Open Access

Weighted Composition Operators from Hardy to Zygmund Type Spaces

Shanli Ye

Corresponding Author

Shanli Ye

Department of Mathematics, Fujian Normal University, Fuzhou 350007, China fjnu.edu.cn

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Zhengyuan Zhuo

Zhengyuan Zhuo

Department of Mathematics, Fujian Normal University, Fuzhou 350007, China fjnu.edu.cn

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First published: 18 April 2013
Citations: 5
Academic Editor: Yansheng Liu

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 š’µĪ±,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 φ : D → D induces the composition operator Cφ on H(D), defined by Cφ(f) = f(φ(z)) for f analytic on D. It is a well-known consequence of Littlewood’s subordination principle that the composition operator Cφ is bounded on the classical Hardy Hp  (0 < p ≤ āˆž) spaces, Bergman Ap  (0 < p ≤ āˆž) spaces, and Bloch spaces (see, e.g., [1–4]).

Let u be a fixed analytic function on the open unit disk. Define a linear operator uCφ on the space of analytic functions on D, called a weighted composition operator, by uCφf = u Ā· (fāˆ˜Ļ†), 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, [6–11]. Also, it has been studied from one Banach space of analytic functions to another; one may see [12–23].

The purpose of this paper is to consider the weighted composition operators from the Hardy space Hp  (0 < p < āˆž) to the Zygmund type spaces š’µĪ±. Our main goal is to characterize boundedness and compactness of the operators uCφ from Hp to š’µĪ± in terms of function theoretic properties of the symbols u and φ.

Now we give a detailed definition of these spaces. For 0 ≤ r < 1, f(z) ∈ H(D), we set
()
For 0 < p ≤ āˆž, the Hardy space Hp consists of those functions f ∈ H(D), for which
()
It is well known that with norm (2) the Hp space is a Banach space if 1 ≤ p ≤ āˆž, for 0 < p < 1, Hp space is a nonlocally convex topological vector space, and is a complete metric for it. For more information about the Hp space, one may see these books, for example, [24, 25].
For α > 0 the α-Bloch space βα consists of all analytic functions f defined on D such that
()
The space š’µĪ± consists of all analytic functions f defined on D such that
()
When α = 1, it is called the Zygmund space. From a theorem by Zygmund (see [26, vol. I, p. 263] or [24, Theorem 5.3]), we see that f ∈ š’µ1 if and only if f is continuous in the close unit disk and the boundary function f(eiĪø) such that
()
When α > 1, from Proposition 8 of [27], we know that š’µĪ± = Ī²Ī±āˆ’1. Then the space š’µĪ± is called a Zygmund type space if 0 < α ≤ 1. However, all results in this paper are valid for all š’µĪ± spaces (α > 0). An analytic function f ∈ H(D) is said to belong to the little Zymund type space š’µĪ±,0 which consists of all f ∈ š’µĪ± satisfying . It can be easily proved that š’µĪ± is a Banach space under the norm
()
And the polynomials are norm-dense in closed subspace š’µĪ±,0. For some other information on this space and some operators on it, see, for example, [28–31].

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 < āˆž, f ∈ Hp; then

()
for every z ∈ D and all nonnegative integer n = 0,1, 2, ….

Proof. We use induction on n. The case n = 0 holds because it is Exercise  5 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 Hāˆž āŠ‚ β1, and . It follows that

()
Let r → 1āˆ’; we have
()
Then the case n = k + 1 holds. Hence (8) holds.

Lemma 3. For 0 < p < āˆž, suppose uCφ : Hp → š’µĪ±,0 is a bounded operator. Then uCφ : Hp → š’µĪ± is a bounded operator.

This is obvious.

3. Boundedness of uCφ from Hp  (0  <  p  < ā€‰āˆž) to š’µĪ± and š’µĪ±,0

In this section we characterize bounded weighted composition operators from the Hardy space Hp  (0 < p < āˆž) to the Zygmund spaces š’µĪ±.

Theorem 4. Let α > 0,  0 < p < āˆž,  and u be an analytic function on the unit disc D and φ an analytic self-map of D. Then uCφ is a bounded operator from Hp to the Zygmund spaces š’µĪ± if and only if the following are satisfied:

()
()
()

Proof . Suppose uCφ is bounded from Hp to the Zygmund spaces š’µĪ±. Then we can easily obtain the following results by taking f(z) = 1 and f(z) = z in Hp, respectively:

()
By (14) and the boundedness of the function φ(z), we get
()
Let f(z) = z2 in Hp again; in the same way we have
()
Using these facts and the boundedness of the function φ(z) again, we get
()
Fix a ∈ D; we take the test functions
()
for z ∈ D. From Lemma 1 we obtain that fa ∈ Hp and with a direct calculation. Since fa(a) = 0,  ,  and , it follows that, for all Ī» ∈ D with |φ(Ī»)| > 1/2, we have
()
Let a = φ(Ī»); it follows that
()
For all Ī» ∈ D with |φ(Ī»)| ≤ 1/2, by (17), we have
()
Hence (12) holds.

Next, fix a ∈ D; we take another test functions

()
for z ∈ D. From Lemma 1 we obtain that ga ∈ Hp and with a direct calculation. Since ga(a) = 0,  ,  and , it follows that, for all Ī» ∈ D with |φ(Ī»)| > 1/2, we obtain that
()
For all Ī» ∈ D with |φ(Ī»)| ≤ 1/2, by (15), we have
()
Hence (13) holds.

Finally, fix a ∈ D, and, for all z ∈ D, let

()
From Lemma 1 we obtain that ha ∈ Hp and with a direct calculation. Since ,  ,  and ha(a) = āˆ’2/(p + 1)(p + 2)(1 āˆ’ |a|2) 1/p, it follows that, for all Ī» ∈ D, we obtain that
()
Then (11) holds.

Conversely, suppose that (11), (12), and (13) hold. For f ∈ Hp, by Lemma 2, we have the following inequality:

()
This shows that uCφ is bounded. This completes the proof of Theorem 4.

Theorem 5. Let α > 0,  0 < p < āˆž, and u be an analytic function on the unit disc D and φ an analytic self-map of D. Then uCφ : Hp → š’µĪ±,0 is a bounded operator provided that the following are satisfied:

()
()
()
Conversely, if uCφ : Hp → š’µĪ±,0 is a bounded operator, then u ∈ š’µĪ±,0, (11), (12), and (13) hold, and the following are satisfied:
()
()

Proof. Assume that (28), (29), and (30) hold. Then for any ϵ > 0, there is a constant Ī“,  0 < Ī“ < 1, such that Ī“ < |z| < 1 implies

()
Then, for any f ∈ Hp, from Lemma 2 we obtain that
()
Hence uCφf ∈ š’µĪ±,0 for all f ∈ š’µĪ±,0. On the other hand, (25), (28), and (29) imply that (11), (12), and (13) hold; then uCφ : Hp → š’µĪ± is bounded by Theorem 4. So uCφ : Hp → š’µĪ±,0 is bounded.

Conversely, assume that uCφ is bounded from Hp to the little Zygmund type space š’µĪ±,0. Then u = uCφ1 ∈ š’µĪ±,0. Also uφ = uCφz ∈ š’µĪ±,0; thus

()
Since |φ| ≤ 1 and u ∈ š’µĪ±,0, we have . Hence (32) holds.

Similarly, uCφz2 ∈ š’µĪ±,0; then

()
By (32), |φ | ≤ 1, and u ∈ š’µĪ±,0, we get that ; that is, (31) holds.

On the other hand, from Lemma 3 and Theorem 4, we obtain that (11), (12), and (13) hold.

4. Compactness of uCφ

In order to prove the compactness of uCφ from Hp to the Zygmund spaces š’µĪ±, we require the following lemmas.

Lemma 6. Let α > 0,  0 < p < āˆž, and u be an analytic function on the unit disc D and φ an analytic self-map of D. Suppose that uCφ is a bounded operator from Hp to š’µĪ±. Then uCφ 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 → āˆž.

The proof is similar to that of Proposition  3.11 in [32]. The details are omitted.

Theorem 7. Let α > 0,0 < p < āˆž, u be an analytic function on the unit disc D and φ an analytic self-map of D. Then uCφ is a compact operator from Hp to š’µĪ± if and only if uCφ is a bounded operator and the following are satisfied:

()

Proof. Suppose that uCφ is compact from Hp to the Zygmund type space š’µĪ±. Let {zn} be a sequence in D such that |φ(zn)| → 1 as n → āˆž. If such a sequence does not exist, then (37) are automatically satisfied. Without loss of generality we may suppose that |φ(zn)| > 1/2 for all n. We take the test functions

()
By a direct calculation, we may easily prove that {fn} converges to 0 uniformly on compact subsets of D and . Then {fn} is a bounded sequence in Hp which converges to 0 uniformly on compact subsets of D. Then by Lemma 6. Note that
()
It follows that
()
Then
()

Next, let

()
By a direct calculation we obtain that gn⇉0  (n → āˆž) on compact subsets of D and . Consequently, {gn} is a bounded sequence in Hp which converges to 0 uniformly on compact subsets of D. Then by Lemma 6. Note that gn(φ(zn)) ≔ 0, and ; it follows that
()
Then lim nā†’āˆž(1āˆ’|zn|2) α(2u′(zn)φ′(zn) + φ′′(zn)u(zn))/(1āˆ’|φ(zn)|2) 1/p+1 = 0.

Finally, let

()
By a direct calculation we obtain that hn⇉0  (n → āˆž) on compact subsets of D and . Consequently, {hn} is a bounded sequence in Hp which converges to 0 uniformly on compact subsets of D. Then by Lemma 6. Note that hn(φ(zn)) = 2/(p + 1)(p + 2)(1āˆ’|φ(zn)|2) 1/p,  , and ; it follows that
()
Then lim nā†’āˆž((1 āˆ’ |zn|2) α|u′′(zn)|/(1āˆ’|φ(zn)|2) 1/p) = 0. The proof of the necessary is completed.

Conversely, Suppose that (37) hold. Since uCφ is a bounded operator, from Theorem 4, we have

()
Let {fn} be a bounded sequence in Hp with and fn → 0 uniformly on compact subsets of D. We only prove by Lemma 6. By the assumption, for any ϵ > 0, there is a constant Ī“,  0 < Ī“ < 1, such that Ī“<|φ(z)| < 1 implies
()
Let K = {w ∈ D:|w | ≤ Ī“}. Note that K is a compact subset of D. Then from Lemma 2 it follows that
()
As n → āˆž,
()
Hence uCφ is compact. This completes the proof of Theorem 7.

In order to prove the compactness of uCφ on the little Zygmund spaces š’µĪ±,0, we require the following lemma.

Lemma 8. Let U āŠ‚ š’µĪ±,0. Then U is compact if and only if it is closed, bounded and satisfies

()

The proof is similar to that of Lemma  1 in [1], but we omit it.

Theorem 9. Let α > 0, 0 < p < āˆž, u be an analytic function on the unit disc D and φ an analytic self-map of D. Then uCφ is compact from Hp to the little Zygmund type spaces š’µĪ±,0 if and only if (28), (29), and (30) hold.

Proof. Assume that (28), (29), and (30) hold. By Theorem 5, we know that uCφ is bounded from Hp to the little Zygmund type spaces š’µĪ±,0. Suppose that f ∈ Hp with ∄f∄p ≤ 1. From Lemmas 1 and 2 we obtain that

()
thus
()
and it follows that
()
Hence uCφ : Hp → š’µĪ±,0 is compact by Lemma 8.

Conversely, suppose that uCφ : Hp → š’µĪ±,0 is compact.

Firstly, it is obvious uCφ is bounded, from Theorem 5 we have u ∈ š’µĪ±,0, and (31), (32) hold. On the other hand, we have

()
for some M > 0 by Lemma 6.

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

()
for |φ(z)| > 1/2. However, if |φ(z)| ≤ 1/2, by (31), we easily have
()

Similarly, note that the functions given in (22) and (25) are in Hp and have norms bounded independently of a, we obtain that

()
for |φ(z)| > 1/2. However, if |φ(z)| ≤ 1/2, from u ∈ š’µĪ±,0 and (32), we easily have
()
This completes the proof of Theorem 9.

Remark 10. From Theorems 5 and 9, we conjecture that uCφ : Hp → š’µĪ±,0 is compact if and only if uCφ : Hp → š’µĪ±,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 Cφ : Hp → š’µĪ±(orā€‰ā€‰š’µĪ±,0).

Corollary 11. Let α > 0,  0 < p < āˆž,  and  φ be an analytic self-map of D. Then Cφ : Hp → š’µĪ± is a bounded operator if and only if the following are satisfied:

()

Corollary 12. Let α > 0,  0 < p < āˆž,  and  φ be an analytic self-map of D. Then Cφ : Hp → š’µĪ± is a compact operator if and only if Cφ is bounded and the following are satisfied:

()

Corollary 13. Let α > 0,  0 < p < āˆž,  and  φ be an analytic self-map of D. Then Cφ : Hp → š’µĪ±,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 ∈ H(D). Taking φ(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 → š’µĪ±(orā€‰ā€‰š’µĪ±,0).

Corollary 14. Let α > 0,  0 < p < āˆž. Then the pointwise multiplier Mu : Hp → š’µĪ± is a bounded operator if and only if

  • (i)

    u = 0 if α < 2 + 1/p;

  • (ii)

    u ∈ Hāˆž if α = 2 + 1/p;

  • (iii)

    sup z∈D(1āˆ’|z|2)ā€‰Ī±āˆ’2āˆ’1/p | u(z)| < āˆž if α > 2 + 1/p.

Corollary 15. Let α > 0,  0 < p < āˆž. Then the pointwise multiplier Mu : Hp → š’µĪ±,0 is a bounded operator if and only if Mu : Hp → š’µĪ±,0 is a compact operator if and only if Mu : Hp → š’µĪ± 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).

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