Weighted Composition Operators from Derivative Hardy Spaces into -th Weighted-Type Spaces
The boundedness, compactness, and the essential norm of weighted composition operators from derivative Hardy spaces into -th weighted-type spaces are investigated in this paper.
Let denote the space of analytic functions on the open unit disk . Let denote the set of all analytic self-maps of . Let . The composition operator with the symbol is defined by
Let and . The weighted composition operator is defined on by
It is important to give function theoretic descriptions when and induce a bounded or compact weighted composition operator on various function spaces. See references [1, 2] for more information of this research field.
For , the Hardy space, denoted by , consists of all functions such that (see )
As usual, denotes the space of bounded analytic functions in . If , we say that belongs to the derivative Hardy space, denoted by . When , the space is a Banach space under the norm defined by
When , is a Hilbert space. In , Roan started the study of composition operators on the space . In , MacCluer investigated composition operators on the space in terms of Carleson measure. The boundedness and compactness of weighted composition operators on were studied in . See references [4–7] and references therein for more study of composition operators and weighted composition operators on the space .
If is a radial, positive, and continuous function on , then is called a radial weight. Let be a radial weight and , the set of all positive integers. Let denote the -th weighted space, which consists of all such that
It is a Banach space with the norm . When and , becomes the Bloch-type space and the Zygmund-type space , respectively. Furthermore, when , is the Bloch space and is the Zygmund space. For some results on the space , see references [2, 8–13].
Let with . Recall that the partial Bell polynomials are defined as follows:where the sum taken over all sequences of nonnegative integers such that the following two conditions hold:
See reference  for more information about Bell polynomials.
In , Stević studied the boundedness and compactness of the composition operator from to on the unit disk. In , Stević studied the boundedness and compactness of weighted composition operators from and the Bloch space to . See references [8–10] for more characterizations for weighted composition operators from and the Bloch space to . In , Zhu and Du studied the boundedness, compactness, and essential norm of weighted composition operators from weighted Bergman spaces with doubling weight to . Recall that the essential norm of a bounded linear operator is its distance to the set of compact operators mapping into , that is,
Here, are Banach spaces and denotes the operator norm. In , Colonna and Tjani studied the boundedness and compactness of weighted composition operators from derivative Hardy spaces to Bloch-type space .
In this paper, we use Bell polynomials to study the boundedness and compactness of weighted composition operators from derivative Hardy spaces to -th weighted-type spaces . Moreover, we give an estimate for the essential norm of weighted composition operators from to .
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 .
In this section, the boundedness of weighted composition operators from to is characterized.
Lemma 1. Suppose and . Then, there exists a positive constant such thatfor every .
Proof. The first inequality follows from the fact that are contained in the disk algebra for . In addition, it is well known that for every , there exists a positive constant such thatwhich implies the second inequality.
For any , , and , setAfter a calculation, for each ,
Lemma 2. Let and . For any , there exist constants such thatMoreover, converges to 0 uniformly in .
Proof. The proof is similar to the proof of Theorem 1 in . Hence, we omit the details of the proof.
For the simplicity of this paper, we defineFrom the definition, we see that, for example,This fact will be used in the proof of the following theorem.
Now, we are in a position to state and prove the first result in this paper.
Theorem 1. Let , , , , and be a weight. Then, the operator is bounded if and only if and
Proof (sufficiency). Let . After a calculation, we have (see, e.g., )Since and , we see that . Hence, by (14) and Lemma 1, we getFrom (18), for each ,Therefore, from (19) and (20) and the fact that , we get that is bounded.
Necessity. Assume that is bounded. It is clear that , and for each and ,by the boundedness of .
Next, we show that for ,Applying the operator for , by (17) and (14), we obtainTherefore, by the triangle inequality and the boundedness of , we obtainNow, we assume that for , To get the desired result, we only need to show thatApplying the operator for , we obtainHence, from the boundedness of and triangle inequality again, we get the desired result.
For any and , by Lemma 2,where are independent of the choice of . From the last inequality and (22), for any , we getTherefore,The proof is complete.
Let . We get the following result, which was first obtained in .
Corollary 1. Let , , , and be a weight. Assume that is bounded if and only if andLet . We get the following result.
Corollary 2. Let , , , and be a weight. Then, the operator is bounded if and only if ,
3. Essential Norm
In this section, we obtain some estimates for the essential norm of the weighted composition operator , and we need the following lemma.
Lemma 3. (see ). 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(1)The point evaluation functionals on are continuous(2)For every sequence in the unit ball of , there exists and a subsequence such that uniformly on (3)The operator is continuous if has the supremum norm and is given the topology of uniform convergence on compact setsThen, is a compact operator if and only if, given a bounded sequence in such that uniformly on , then\ as .
Lemma 5. Let , , and be a weight. If is a bounded linear operator from into , then is compact if and only if as for any sequence in bounded in norm which convergences to 0 uniformly in .
Theorem 2. Let , , , , and be a weight such that is bounded. Then,
Proof. First, we prove thatIf , there is a number such that . In this case, (32) is vacuously satisfied and the desired result follows.
Assume that . Let be a sequence in such that as . Since is bounded, for any compact operator and , by using Lemma 2 and Lemma 5, we obtainHence,which implies thatas desired.
Now, we show thatLet and define . Then, is compact and . It is clear that uniformly on compact subsets of as . Let be a sequence such that as . Then, for any positive integer , the operator is compact. By the definition of the essential norm, we getHence, it is sufficient to show thatFor any such that ,where such that for all . Since for any nonnegative integer , uniformly on compact subsets of as . It is clear thatFrom Lemma 4,whileFor any , by Lemma 1 and Lemma 2,Taking the limit as , by Lemma 1, we getBy the same reason, we haveHence, by (40)–(43), (45), and (46), we obtainHence, from the last two inequalities, we obtain (39). The proof is complete.
From Theorem 2 and the well-known result that if and only if is compact, we obtain the following corollary.
Corollary 3. Let , , , , and be a weight. Assume that is bounded, then the operator is compact if and only ifLet . We get the following result, which was first obtained in .
Corollary 4. Let , , , and be a weight such that is bounded. Then, the operator is compact if and only ifLet . We get the following result.
Corollary 5. Let , , , and be a weight such that is bounded. Then, the operator is compact if and only if
No data were used to support this study.
Conflicts of Interest
The author declares that there are no conflicts of interest.
C. Cowen and B. MacCluer, Composition Operators on Spaces of Analytic Functions, CRC Press, Boca Raton, FL, USA, 1995.
K. Zhu, “Operator theory in function spaces,” Mathematical Surveys and Monographs, vol. 138, American Mathematical Society, Providence, RI, USA, 2ed edition, 2007.View at: Google Scholar
P. Duren, Theory of Hp Spaces, Academic Press, New York, NY, USA, 1970.
R. Roan, “Composition operators on the space of functions with -derivative,” Houston Journal of Mathematics., vol. 4, pp. 423–438, 1978.View at: Google Scholar
B. Maccluer, “Composition operators on ,” Houston Journal of Mathematics., vol. 13, pp. 245–254, 1987.View at: Google Scholar
M. Contreras and A. Hernandez-Diaz, “Weighted composition operators on spaces with derivative in a Hardy space,” Journal of Operator Theory, vol. 52, pp. 173–184, 2004.View at: Google Scholar
S. Stević, “Composition followed by differentiation from and the Bloch space to - th weighted-type spaces on the unit disk,” Applied. Mathematics and Computation, vol. 216, no. 12, pp. 3450–3458, 2010.View at: Google Scholar
L. Comtet, Advanced Combinatorics, D. Reidel, Dordrecht, Netherlands, 1974.