Composition Operator on Bergman-Orlicz Space
© Z. Jiang and G. Cao. 2009
Received: 19 May 2009
Accepted: 22 October 2009
Published: 25 November 2009
Let denote the open unit disk in the complex plane and let denote the normalized area measure on . For and a twice differentiable, nonconstant, nondecreasing, nonnegative, and convex function on , the Bergman-Orlicz space is defined as follows Let be an analytic self-map of . The composition operator induced by is defined by for analytic in . We prove that the composition operator is compact on if and only if is compact on , and has closed range on if and only if has closed range on .
Let be the open unit disk in the complex plane and let be an analytic self-map of . The composition operator induced by is defined by for analytic in . The idea of studying the general properties of composition operators originated from Nordgren . As a sequence of Littlewood's subordinate theorem, each induces a bounded composition operator on the Hardy spaces for all ( ) and the weighted Bergman spaces for all ( ) and for all ( ). Thus, boundedness of composition operators on these spaces becomes very clear. Nextly, a natural problem is how to characterize the compactness of composition operators on these spaces, which once was a central problem for mathematicians who were interested in the theory of composition operators. The study of compact composition operators was started by Schwartz, who obtained the first compactness theorem in his thesis , showing that the integrability of over implied the compactness of on . The work was continued by Shapiro and Taylor , who showed that was not compact on whenever had a finite angular derivative at some point of . Moreover, MacCluer and Shapiro  pointed out that nonexistence of the finite angular derivatives of was a sufficient condition for the compactness of on but it failed on . So looking for an appropriate tool of characterizing the compactness of on was difficult at that time. Fortunately, Shapiro  developed relations between the essential norm of on and the Nevanlinna counting function of , and he obtained a nice essential norm formula of in 1987. As a result, he completely gave a characterization of the compactness of in terms of the function properties of .
Another solution to the compactness of on was done by the Aleksandrov measures which was introduced by Cima and Matheson . It is well known that the harmonic function can be expressed by the Possion integral
The study of compactness of composition operators is also an important subject on other analytic function spaces, and we have chosen two typical examples above, and for more related materials one can consult [7, 8]. Another natural interesting subject is the composition operator with closed range. Considering angular derivatives of , it is known that is compact on if and only if fails to have finite angular derivatives on , in this case, does not have closed range since is not a finite rank operator. And if has finite angular derivatives on , then is necessarily a finite Blaschke product and hence one can easily verify that has closed range on . Zorboska has given a necessary and sufficient condition for with closed range on , and she also has done on . Luecking  considered the same question on Dirichlet space after Zorboska's work. Recently, Kumar and Partington  have studied the weighted composition operators with closed range on Hardy spaces and Bergman spaces.
This paper will study the compactness of composition operator on Bergman-Orlicz space. We are mainly inspired by the following results.
(ii)A composition operator was compact on the Nevanlinna class if and only if it was compact on .
(iii)If a composition operator was compact on for some , then it was compact on for all . Moreover, paper  compared the compactness of composition operators on Hardy-Orlicz spaces and on Hardy spaces. All these results lead us to wonder whether there is a equivalence for the compactness of on and on the Bergman-Orlicz space, and whether there is a equivalence for the closed range of on and on the Bergman-Orlicz space. In this paper, we are going to give affirmative answers for the proceeding questions.
or if and only if
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, we write and say that is asymptotically equivalent to .
In this section we will prove several auxiliary results which will be used in the proofs of the main results in this paper.
the proof is complete.
Lemma 2.2 (see ).
Lemmas 2.1 and 2.2(see )can lead to the following corollary.
In this section, we are going to investigate the equivalence between compactness of composition operator on the Bergman-Orlicz space and on the weighted Bergman space . The following lemma characterizes the compactness of on in terms of sequential convergence, whose proof is similar to that in [7, Proposition 3.11].
In order to characterize the compactness of , we need to introduce the notion of Carleson measure. For and we define . A positive Borel measure on is called a Carleson measure if . Moreover, if satisfies the additional condition , is called a vanishing Carleson measure (see  for the further information of Carleson measure). The following result for the compactness of on is useful in the proof of Theorem 3.3.
4. Closed Range
In this section we will develop a relatively tractable if and only if condition for the composition operator on with closed range. Considering that any analytic automorphism of has the form , where and . By , we have the following lemma.
The converse can be derived from modification of , so we omit it here.
From , we find that the composition operator has closed range on the weighted Bergman space if and only if there are positive constants and such that for all . Thus, we have the following fact.
From the perspective of closed subspace, we will see the following special setting. Let be a -sequence in . That is, there is with for every . We also assume that is separated for some fixed , that is, for all . Using the subharmonicity of for analytic function , it is easy to see that
The authors are extremely thankful to the editor for pointing out several errors. This work was supported by the Science Foundation of Sichuan Province (no. 20072A04) and the Scientific Research Fund of School of Science SUSE.
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