Journal of Function Spaces

Volume 2018, Article ID 3485962, 9 pages

https://doi.org/10.1155/2018/3485962

## Sharp Weighted Bounds for Multilinear Fractional Type Operators Associated with Bergman Projection

^{1}School of Mathematical Sciences, Beijing Normal University, Laboratory of Mathematics and Complex Systems, Ministry of Education, Beijing 100875, China^{2}College of Mathematics, Physics and Information Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, China

Correspondence should be addressed to Senhua Lan; moc.anis@nalauhnes

Received 14 March 2018; Accepted 22 April 2018; Published 3 June 2018

Academic Editor: Dashan Fan

Copyright © 2018 Juan Zhang et al. 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.

#### Abstract

We first introduce the multiple weights which are suitable for the study of Bergman type operators. Then, we give the sharp weighted estimates for multilinear fractional Bergman operators and fractional maximal function.

#### 1. Introduction

Let be the area measure in the upper half plane and for . Then, the Bergman space () is defined to be the space of analytic functions in . The well-known Bergman projection and its maximal operator have the following integral representations: where is an appropriate real constant. It is well known that [1] both and map boundedly onto for . The boundedness of and in weighted spaces was studied by Békollé and Bonami in [2, 3]. They showed that the following assertions are equivalent: (i): ,(ii): ,(iii),

where is the family of positive locally integrable functions for which the following inequality holds: Here is denoted to be the Carleson cube associated with the interval (see (18)) and . Later on, for , Pott and Reguera [4] demonstrated the sharp Békollé estimates for the maximal Bergman projection as follows:where the constant depends only on , and the estimate is sharp in the sense that the power of is optimal. Since then, several nice works have been done in the direction of the sharp estimate of Bergman type operators, such as [5–7].

It is easy to see that the maximal Bergman operator coincides with the following operators in the case of : The operator and the following operator appear naturally in the problem of off-diagonal weighted inequalities for the Bergman operator [8].Obviously, is pointwise dominated by .

In order to state some known results, we first introduce the weights class . Let be a weight function and let ; we say if the following quantity is finite: The weights class is an extension and an analogue of the weight class introduced by Muckenhoupt and Wheeden in [9]. Recently, Sehba [8] showed that is bounded from into if and only if for , , or . Moreover, it holds thatand the power of in the above inequality is sharp. In the case , one may note that formula (7) coincides with formula (3). We summarize the results in [8, 10] for the estimates of and as follows.

Theorem A (see [8]). *Let , , , , and . Then is bounded from into if and only if . Moreover, and the power of is sharp.*

Theorem B (see [10]). *Let , , , and . If then where the exponent of is sharp.*

In order to state our results, we first introduce some definitions.

*Definition 1 (multilinear weights ). *Let and . Suppose that each () is a positive locally integrable function defined on . Given , , and . A multiple weight associated with is said to satisfy the condition if the following inequality holds:

*Definition 2 (multilinear fractional Bergman operator). *For , , and , the multilinear positive fractional Bergman operator is defined by

*Definition 3 (multilinear fractional maximal function). *For , , , and , the multilinear fractional maximal function is defined by Note that if , then denote ; it is just the multilinear version of Bergman operators. This paper will be devoted to investigate the sharp weighted bounds for and . We summarize our results as follows.

Theorem 4. *Let , , , and . Suppose that , , and Then, the following inequality holds: and the exponent of is sharp.*

Theorem 5. *Let , , , and . Suppose that , for some and . If , then it holds that and the exponent of is sharp.*

Although the estimate in Theorem 5 is sharp, it can be improved whenever mixed estimates are invoked.

Theorem 6. *Let , , , , and . If is an tuple of weights satisfying for all and , then *

As a consequence of Theorem 6, if and , then . The following Corollary 7 follows immediately from Theorem 6.

Corollary 7. *Let , , , , and . Suppose that and . Then we have the following: and the exponents are sharp.*

Throughout the paper, the symbol means that , where is used to denote a constant which is independent of the main parameters.

#### 2. Notions and Preliminaries

We begin this section with some basic concepts and notions that will be used in our paper. The space we will focus on is the upper half of complex plane, . For any subset of , we denote . For an interval , the Carleson cube associated with will be denote by , and the upper half associated with will be denoted by ; namely,In order to establish the sparse domination for the Bergman type operators, let us first introduce the following system of dyadic grids [11], for . We need the following well-known lemma given by Pott and Reguera [4].

Lemma 8 (see [4]). *Given an interval , there is a dyadic interval , for some , such that and .*

For any locally integrable function and , it follows that where is defined in the same way as but the supremum is taken only over dyadic intervals of the dyadic grid .

We say that is an -sparse family, if for any , there exists , such that and the sets are pointwise disjoint. We need one more definition.

*Definition 9. *Let be one of the dyadic grids described above, and a sparse family in , we define the dyadic fractional Bergman operators byThen we have the following sparse domination for , which will be used in proving Theorems 4 and 5.

Proposition 10. *Let , ; then, it holds that*

*Proof. *Let us fix , then there exists such that It is easy to see that there exists an interval such that ; by Lemma 8, we have and . Now it follows that Applying the same decomposition to the case yields that

#### 3. Lemmas and the Proof of Theorems

We begin by preparing some lemmas.

Lemma 11 (see [10]). *Let , , , and be a weight. Assume that is such that . Then there exists a constant such that *

Lemma 12. *Let , , , and . Suppose that , , and ; then we have*

*Proof. *Let ; we have Given , we recall that it is upper-half is the set . It is clear that if is a dyadic grid in , then the family forms a tiling of . As , and , i.e., , .

We remark that Then, it follows thatSince , we deduce that Hence, we have Combining the former estimate, we get the desired result,

Lemma 13. *Let , , , and . Suppose that , for some , and . Furthermore, assume that is a sparse family. Then we have *

*Proof. *Let ; one may obtain that By (31) and the fact , we have Let , by the Hölder inequality, we obtain

We are now ready to prove Theorem 4.

*Proof of Theorem 4. *We know that when , we can use the duality. However, when , we cannot use the method of the duality. Thus, we need to consider these two cases separately, and here we will borrow some ideas from [12, 13].* (i) The Case *. By (22), it is sufficient to show that If , then using Lemma 12, we may get the desired result. If , we assume that . By duality, we have *(ii) The Case *. In this case, we first notice that By (23), it is enough to show that By Lemma 13, it yields that

*Proof of Theorem 5. *Let and . Then we have , where and is a family of dyadic cube which is maximal with respect to the inclusion and enjoys the properties that Define ; then it is easy to see that the sets are disjoint. By the Hölder inequality, it follows that Hence, we obtain