Research Article | Open Access

Feng Liu, Huoxiong Wu, Daiqing Zhang, "On the Multilinear Singular Integrals and Commutators in the Weighted Amalgam Spaces", *Journal of Function Spaces*, vol. 2014, Article ID 686017, 12 pages, 2014. https://doi.org/10.1155/2014/686017

# On the Multilinear Singular Integrals and Commutators in the Weighted Amalgam Spaces

**Academic Editor:**Yoshihiro Sawano

#### Abstract

This paper is concerned with the norm estimates for the multilinear singular integral operators and their commutators formed by BMO functions on the weighted amalgam spaces . Some criterions of boundedness for such operators in are given. As applications, the norm inequalities for the multilinear Calderón-Zygmund operators and multilinear singular integrals with nonsmooth kernels as well as the corresponding commutators on are obtained.

#### 1. Introduction

Let () be the -dimensional Euclidean space equipped with the Euclidean norm and the Lebesgue measure . For , ; the amalgam spaces of and are denoted by the set of all measurable functions , which are locally in and satisfy where for and . We remark that the amalgam spaces were introduced by Fofana in [1] in connection with the study of the continuity of the fractional maximal operator of Hardy-Littlewood and of the Fourier transformation in . In [1], Fofana also considered the subspace of , which consists of measurable functions such that for , and a suitable modification version for or .

By the definitions, it is clear (also see [1]) that , , where , with and , is the classical Morrey space that consists of measurable functions such that

In this paper, we focus on the weighted version of . Precisely, letting be a weight on and , we define the weighted amalgam spaces as the space of all measurable functions satisfying and a suitable modification version for or , where is the weighted Lebesgue space.

It is easy to check that when and , the space is nothing but the weighted Morrey space , which is the set of all measurable functions such that (see [2]) As is well known, the boundedness of the classical operators in the harmonic analysis on the weighted Morrey spaces has extensively been studied (see [2–6] and references therein). In particular, Wang and Yi [6] recently showed that the -linear commutators and the iterated commutators of the -linear Calderón-Zygmund operators are bounded on weighted Morrey spaces.

Based on the above, we feel that it is natural and interesting to study the boundedness of the classical operators in harmonic analysis on the amalgam spaces and the weighted versions. Indeed, a lot of attention has recently been given to this topic (e.g., see [7–10]). Here, we will continue the investigation along this line. The main purpose of this paper is to study the boundedness of the multilinear operators on the weighted amalgam spaces .

Let be a locally integral function defined off the diagonal in and let be an -linear operator associated with the kernel in the following way: where , in with .

For , we define the -linear commutator of denoted by as follows: where each term is the commutator of and in the th entry of ; that is and , where is a smooth function with compact support on . The iterated commutator is defined by

If is associated with a distribution kernel, which coincides with the above function , then we have, at a formal level,

Also, we recall the definitions of the classical Muckenhoupt classes weights and the multilinear conditions for multiple weights.

*Definition 1. *A weighted on , that is, a positive locally integrable function on , belongs to for if there exists a constant such that
The infimum of these constants is called the constant of and denoted by . A weight belongs to the class if there exists a constant such that
and the infimum of these constants is called the constant of and is denoted by .

*Definition 2. *Let with and , , and . Let and . Set
We say that satisfies the condition if
where for .

Obviously, for , is the classical Muckenhoupt classes condition. It is not difficult to check that for (see [11]),

which implies that something more general happens for the classes. Also, the authors in [11] showed that the conditions are the largest classes of weights because all -linear Calderón-Zygmund operators are bounded on the weighted Lebesgue spaces.

To state our main results, we still need to recall and introduce some notations. For fixed and , we set . For any , let and be the characteristic function of the set . Given any positive integer and , we denote by the family of all finite subset of of different elements. For any , we also denote the complementary sequence of by given by . We remark that if and only if . Letting for a fixed and , we set and if and if . Now we can formulate our main results as follows.

Theorem 3. *Let with and be an -linear operator. Let satisfy , , , and . Assume that for with and . If maps to , then the inequality
**
holds provided that for any ball in , any and , there exist constants and such that for a.e. ,
*

Theorem 4. *Let with , , satisfy , , , and . Assume that for with , , and . If
**
then the inequality
**
holds provided that for any ball in , any , , , and , there exist constants and such that for a.e. ,
*

Theorem 5. *Let with , , satisfy , , , and . Suppose that for with , , and . If
**
then the inequality
**
holds provided that for any ball in , any and , there exist constants and such that for a.e. ,
**
where for any , and .*

Theorem 6. *Let be an -linear operator with kernel satisfying
**
Let satisfy , , , and . Assume that , , and . Then these inequalities (17), (20)-(21), and (24) hold.*

*Remark 7. *We remark that for , Theorems 3–6 are also true, just with the restrictive condition: . Moreover, for , that is, , we can remove the restrictive condition in Theorems 3–6. See also [12, Theorem 3.5] for the unweighted case.

The rest of this paper is organized as follows. In Section 2, we will give the proofs of our main results. Some applications will be given in Section 3. Throughout this paper, the letter , sometimes with additional parameters, will stand for positive constants, not necessarily the same one at each occurrence but independent of the essential variables. In what follows, we use the convention and .

#### 2. The Proofs of Main Results

Let us begin with a lemma, which will be used in the proofs of our main results.

Lemma 8 (cf. [6, Lemma 3.1]). *Let , and with . Assume that and , then for any ball , there exists a constant such that
*

*Proof of Theorem 3. *For fixed , we can write
The boundedness of from to , (17) and Hölder’s inequality lead to
Note that for any and , there exists a constant that depends only on , , such that
Hence, multiplying both sides of (28) by , note that and ; by Lemma 8 and (29) we obtain
which combined with the fact that for all leads to
Theorem 3 is proved.

*Proof of Theorem 4. *For fixed , by linearity we can write
Invoking (18), (20)-(21) and Hölder’s inequality, we have
By a similar argument as in getting (31), we can conclude that
which completes the proof of Theorem 4.

*Proof of Theorem 5. *For fixed , we can write
Applying (22), (24) and Hölder’s inequality, we get that
By similar arguments as in getting (31) again, we can deduce that
This completes the proof of Theorem 5.

*Proof of Theorem 6. *For fixed , it is easy to check that
Since , we have for any and ,
By Hölder’s inequality, (25) and (38), writing , we have
It follows from (39)–(42) that
This implies (17) in the case of that and or and .

For , and , we have from (25) and (38) that
Since , thus with . By the properties of functions in and Hölder’s inequality, we have for any ball and ,
It follows from (39) and (45) that for any and ,
Let for some . We now consider two cases:*Case 1* (). We have