Research Article | Open Access

# A Class of Unbounded Fourier Multipliers on the Unit Complex Ball

**Academic Editor:**Anna Mercaldo

#### Abstract

We introduce a class of Fourier multiplier operators on *n*-complex unit sphere, where the symbol . We obtained the Sobolev boundedness of . Our result implies that the operators take a role of fractional differential operators on .

#### 1. Introduction

In this paper, we introduce a class of unbounded holomorphic Fourier multipliers on complex unit sphere. We further study the boundedness of on Sobolev spaces. Our results generalize the theory of Fourier multipliers on Lipschitz curves in to complex unit sphere . We refer the reader to Gaudry et al. [1], McIntosh and Qian [2], and Qian [3, 4] for further information on multipliers on Lipschitz curves.

Our motivation originates from the following example on the unit sphere in . The explicit formula of the Cauchy-Szegö kernel is as follows: Let denote the orthonormal system in the space of holomorphic functions in . The following result is well-known: See Theorem 1 and (16) below for details. Formally, (2) can be seen as the special case of (4) below. Let be the sector defined as Assume that (1)is holomorphic on ;(2) is bounded near the origin;(3) for .

We consider the function: If , then (4) becomes (2). For , Cowling and Qian [5] introduced a class of bounded holomorphic multipliers on . In this paper, we consider the case . For this case, is unbounded on . We prove that if , then See Theorem 5.

In Section 4, we introduce a class of Fourier multipliers with . Unlike the ones of Cowling and Qian [5], our multipliers are unbounded on . Take . Plancherel's theorem implies that is not bounded on . Hence for such , we need to consider their boundedness on some function spaces with higher regularity. Let . We prove that if , is bounded from Sobolev space to Sobolev space , . Our result implies that the operators take a role of fractional differential operators on . See Theorem 11.

The rest of this paper is organized as follows. In Section 2, we state some basic preliminaries and notations which will be used in the sequel. In Section 3, we estimate the kernels generated by holomorphic multipliers . The Sobolev boundedness of the operators is given in Section 4.

*Notations*. represents that there is a constant such that whose right inequality is also written as . Similarly, one writes for .

#### 2. Preliminaries and Notations

In this section, we state some preliminaries and notations and refer the reader to Gong [6], Hua [7], and Rudin [8] for further information. We use as a general element of ; that is, , , , . Denote . The notation is considered to be a row vector. Denote by the open unit ball , where . The unit sphere in is denoted by The open ball centered at with radius will be denoted by . A general element on is usually denoted by . The constant involved in the Cauchy-Szegö kernel is the surface area of and is equal to . For , we use the notation . The theory developed in this paper is relevant to the radial Dirac operator

Now we state some basis knowledge of basic functions in the space of holomorphic function in and some relevant function spaces on . We refer to Hua [7] for details. Let be a nonnegative integer. We consider the column vector with components The dimension of is

Let and be the Lebesgue volume element of and the Lebesgue area element of , respectively. Define It is easy to prove that and are positive definite Hermitian matrices of order . There exists a matrix such that where is a diagonal matrix and is the identity matrix. Set Denote by the components of the vectors . From (11), we can see that The following theorem is well known.

Theorem 1. *The system of functions
**
is a complete orthonormal system in the space of holomorphic functions in . The system is orthonormal, but it is not complete in the space of continuous functions on .*

The explicit formula of the Cauchy-Szegö kernel on was first deduced in Hua [7] by using the system and the relation For , the nonisotropic distance is defined as It can be easily shown that is a metric on . For and , we define the ball corresponding to as The complement set of in is denoted by .

Set If , then where are the Fourier coefficients of : and for any positive integer , the series is uniformaly and absolutely convergent in any compact ball contained in in which is defined.

Denote by the unitary group of consisting of all unitary operators on the Hilbert space under the complex inner product . These are the linear operators that preserve inner products: Clearly, is a compact subset of . It is easy to verify that is invariant under . If , then is defined by its values on . In Section 3, we treat as identical to .

#### 3. The Kernel Generated by Holomorphic Multipliers

Set

The following function space is relevant.

*Definition 2. *Let . is defined as the set of all holomorphic functions in such that (a) is bounded for ;(b), , .

*Remark 3. *The classes are generalizations of which is introduced by McIntosh and his collaborators. We refer to Li et al. [9], McIntosh [10], McIntosh and Qian [2], Qian [11], and the reference therein for further information on .

Let

Lemma 4. *Let , . Then can be holomorphically extended to . Moreover, for and ,
**
where ; are the constants in Definition 2.*

*Proof. *Let
and is the ray , , where is chosen so that . Define
where is exponentially decaying as along . Then we get
which implies . Define
It is easy to see that is holomorphically and -periodically defined in the described region, and . Let
For , we write , where . Then . This implies that and . Therefore, (29) gives

Take the ball
Applying Cauchy's integral formula, we obtain
For any , we have . Then we have

Theorem 5. *Let and
**
Then
**
is holomorphically defined for , such that , where is the function defined in Lemma 4, Moreover, for and ,
**
where , are the constant in the definition of the function space .*

*Proof. *Recall that
Then we have
Therefore,

By [4, Theorem 3], we could obtain the following result.

Theorem 6. *Let be a negative integer. If ,
**
then
*

*Proof. *The proof is similar to Theorem 5. we omit it.

#### 4. Sobolev Spaces and Unbounded Fourier Multipliers

##### 4.1. Integral Representation of Multipliers

Given , we define the Fourier multiplier operator by where are the Fourier coefficients of the test function .

For the above operator , the Plemelj type formula holds.

Theorem 7. *Let . Take , where . Operator has a singular integral expression. For ,
**
where is a bounded function of and .*

*Proof. *Let
where
We can see that
which implies that . Then, we have
By integration by parts,
For any , we have
where

For , we have
Now we consider . Let . For , write
For such , . Without loss of generality, assume . We get
which implies that
The above estimate implies
Since
we obtain and then
Denote
Since and , we get . It is easy to see
that is, . Because
then we have , so
Since , then
that is
Since , we have
For ,
For ,
Then we get
For , we have
Then, we get
Now we prove if , has a limit uniformly bounded for near . Integrating as before, we have
Let . Then . We get
By integration by parts, the inside integral with respect to the variable becomes
We first estimate ,

Because ,
Since , we have
So
For , when , we obtain
On the other hand,
that implies

##### 4.2. Sobolev Spaces on via Fourier Multipliers

Sobolev spaces on the -complex unit sphere are defined as follows. We define the fractional integral operator on as follows. Let For , the operator is defined by

For , we can see that the operators become the ordinary differential operators with higher orders.

Theorem 8. *Let . on .*

*Proof. *Without loss of generality, we assume that . Then
where are the Fourier coefficients of :
So

*Definition 9. *Let . The Sobolev norm on is defined as
The Sobolev spaces on are defined as the closure of under the norm , that is, .

*Remark 10. *By the Plancherel theorem, if and only if

Now we consider the Sobolev boundedness of .

Theorem 11. *Given and . The Fourier multiplier operator is bounded from to .*

*Proof. *Write
By the orthogonality of , we can see that . Let . Because , we can see that . This implies that
Finally, by [5, Theorem 3], we can see that
This completes the proof of Theorem 11.

#### Conflict of Interests

The authors declare that they have no conflict of interests in this submitted paper.

#### Acknowledgments

This project is supported by NSFC Grant no. 11171203; New Teacher’s Fund for Doctor Stations, Ministry of Education, Grant no. 20114402120003; Guangdong Natural Science Foundation S2011040004131; Foundation for Distinguished Young Talents in Higher Education of Guangdong, China, LYM11063. Tao Qian is supported by MYRG116(Y1-L3)-FST13-QT; MYRG115(Y1-L4)-FST13-QT; FDCT 098/2012/A3.

#### References

- G. Gaudry, T. Qian, and S. Wang, “Boundedness of singular integral operators with holomorphic kernels on star-shaped closed Lipschitz curves,”
*Colloquium Mathematicum*, vol. 70, no. 1, pp. 133–150, 1996. View at: Google Scholar | Zentralblatt MATH | MathSciNet - A. McIntosh and T. Qian, “Convolution singular integral operators on Lipschitz curves,” in
*Harmonic Analysis*, vol. 1494 of*Lecture Notes in Mathematics*, pp. 142–162, Springer, Berlin, Germany, 1991. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - T. Qian, “Singular integrals with holomorphic kernels and ${H}_{\infty}$-Fourier multipliers on star-shaped closed Lipschitz curves,”
*Studia Mathematica*, vol. 123, no. 3, pp. 195–216, 1997. View at: Google Scholar | Zentralblatt MATH | MathSciNet - T. Qian, “A holomorphic extension result,”
*Complex Variables. Theory and Application*, vol. 32, no. 1, pp. 59–77, 1997. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - M. Cowling and T. Qian, “A class of singular integrals on the $n$-complex unit sphere,”
*Science in China. Series A*, vol. 42, no. 12, pp. 1233–1245, 1999. View at: Publisher Site | Google Scholar | MathSciNet - S. Gong,
*Integrals of Cauchy Type on the Ball*, vol. 1 of*Monographs in Analysis*, International Press, Cambridge, Mass, USA, 1993. View at: MathSciNet - L. K. Hua,
*Harmonic Analysis of Functions of Several Complex Variables in the Classical Domains*, American Mathematical Society, Providence, RI, USA, 1963. View at: MathSciNet - W. Rudin,
*Function theory in the unit ball of*, Grundlehren der Mathematischen Wissenschaften, Springer, Berlin, Germany, 2008. View at: MathSciNet*ℂ*^{n} - C. Li, A. McIntosh, and S. Semmes, “Convolution singular integrals on Lipschitz surfaces,”
*Journal of the American Mathematical Society*, vol. 5, no. 3, pp. 455–481, 1992. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - A. McIntosh, “Operators which have an ${H}_{\infty}$ functional calculus,” in
*Proceedings of the Mini Conference on Operator Theory and Partial Differential Equations, Center for Mathematical Analysis*, vol. 14, ANU, Canberra, Australia. View at: Google Scholar - T. Qian, “Fourier analysis on starlike Lipschitz surfaces,”
*Journal of Functional Analysis*, vol. 183, no. 2, pp. 370–412, 2001. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet

#### Copyright

Copyright © 2014 Pengtao Li 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.