Abstract and Applied Analysis

Volume 2014 (2014), Article ID 265378, 13 pages

http://dx.doi.org/10.1155/2014/265378

## Some Function Spaces via Orthonormal Bases on Spaces of Homogeneous Type

^{1}Department of Mathematics, Sichuan University, Chengdu 610064, China^{2}Department of Mathematics, Macquarie University, NSW 2109, Australia^{3}Department of Mathematics, China University of Mining & Technology Beijing, Beijing 100083, China

Received 20 June 2014; Accepted 26 August 2014; Published 16 December 2014

Academic Editor: Daoyi Xu

Copyright © 2014 Chuang Chen 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

Let be a space of homogeneous type in the sense of Coifman and Weiss, where the quasi-metric *d* may have no regularity and the measure *μ* satisfies only the doubling property. Adapting the recently developed randomized dyadic structures of *X* and applying orthonormal bases of constructed recently by Auscher and Hytönen, we develop the Besov and Triebel-Lizorkin spaces on such a general setting. In this paper, we establish the wavelet characterizations and provide the dualities for these spaces. The results in this paper extend earlier related results with additional assumptions on the quasi-metric *d* and the measure *μ* to the full generality of the theory of these function spaces.

#### 1. Introduction

The aim of this paper is to introduce the Besov and Triebel-Lizorkin spaces on spaces of homogeneous type in the sense of Coifman and Weiss without any additional assumption on the quasi-metric and the doubling measure. The main tool used in this paper is the remarkable orthonormal basis constructed recently by Auscher and Hytönen [1]. It is well known that function spaces play an important role in both classical and modern analysis, ordinary and partial differential equations, and approximation theory, and so forth. Since the seventies of last century, classical theory of the Besov spaces [2] as well as the Triebel-Lizorkin spaces [3, 4] has been developed rapidly from Euclidean spaces to spaces of homogeneous type in the sense of Coifman and Weiss with some additional assumptions on the quasi-metric and the doubling measure. See, for example, [5, 6] and references therein.

Let us recall briefly spaces of homogeneous type introduced by Coifman and Weiss [7]. A function is called a quasi-metric on a set if satisfies the following: (1) for all ; (2) if and only if ; and (3) the quasi-triangle inequality holds: there is a constant such that
for all , and . In addition, the quasi-metric ball with center and radius is defined by . We say that a nonzero measure satisfies the doubling condition if there is a constant such that, for all and all ,
We point out that the doubling condition (2) implies that there exist positive constants (the* upper dimension* of ) and such that, for all and ,

A space is said to be the space of homogeneous type in the sense of Coifman and Weiss [7] if is a quasi-metric on and satisfies the doubling condition. Such spaces have many applications in the theory of singular integrals and function spaces [8]. Unfortunately, for some applications, additional assumptions were imposed on these general spaces due to the facts that the original quasi-metric may have no (Hölder) regularity and quasi-metric balls, even Borel sets, may not be open.

A recent work on the Besov spaces and the Triebel-Lizorkin spaces on spaces of homogeneous type in the sense of Coifman and Weiss was developed by Han et al. [6]. More precisely, in [6], in order to apply Coifman’s construction for the approximation to the identity, they need that the quasi-metric satisfies the Hölder regularity and the doubling measure is required to satisfy the reversed doubling condition; that is, there are constants and such that for all , , and . This assumption ensures that which is the key to develop the theory of the Besov and Triebel-Lizorkin spaces on spaces of homogeneous type. The main tool used in [6] is the so-called frame developed by Han [9] (see also [10]) because the Fourier transform is missing in general spaces of homogeneous type and wavelets were not available at that time. However, things seem to be changed after the quite recent work of Auscher and Hytönen [1], where remarkable orthonormal bases (wavelets) were constructed by using spline functions with the original quasi-metric and the doubling measure. These orthonormal bases open the door for developing function spaces on these general settings. See the very recent work in [11] for the theory of product , , and duality on such general spaces. Motivated by the work in [11], in this paper, we develop the theory of the Besov and Triebel-Lizorkin spaces on spaces of homogeneous type in the sense of Coifman and Weiss without any additional assumption on the quasi-metric and the doubling measure. Therefore, results in this paper extend earlier related results with additional assumptions on the quasi-metric and the measure to the full generality of the theory of the Besov and Triebel-Lizorkin spaces.

This paper is organized as follows. In Section 2 we give some notations and preliminaries, including a useful almost orthogonality estimate (see Lemma 7). Wavelet characterizations and dualities of the inhomogeneous Besov and Triebel-Lizorkin spaces are given in Section 3. We introduce homogeneous Besov and Triebel-Lizorkin spaces and discuss relationships between homogeneous and inhomogeneous Besov and Triebel-Lizorkin spaces in Section 4.

#### 2. Preliminaries and Notations

Throughout this paper, denotes a space of homogeneous type with quasi-triangle constant and . By we denote the measure of , the ball centered at with radius , and by we denote the measure of , the ball centered at with radius . In addition, we use the notation to mean that there is a constant such that and the notation to mean that . The implicit constants, , are meant to be independent of other relevant quantities.

We first recall the orthonormal basis of constructed by Auscher and Hytönen [1]. Let be spline functions constructed in [1, Section 3] and let be the closed linear span in of . By [1, Theorem 5.1] we observe that Further on, by writing as the orthogonal (in ) complement of in , Auscher and Hytönen pointed out the following result.

Theorem 1 (Theorems 6.1 and 7.1, [1]). *Let . There exists an orthonormal basis of satisfying the following exponential decay:
**
and the Hölder regularity of order :
**
whenever for some . Here is a fixed parameter small enough, say , and , are two positive constants independent of , , , and . Clearly, is an orthonormal basis of .**Also, there exists an orthonormal basis of satisfying the following exponential decay:
**
the Hölder regularity of order :
**
whenever for some , and the cancellation property:
**
where is given above and , are two positive constants independent of , , , and . Clearly, is an orthonormal basis of . In what follows, we also refer to the functions as wavelets.*

In order to introduce the (inhomogeneous) Besov and Triebel-Lizorkin spaces, we need the following concepts of test functions and distributions. Refer to [12] for details and also see [11].

*Definition 2. *For fixed , , , where is given in Theorem 1, and . A function is said to be a test function of type centered at with width if satisfies the following decay and Hölder regularity properties:(i) for all ,(ii) for all , such that .

If is a test function of type , we write . The norm of on is defined by

We denote . It is easy to check that with equivalent norms for any fixed and . Furthermore, it is also easy to see that is a Banach space with respect to the norm on . For given , let be the completion of the space in with , . Obviously, . In addition, we say that if and there is a sequence such that as . For given , we define . Obviously, is a Banach space with respect to the norm . As usual, we denote by the dual space of consisting of all continuous linear functionals on .

Han et al. [11] showed that the wavelets constructed by Auscher and Hytönen [1] are test functions defined above.

Lemma 3 (see [11], Theorem 3.3). *Let be given in Theorem 1 with the Hölder regularity of . Then for each .*

By analogous arguments, we can obtain the next result.

Proposition 4. *Let be given in Theorem 1 with Hölder regularity of . Then for each .*

Let and be orthogonal projections onto and , respectively. We next give some estimates on the kernels of operators and .

Lemma 5. *The kernel is symmetric in and , and, for any , satisfies that*(i)*for all , ,
*(ii)*for ,
*(iii)*for and ,
*

*Proof. *Note that, for any , and with , from the doubling property on the measure it follows that
Also, note that the kernel is symmetric in and due to [1, Lemma 10.1]. Thus, it suffices to prove (13) for the cases that and . Again, by [1, Lemma 10.1] we have
for all , where the constant is independent of . If , then and hence
On the other hand, if , then (17) implies that
By letting , we obtain (13), immediately.

The proof of (14) is based on the estimates
given in [1, Lemma 10.1]. Indeed, since implies that and that , it follows from (17) and (21) that
Applying yields (14), immediately. The proof of (15) is similar to that of (13) and (14), and we omit it here.

*Lemma 6 (see [11], Lemma 3.6). The kernel is symmetric in and and, for any , satisfies that(i)for all , ,
(ii)for ,
(iii)for and *

*Let be another orthonormal basis of given by the algorithm in Theorem 1, and let be the projection onto with respect to the basis . Finally, we give the almost orthogonality estimate of the kernel of operator to end this section. The proof is similar to that in [6, Lemma 3.1], and we omit it here.*

*Lemma 7. Let , and let be the order of the Hölder regularity given in (8). There exists a constant , dependent only on , such that, for all ,
(ii) for all ,
whenever . The same estimate holds with and interchanged.*

*3. Inhomogeneous Besov and Triebel-Lizorkin Spaces on Spaces of Homogeneous Type*

*3. Inhomogeneous Besov and Triebel-Lizorkin Spaces on Spaces of Homogeneous Type*

*In this section, we introduce the inhomogeneous Besov and Triebel-Lizorkin spaces on spaces of homogeneous type and give wavelet characterizations of corresponding spaces.*

*We would like to point out that the wavelet expansion (Theorem 9) is the key to develop the theory of the Besov and Triebel-Lizorkin spaces. As defined previously, by and we denote the orthogonal projections onto and , respectively. These two projections provide us with regular Littlewood-Paley decompositions for spaces of homogeneous type.*

*Theorem 8 (Theorem 10.2, [1]). Let . Then
which gives homogeneous Littlewood-Paley decomposition, while
provides inhomogeneous Littlewood-Paley decomposition.*

*Furthermore, following the proof of Theorem 3.4 in [11], we can show that such wavelet expansions also hold in the space of distributions.*

*Theorem 9. Let , . Then wavelet expansion (29) holds in with and and hence holds in with and as well.*

*Proof. *Since and (29) holds in the sense of , (29) holds for such test function in the (a.e.) pointwise sense (for some subsequence with respect to the convergence).

By analogous arguments given in [11, Theorem 3.4], we can show the convergence of and in by using the size and smooth conditions of kernels and , respectively. Therefore,
in the sense of . The second equality of (29) holds for the same reason.

*We now introduce the inhomogeneous Besov and Triebel-Lizorkin spaces on spaces of homogeneous type.*

*Definition 10. *Let , and . The inhomogeneous Besov space is defined by
where

For and , the inhomogeneous Triebel-Lizorkin space is defined by
where

*Remark 11. *Observe that
as well as

*The next result shows that the Besov and Triebel-Lizorkin norms given in Definition 10 are both independent of choice of the orthonormal basis of and hence, they are independent of the choice of operators and with . Thus, the Besov and Triebel-Lizorkin spaces given above are both well defined. To see this, let be another orthonormal basis of given in Theorem 1, and let and be projections onto and , respectively, with respect to the basis .*

*Proposition 12. Let , and let . For all , , then
and for all and *

*Proof. *Clearly, the equivalence of and is a direct consequence of the size condition (13) and the wavelet expansion (29). We next show that
Indeed, by the wavelet expansion (29) and almost orthogonal estimate (26), we have
where denotes the Hardy-Littlewood maximal operator. Thus,
due to the fact that . This proves (39), and hence, we obtain (37) symmetrically.

We now prove (38). Clearly, by symmetry, it suffices to show that
To this end, from (40) it follows that
due to the fact that and for . Thus, applying the Fefferman-Stein vector-valued maximal inequality, we obtain (42), immediately. The proof is complete.

*Let be an orthonormal basis of and let be an orthonormal basis of as shown in Theorem 1. Applying the wavelet expansion, given in Theorem 9,
we now give the wavelet characterizations of the inhomogeneous Besov and Triebel-Lizorkin spaces.*

*Theorem 13. Let and let , be that given in Theorem 1 with . For , , then
and for , , then*

*Proof. *Observe that, by analogous arguments in the proof of (26), we have
In order to prove (45), by Remark 11, it suffices to show that
By the wavelet expansion (44), we have

so that
By analogous arguments, we also have
and hence,
This implies that

On the other hand, by using the wavelet expansion and the size condition, we have
as well asand hence, by using the Hölder inequality, we have further that
This completes the proof of (48).

Analogously, we can also obtain (46) by using the Fefferman-Stein vector-valued maximal inequality.

*As an application of wavelet characterizations, we now turn to discuss the dual spaces of the inhomogeneous Besov and Triebel-Lizorkin spaces. For convenience, we rewrite (44) as
where , , , and for . The following concepts of spaces of sequences were first introduced by Frazier and Jawerth [13].*

*Definition 14. *Let , and let and , . The space of sequences is defined by with
and, for , the space is defined by with

*Remark 15. *It is easy to see that and due to (57), (45), and (46).

*The next result shows that is dense in as well as .*

*Proposition 16. Let , , and . Then is dense in both for , and for and .*

*Proof. *Let . Since , to show the conclusion, it suffices to show that is dense in and . Let . For any fixed , write
By the estimates (26) and (27), we can show that and . Further on, we can also show that
This is a direct consequence of the wavelet expansion (29) and estimate (26). Indeed,
where is defined as the same as for . By wavelet expansion (29), we then have
This implies that in as , and hence, is dense in . By analogous arguments, we can also show that is dense in .

*We are ready to give the dualities of the Besov and Triebel-Lizorkin spaces.*

*Theorem 17. Let , and , and let . Then . More precisely, given , then defines a linear functional on with such that
and this linear functional can be extended to with norm at most .*

Conversely, if is a linear functional on , then there exists a unique such that defines a linear functional on with , and is the extension of with .

*Remark 18. *Theorem 17 also holds with and replaced by and , respectively.

*Proof of Theorem 17. *(i)Suppose that and . By Remark 15, we observe that
is an element of , and hence, . Also,
is an element of , and hence, . Write now
By the Hölder inequality and Minkowski inequality, we have
and hence, is a linear functional defined on . Clearly, has bounded extension to due to the density of in . This proves that .(ii)Suppose that . We need to prove that there exists a such that
Note that
and define a linear functional on by
From (71) it follows that
By the Hahn-Banach theorem, can be extended to as a continuous linear functional. In addition, by arguments analogous to that in ([13, Remark 5.11]), we can show that , and hence, there exists a unique sequence such that and
Thus, for each , we have
Let . By the orthogonality of , we have