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Journal of Function Spaces and Applications

Volumeย 2012ย (2012), Article IDย 523586, 41 pages

http://dx.doi.org/10.1155/2012/523586

## Homogeneous Besov Spaces on Stratified Lie Groups and Their Wavelet Characterization

^{1}Lehrstuhl A fรผr Mathematik, RWTH Aachen University, D-52056 Aachen, Germany^{2}Department of Mathematics and Computer Sciences, City University of New York (CUNY), Queensborough College, 222-05 56th Avenue Bayside, NY 11364, USA

Received 17 January 2012; Accepted 29 January 2012

Academic Editor: Hans G.ย Feichtinger

Copyright ยฉ 2012 Hartmut Fรผhr and Azita Mayeli. 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 establish wavelet characterizations of homogeneous Besov spaces on stratified Lie groups, both in terms of continuous and discrete wavelet systems. We first introduce a notion of homogeneous Besov space in terms of a Littlewood-Paley-type decomposition, in analogy to the well-known characterization of the Euclidean case. Such decompositions can be defined via the spectral measure of a suitably chosen sub-Laplacian. We prove that the scale of Besov spaces is independent of the precise choice of Littlewood-Paley decomposition. In particular, different sub-Laplacians yield the same Besov spaces. We then turn to wavelet characterizations, first via continuous wavelet transforms (which can be viewed as continuous-scale Littlewood-Paley decompositions), then via discretely indexed systems. We prove the existence of wavelet frames and associated atomic decomposition formulas for all homogeneous Besov spaces with and .

#### 1. Introduction

To a large extent, the success of wavelets in applications can be attributed to the realization that wavelet bases are universal unconditional bases for a large class of smoothness spaces, including all homogeneous Besov spaces. Given a wavelet orthonormal basis (consisting of sufficiently regular wavelets with vanishing moments) and , the expansion converges not only in , but also in any other Besov space norm , as soon as is contained in that space. Furthermore, the latter condition can be read off the decay behaviour of the wavelet coefficients associated to in a straightforward manner.

This observation provided important background and heuristics for many wavelet-based methods in applications such as denoising and data compression, but it was also of considerable theoretical interest, for example, for the study of operators. In this paper we provide similar results for simply connected stratified Lie groups. To our knowledge, studies of Besov spaces in this context have been largely restricted to the inhomogeneous cases. The definition of inhomogeneous Besov spaces on stratified Lie groups was introduced independently by Saka [1], and in a somewhat more general setting by Pesenson [2, 3]. Since then, the study of Besov spaces on Lie groups remained restricted to the inhomogeneous cases [4โ8], with the notable exception of [9] which studied homogeneous Besov spaces on the Heisenberg group. A further highly influential source for the study of function spaces associated to the sub-Laplacian is Follandโs paper [10].

The first wavelet systems on stratified Lie groups (fulfilling certain technical assumptions) were constructed by Lemariรฉ [11], by suitably adapting concepts from spline theory. Lemariรฉ also indicated that the wavelet systems constructed by his approach were indeed unconditional bases of Sakaโs inhomogeneous Besov spaces. Note that an adaptation, say, of the arguments in [12] for a proof of such a characterization requires a sampling theory for bandlimited functions on stratified groups, which was established only a few years ago by Pesenson [13]; see also [14].

More recent constructions of both continuous and discrete wavelet systems were based on the spectral theory of the sub-Laplacian [15]. Given the central role of the sub-Laplacian both in [8, 15], and in view of Lemariรฉโs remarks, it seemed quite natural to expect a wavelet characterization of homogeneous Besov spaces, and it is the aim of this paper to work out the necessary details. New results in this direction were recently published in [16โ18].

The paper is structured as follows. After reviewing the basic notions concerning stratified Lie groups and their associated sub-Laplacians in Section 2, in Section 3 we introduce a Littlewood-Paley-type decomposition of functions and tempered discributions on . It is customary to employ the spectral calculus of a suitable sub-Laplacian for the definition of such decompositions, see, for example, [8], and this approach is also used here (Lemma 3.7). However, this raises the issue of consistency: the spaces should reflect properties of the group, not of the sub-Laplacian used for the construction of the decomposition. Using a somewhat more general notion than the -functions in [12] allows to establish that different choices of sub-Laplacian result in the same scale of Besov spaces (Theorem 3.11). In Section 4, we derive a characterization of Besov spaces in terms of continuous wavelet transform, with a wide variety of wavelets to choose from (Theorem 4.4). As a special case one obtains a characterization of homogeneous Besov spaces in terms of the heat semigroup. (See the remarks before Theorem 4.4.)

In Section 5, we study discrete characterizations of Besov spaces obtained by sampling the Calderรณn decomposition. For this purpose, we introduce the coefficient space . The chief result is Theorem 5.4, establishing that the wavelet coefficient sequence of lies in . Section 5 introduces our most important tool to bridge the gap between continuous and discrete decompositions, namely, oscillation estimates.

We then proceed to study wavelet synthesis and frame properties of the wavelet system. Our main result in this respect is that for all sufficiently dense regular sampling sets , the discrete wavelet system obtained by shifts from and dilations by powers of 2 is a *universal Banach frame* for all Besov spaces. In other words, the wavelet system allows the decomposition
converging unconditionally in whenever , with coefficients depending linearly and boundedly on , and satisfying the norm equivalence

#### 2. Preliminaries and Notation

Following the terminology in [19], we call a Lie group stratified if it is connected and simply connected, and its Lie algebra decomposes as a direct sum , with for and . Then is nilpotent of step and generated as a Lie algebra by . Euclidean spaces and the Heisenberg group are examples of stratified Lie groups.

If is stratified, its Lie algebra admits a canonical (natural) family of dilations, namely, which are Lie algebra automorphisms. We identify with through the exponential map. Hence is a Lie group with underlying manifold , for some , and the group product provided by the Campbell-Baker-Hausdorff formula. The dilations are then also group automorphisms of . Instead of writing for and , we simply use , whenever a confusion with the Lie group product is excluded. After choosing a basis of obtained as a union of bases of the , and a possible change of coordinates, one therefore has for and that for integers , according to .

Under our identification of with , polynomials on are polynomials on (with respect to any linear coordinate system on the latter). Polynomials on are written as
where are the coefficients with respect to a suitable basis , and the monomials associated to the multi-indices . For a multi-index , define
A polynomial of the type (2.3) is called *of homogeneous degree * if holds, for all multiindices with . We write for the space of polynomials of homogeneous degree .

We let denote the space of Schwartz functions on . By definition,. Let and denote the space of distributions and distributions modulo polynomials on , respectively. The duality between the spaces is denoted by the map . Most of the time, however, we will work with the sesquilinear version , for and .

Left Haar measure on is induced by Lebesgue measure on its Lie algebra, and it is also right-invariant. The number will be called the *homogeneous dimension* of . (For instance, for and we have and , respectively.) For any function on and , the -normalized dilation of is defined by
Observe that this action preserves the -norm, that is, . We fix a homogeneous quasi-norm on which is smooth away from 0 with, for all , , for all , with if , and fulfilling a triangle inequality , with constant . Confer [19] for the construction of homogeneous norms, as well as further properties.

Moreover, by [19, Propositionโโ1.15], for any , there is a finite such that for all .

Our conventions for left-invariant operators on are as follows. We let denote a basis of , obtained as a union of bases of the . In particular, , for , is a basis of . Elements of the Lie algebra are identified in the usual manner with left-invariant differential operators on . Given a multi-index , we write for . A convenient characterization of Schwartz functions in terms of left-invariant operators states that if and only if, for all , , where In addition, the norms induce the topology of (see [19]).

The sub-Laplacian operator on can be viewed as the analog of the Laplacian operator on defined by . Using the above conventions for the choice of basis and , the sub-Laplacian is defined as . Note that a less restrictive notion of sub-Laplacians can also be found in the literature (e.g., any sum of squares of Lie algebra generators); we stress that the results in this paper crucially rely on the definition presented here. A linear differential operator on is called homogenous of degree if for any on . By choice of the for , these operators are homogeneous of degree 1; it follows that is homogenous of degree 2, and is homogenous of degree . Furthermore, any operator of the form is homogeneous of degree .

When restricted to , is formally self-adjoint: for any , . (For more see [15].) Its closure has domain , where we take in the sense of distributions. From this fact it quickly follows that this closure is self-adjoint and is in fact the unique self-adjoint extension of ; we denote this extension also by the symbol .

Suppose that has spectral resolution where is the projection measure. For a bounded Borel function on , the operator is a bounded integral operator on with a convolution distribution kernel in denoted by , and An important fact to be used later on is that for rapidly decaying smooth functions, , the kernel associated to is a Schwartz function. For a function on we define and . For , the adjoint of the convolution operator is provided by .

#### 3. Homogeneous Besov Spaces on Stratified Lie Groups

In this section we define homogeneous Besov spaces on stratified Lie groups via Littlewood-Paley decompositions of distributions as

where is a dilated copy of a suitably chosen Schwartz function . In the Euclidean setting, it is customary to construct by picking a dyadic partition of unity on the Fourier transform side and applying Fourier inversion. The standard way of transferring this construction to stratified Lie groups consists in replacing the Fourier transform by the spectral decomposition of a sub-Laplacian , see Lemma 3.7. However, this approach raises the question to what extent the construction depends on the choice of . It turns out that the precise choice of sub-Laplacian obtained from a basis of is irrelevant. In order to prove this, we study Littlewood-Paley decompositions in somewhat different terms. The right setting for the study of such decompositions is the space of tempered distributions modulo polynomials, and the easiest approach to this convergence is via duality to a suitable space of Schwartz functions.

*Definition 3.1. *Let . A function has polynomial decay order if there exists a constant such that, for all ,
has vanishing moments of order , if one has
with absolute convergence of the integral.

Under our identification of with , the inversion map is identical to the additive inversion map. That is, , and it follows that for all . Thus, if has vanishing moments of order , then for all that is, has vanishing moments of order as well.

Vanishing moments are central to most estimates in wavelet analysis, by the following principle: in a convolution product of the type , vanishing moments of one factor together with smoothness of the other result in decay. Later on, we will apply the lemma to Schwartz functions , where only the vanishing moment assumptions are nontrivial. The more general version given here is included for reference.

Lemma 3.2. *Let be arbitrary.*

(a)*Let , such that is of decay order , for all with . Let have vanishing moments of order and decay order . Then there exists a constant, depending only on the decay of and , such that
In particular, if ,
*(b)*Now suppose that , with of decay order for all with . Let have vanishing moments of order and decay order . Then there exists a constant, depending only on the decay of and , such that
In particular, if ,
*

* Proof. *First, let us prove (a). Let . For , let denote the left Taylor polynomial of with homogeneous degree , see [19, Definitionโโ1.44]. By that result,
with suitable positive constants and . We next use the homogeneity properties of the partial derivatives [19, page 21], together with the decay condition on to estimate for with
where the penultimate inequality used [19, 1.10], and the final estimate used . Thus,
Next, using vanishing moments of ,
and the integral is finite by [19, 1.15]. This proves (3.5), and (3.6) follows by
using .

For part (b), we first observe that
Our assumptions on allow to invoke part (a) with replacing , and (3.7) follows immediately. (3.8) is obtained from this by straightforward integration.

We let denote the space of Schwartz functions with all moments vanishing. We next consider properties of as a subspace of with the relative topology.

Lemma 3.3. * is a closed subspace (in particular complete) of , with , as well as for all . The topological dual of , , can be canonically identified with the factor space .*

* Proof. *By definition, is the intersection of kernels of a family of tempered distributions, hence a closed subspace. For and , one has by unimodularity of that , since is a polynomial. But then, for any and , one has for all polynomials on that
since implies (translation on is polynomial). Thus . All further properties of follow from the corresponding statements concerning . For identification of with the quotient space , we first observe that a tempered distribution vanishes on if and only if its (Euclidean) Fourier transform is supported in , which is well known to be the case if and only if is a polynomial. Using this observation, we map to , where is a continuous extension of to all of ; such an extension exists by the Hahn-Banach theorem. The map is well defined because the difference between two extensions of annihilates and hence is a polynomial. Linearity follows from well-definedness. Furthermore, the inverse of the mapping is clearly obtained by assigning to the restriction .

In the following, we will usually not explicitly distinguish between and its equivalence class modulo polynomials, and we will occasionally write . The topology of is just the topology of pointwise convergence on the elements of . For any net , holds if and only if , for all . We next study convolution on .

Lemma 3.4. *For every , the map is a well-defined and continuous operator . If , the associated convolution operator is a well-defined and continuous operator .*

* Proof. *Note that . Hence induces a well-defined canonical map . Furthermore, is continuous on , as a consequence of [19, Propositionโโ1.47]. Therefore, for any net and any , the fact that allows to write
showing in .

For , the fact that makes the mapping well-defined modulo polynomials. The continuity statement is proved by (3.16), with assumptions on and switched.

The definition of homogeneous Besov spaces requires taking -norms of elements of . The following remark clarifies this.

*Remark 3.5. *Throughout this paper, we use the canonical embedding . For , this gives rise to an embedding , using that . Consequently, given , we let
assigning the value otherwise. Here the fact that guarantees that the decomposition is unique, and thus (3.17) well-defined.

By contrast, can only be defined on , if we assign the value to .

Note that with these definitions, the Hausdorff-Young inequality remains valid for all , and all (for ), respectively, (for ). For , this is clear. For , note that if , then with .

We now introduce a general Littlewood-Paley-type decomposition. For this purpose we define for ,

*Definition 3.6. *A function is called LP-admissible if for all ,
holds, with convergence in the Schwartz space topology. Duality entails the convergence
for all .

The following lemma yields the chief construction of LP-admissible functions.

Lemma 3.7. *Let be a function in with support in such that and on . Take . Thus, , with support in the interval , and
**
Pick a sub-Laplacian , and let denote the convolution kernel associated to the bounded left-invariant operator . Then is LP-admissible, with .*

* Proof. *Let us first comment on the properties of that are immediate from the construction via spectral calculus: follows from [20] and vanishing moments by [15, Propositionโโ1].

Now let . First note that 2-homogeneity of implies that the convolution kernel associated to coincides with . Then, by the spectral theorem and (3.21),
holds in -norm.

For any positive integer ,
where is the convolution kernel of . Since is a Schwartz function, it follows by [19, Proposition (1.49)] that , for , for all , with convergence in and a suitable constant .

We next show that in , as , for any . Fix a multi-index and with . Then left-invariance and homogeneity of yield
Here the first inequality is an application of (3.5); the constant can be estimated in terms of , for sufficiently large. But this proves in the Schwartz topology.

Summarizing, in , and in addition by (3.22), in , whence follows.

Note that an LP-admissible function as constructed in Lemma 3.7 fulfills the convenient relation which follows from .

*Remark 3.8. *By spectral calculus, we find that , with . In particular, the decomposition
shows that is dense.

We now associate a scale of homogeneous Besov spaces to the function .

*Definition 3.9. *Let be LP-admissible, let , , and . *The homogeneous Besov space associated to ** is defined as*
with associated norm

*Remark 3.10. *The definition relies on the conventions regarding -norms of distributions (modulo polynomials), as outlined in Remark 3.5. Definiteness of the Besov norm holds because of (3.20).

The combination of Lemma 3.7 with Definition 3.9 shows that we cover the homogeneous Besov spaces defined in the usual manner via the spectral calculus of sub-Laplacians. Hence the following theorem implies in particular that different sub-Laplacians yield the same homogeneous Besov spaces (at least within the range of sub-Laplacians that we consider).

Theorem 3.11. *Let be LP-admissible. Let andโ . Then, , with equivalent norms.*

* Proof. *It is sufficient to prove the norm equivalence, and here symmetry with respect to and immediately reduces the proof to showing, for a suitable constant ,
in the extended sense that the left-hand side is finite whenever the right-hand side is. Hence assume that ; otherwise, there is nothing to show. In the following, let ().

By LP-admissibility of ,
with convergence in . Accordingly,
where the convergence on the right-hand side holds in , by Lemma 3.4. We next show that the right-hand side also converges in . For this purpose, we observe that
where is a fixed integer. For , this follows directly from (3.8), using , and vanishing moments of , whereas for , the vanishing moments of allow to apply (3.6).

Using Youngโs inequality, we estimate with from above that
Next observe that
By assumption, the sequence is in , in particular, bounded. Therefore, yields that (3.34) converges. But then the right-hand side of (3.31) converges unconditionally with respect to . This limit coincides with the -limit (which because of is even a -limit), yielding , with
Now an application of Youngโs inequality for convolution over , again using , provides (3.29).

As a consequence, we write , for any LP-admissible . These spaces coincide with the homogeneous Besov spaces for the Heisenberg group in [9], and with the usual definitions in the case .

In the remainder of the section we note some functional-analytic properties of Besov spaces and Littlewood-Paley-decompositions for later use.

Lemma 3.12. *For all and all , one has continuous inclusion maps , as well as , where the latter denotes the dual of . For , is dense.*

* Proof. * We pick as in Lemma 3.7 and define for . For the inclusion , note that (3.6) and (3.8) allow to estimate for all and that
Here the constant is a suitable multiple of , for sufficiently large. But this implies that continuously.

For the other embedding, repeated applications of Hรถlderโs inequality yield the estimate
valid for all and . Here are the conjugate exponents of , respectively. But this estimate implies continuity of the embeddings and .

For the density statement, let , and . For convenience, we pick according to Lemma 3.7. Since , there exists such that
Next define

Let . By assumption on and Youngโs inequality, , and since , there exists with . Let , then , and for ,
For , the construction of and implies that , whereas for , one has . As a consequence, one finds for
and for
For , one finds
with some constant depending only on . For instance, for ,
A straight forward application of triangle and Youngโs inequality yields
Similar considerations applied to yield
Now summation over yields
as desired.

*Remark 3.13. *Let be as in Lemma 3.7. As a byproduct of the proof, we note that the space
is dense in as well as , if . In , the decomposition
holds with finitely many nonzero terms.

We next extend the Littlewood-Paley decomposition to the elements of the Besov space. For simplicity, we prove the result only for certain LP-admissible functions.

Proposition 3.14. *Let , and let be an LP-admissible vector constructed via Lemma 3.7. Then the decomposition (3.19) converges for all in the Besov space norm.*

* Proof. *Consider the operators ,
By suitably adapting the arguments proving the density statement of Lemma 3.12, it is easy to see that the family of operators is bounded in the operator norm. As noted in Remark 3.13, the strongly converges to the identity operator on a dense subspace. But then boundedness of the family implies strong convergence everywhere.

A further class of spaces for which the decomposition converges is .

Proposition 3.15. *Let , and let be an LP-admissible vector constructed via Lemma 3.7. Then the decomposition (3.19) converges with respect to , for all .*

* Proof. *Let the operator family be defined as in the previous proof. Then, , and Youngโs inequality implies that the sequence of operators is norm-bounded. It therefore suffices to prove the desired convergence on the dense subspace . By [19, Propositionโโ1.49],. Furthermore, for ,
and thus
Again by [19, Propositionโโ1.49],, in particular,
Hence, , and the case yields .

Theorem 3.16. * is a Banach space.*

* Proof. *Completeness is the only issue here. Again, we pick as an LP-admissible vector via Lemma 3.7. Suppose that is a Cauchy sequence. As a consequence, one has in particular, for all , that is a Cauchy sequence, hence , for a suitable . Furthermore, the Cauchy property of implies that
is a Cauchy sequence. On the other hand, the sequence converges pointwise to , whence
We define
Now, using (3.56) and , where are the conjugate exponents of , respectively, a straightforward calculation as in the proof of Lemma 3.12 shows that the sum defining converges in . Furthermore, (3.56) and (3.25) easily imply that . Finally, for the proof of , we employ (3.25) together with the equality , to show that
Summarizing, the sequence is a Cauchy sequence, converging pointwise to 0. But then follows.

#### 4. Characterization via Continuous Wavelet Transform

The following definition can be viewed as a continuous-scale analog of LP-admissibility.

*Definition 4.1. * is called *-admissible*, if for all ,
holds with convergence in the Schwartz topology.

The next theorem reveals a large class of -admissible wavelets. In fact, all the wavelets studied in [15] are also -admissible in the sense considered here. Its proof is an adaptation of the argument showing [15, Theoremโโ1].

We let

Theorem 4.2. *Let , and let be the distribution kernel associated to the operator . Then is -admissible up to normalization.*

* Proof. *The main idea of the proof is to write, for ,
with suitable . Once this is established, for follows by [19, Proposition (1.49)], with convergence in the Schwartz topology. Moreover, entails that in the Schwartz topology: given any and with associated left-invariant differential operator , we can employ (3.5) to estimate
which converges to zero for , as soon as and . But this implies in , by [19].

Thus it remains to construct . To this end, define
which is clearly in , and let denote the associated convolution kernel of . By the definition, . Let be in , and let denote the scalar-valued Borel measure associated to by the spectral measure. Then, by spectral calculus and the invariance properties of ,
as desired.

Hence, by [15, Corollaryโโ1] we have the following.

Corollary 4.3. *
(a) There exist -admissible .**
(b) There exist -admissible with vanishing moments of arbitrary finite order.*

Given a tempered distribution and a -admissible function , the continuous wavelet transform of is the family of convolution products. We will now prove a characterization of Besov spaces in terms of the continuous wavelet transform.

Another popular candidate for defining scales of Besov spaces is the heat semigroup; see for example, [1] for the inhomogeneous case on stratified groups, or rather [21] for the general treatment. In our setting, the heat semigroup associated to the sub-Laplacian is given by right convolution with , where is the kernel of with . Theorem 4.2 implies that is -admissible; it can be viewed as an analog of the well-known *Mexican Hat* wavelet. (For general stratified Lie groups, this class of wavelets was studied for the first time in [15].) The wavelet transform of associated to is then very closely related to the -fold time derivative of the solution to the heat equation with initial condition . By choice of ,
denotes the solution of the heat equation associated to , with initial condition . A formal calculation using left invariance of then yields
Thus the following theorem also implies a characterization of Besov spaces in terms of the heat semigroup.

Theorem 4.4. *Let be -admissible, with vanishing moments of order . Then, for all with , and all , , the following norm equivalence holds:
**
Here the norm equivalence is understood in the extended sense that one side is finite if and only if the other side is. If , the equivalence is also valid for the case .*

* Proof. *The strategy consists in adapting the proof of Theorem 3.11 to the setting where one summation over scales is replaced by integration. This time, however, we have to deal with both directions of the norm equivalence. In the following estimates, the symbol denotes a constant that may change from line to line, but in a way that is independent of .

Let us first assume that
for , , for a -admissible function with vanishing moments (, if ). Let be LP-admissible. Then, for all ,
holds in , by Lemma 3.4.

We next prove that the right-hand side of (4.11) converges in . For this purpose, introduce
We estimate
where we used that is scaling invariant. Note that the last inequality is Hรถlderโs inequality for . In this case, taking th powers and summing over yields
Using vanishing moments and Schwartz properties of and , we can now employ (3.6) and (3.8) to obtain
with a constant independent of . But then, since , we may proceed just as in the proof of Theorem 3.11 to estimate the integrand in (4.15) via
Summarizing, we obtain
In particular, . But then the right-hand side of (4.11) converges to in . The Minkowski inequality for integrals yields , and thus
as desired. In the case , (4.16) yields that
Thus, by (4.13),
The remainder of the argument is the same as for the case .

Next assume . Then, for all and ,
with convergence in ; for , convergence holds even in . As before,
Again, we have with a constant independent of . Hence, one concludes in the same fashion as in the proof of Theorem 3.11 that, for all ,
again with a constant independent of . In the case , this finishes the proof immediately, and for , we integrate the th power over and sum over to obtain the desired inequality.

*Remark 4.5. *Clearly, the proof of Theorem 4.4 can be adapted to consider discrete Littlewood-Paley decompositions based on integer powers of any instead of . Thus consistently replacing powers of 2 in Definitions 3.6 and 3.9 by powers of results in the same scale of Besov spaces.

As an application of the characterization via continuous wavelet transforms, we exhibit certain of the homogeneous Besov spaces as homogeneous Sobolev spaces, and we investigate the mapping properties of sub-Laplacians between Besov spaces of different smoothness exponents.

Lemma 4.6. *, with equivalent norms.*

* Proof. *Pick by Lemma 3.7. Then spectral calculus implies that for all
Since is dense in both spaces, and both spaces are complete, it follows that .

The next lemma investigates the mapping properties of sub-Laplacians between Besov spaces of different smoothness exponents. Its proof is greatly facilitated by the characterization via continuous wavelet transforms.

Lemma 4.7. *Let denote a sub-Laplacian. For all , , and ,
**
in the extended sense that one side is infinite if and only if the other side is. In particular, is a bijection, and it makes sense to extend the definition to negative . Thus, for all ,
**
is a topological isomorphism of Banach spaces.*

* Proof. *Pick a nonzero real-valued , an integer and let denote the distribution kernel of . Hence is admissible by Theorem 4.2, with vanishing moments of order and . On , the convolution operator can be written as with a suitable function . For , spectral calculus implies
where we employed left invariance to pull past in the convolution. Note that up to normalization, is admissible with vanishing moments of order . Thus, applying Theorem 4.4, we obtain
Now assume that . Then, combining the density statements from Lemma 3.12 and Remark 3.8, we obtain a sequence with in ; thus also with convergence in . The norm equivalence and completeness of yield that , for suitable . Again, this implies convergence in . Since is continuous on that space, it follows that , establishing that . Since any distribution annihilated by is a polynomial, this finally yields , and follows by taking limits. A similar but simpler argument establishes the norm equivalence under the assumption that .

This observation shows that we can regard certain Besov spaces as homogeneous Sobolev spaces, or, more generally, as generalizations of Riesz potential spaces.

Corollary 4.8. *For all : .*

As a further corollary, we obtain the following interesting result relating two sub-Laplacians and . For all , the operator

is densely defined and has a bounded extension with bounded inverse. More general analogues involving more than two sub-Laplacians are also easily formulated. For the Euclidean case, this is easily derived using the Fourier transform, which can be viewed as a joint spectral decomposition of commuting operators. In the general, nonabelian case however, this tool is not readily available, and we are not aware of a direct proof of this observation, nor of a previous source containing it.

#### 5. Characterization of Besov Spaces by Discrete Wavelet Systems

We next show that the Littlewood-Paley characterization of can be discretized by sampling the convolution products over a given discrete set . This is equivalent to the study of the analysis operator associated to a discrete wavelet system , defined by Throughout the rest of the paper, we assume that the wavelet has been chosen according to Lemma 3.7 and .

We first define the discrete coefficient spaces which will be instrumental in the characterization of the Besov spaces.

*Definition 5.1. *Fix a discrete set . For a family of complex numbers, we define