# Titchmarsh theorems for Fourier transforms of Hölder–Lipschitz functions on compact homogeneous manifolds

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## Abstract

In this paper we extend classical Titchmarsh theorems on the Fourier transform of Hölder–Lipschitz functions to the setting of compact homogeneous manifolds. As an application, we derive a Fourier multiplier theorem for \(L^2\)-Hölder–Lipschitz spaces on compact Lie groups. We also derive conditions and a characterisation for Dini–Lipschitz classes on compact homogeneous manifolds in terms of the behaviour of their Fourier coefficients.

## Keywords

Compact homogeneous manifolds Compact Lie groups Lipschitz functions## Mathematics Subject Classification

Primary 43A30 43A85 Secondary 22E30## 1 Introduction

*f*satisfies the Hölder condition of order \(0<\alpha \le 1\) then

*f*satisfies the Hölder condition of order \(\alpha >\frac{1}{2}\) then its Fourier series converges absolutely. It was also established by Bernstein that the index \(\frac{1}{2}\) is sharp. An introduction to the classical topics discussed above can be found in [34].

The Hausdorff–Young inequality on a locally compact abelian group \(G_1\) states that \(\widehat{f}\in L^q\) if \(f\in L^p(G_1)\) for \(1\le p\le 2\) and \(\frac{1}{p}+\frac{1}{q}=1\). In [37], Edward Charles Titchmarsh studied the problem of how much this fact can be strengthened if *f* additionally satisfies a Lipschitz condition in the case of \(G_1=\mathbb {R}\). In fact, Titchmarsh showed that the information on \(\widehat{f}\) can be significantly improved. In [39, 41, 43], M. Younis extended the Titchmarsh theorems to the case of the circle and to the very specific case of compact groups, namely, the 0-dimensional groups. Other studies of Lipschitz conditions have been done in [19] in term of the modulus of continuity. However, our aim in this paper is to derive *suitable characterisations in terms of the Fourier coefficients of the functions.*

The aim of this paper is to extend the classical Titchmarsh theorems to the setting of general compact homogeneous manifolds. In particular, this includes higher dimensional tori, compact Lie groups, real, complex or quaternionic spheres, projective spaces, and many others. As an application of such an extension, we derive a Fourier multiplier theorem for \(L^2\)-Lipschitz spaces.

The analysis of various spaces and functional inequalities on compact Lie groups and compact homogeneous manifolds, despite its own interest and long history, has also a range of applications linking the local properties of functions with the global spectral properties of the group and to its representation theory, see e.g. Pesenson [21] for Bernstein–Nikolskii inequalities and links to the approximation theory, [22] for links to frames construction and Geller and Pesenson [17] for Besov spaces, Kushpel [18] for cubature formulae, [5] for Gevrey spaces with further links to weakly hyperbolic partial differential equations involving Laplacian and sub-Laplacian, [20] for Nikolskii inequality and Besov spaces, as well as further references in these papers. The convergence of the Fourier series of functions on compact Lie groups has a long history, see e.g., to mention only very few, Taylor [36], Sugiura [35], and some review in Faraut’s book [12]. We can also refer to [2] for conditions for Lipschitz functions on compact totally disconnected groups, [23] on symmetric spaces of rank 1, [25] for Fourier series on Vilenkin groups (compact, metrisable, 0-dimensional, abelian groups), and results for Lipschitz multipliers in [4, 24, 26, 27] on compact 0-dimensional, locally compact abelian, and abelian 0-dimensional groups, respectively.

We note that for convolution operators (on compact Lie groups) the rate of decay of Fourier coefficients of the kernel is also related to the Schatten–von Neumann properties of the operator, see e.g. [6, 8].

We also note that anisotropic Hölder spaces (with respect to the group structure) have been also analysed on the Heisenberg group in [16] (see also [15]) with applications to partial differential equations, and some Fourier transform description was considered in [42]. Anisotropic Hölder spaces on more general Carnot groups were recently applied in [29] to questions in the potential theory.

### Theorem A

Let \(0<\alpha \le 1\) and \(1<p\le 2\). If \(f\in {{\,\mathrm{Lip}\,}}_{{{{\mathbb {T}}}^1}}(\alpha ;p)\), then \({\widehat{f}}\in \ell ^{\beta }({{{\mathbb {Z}}}})\) for \(\frac{p}{p+\alpha p-1}<\beta \le q\), \(\frac{1}{p}+\frac{1}{q}=1\).

In order to illustrate the improvement in the conclusion \({\widehat{f}}\in \ell ^{\beta }({{{\mathbb {Z}}}})\) in Theorem A in comparison to the Hausdorff–Young theorem, consider for instance \(f\in {{\,\mathrm{Lip}\,}}_{{{{\mathbb {T}}}^1}}(\frac{1}{2};2)\), then one has \(\frac{p}{p+\alpha p-1}=1\), and thus \({\widehat{f}}\in \ell ^{\beta }({{{\mathbb {Z}}}})\) for all \(\beta \in (1,2]\).

In Theorem A\('\) and Theorem 3.2 we extend Theorem A to the setting of general compact homogeneous manifolds, also slightly sharpening the conclusion.

The second Titchmarsh theorem (Titchmarsh [37, Theorem 85]) gives the necessary and sufficient conditions on the remainder of the series of Fourier coefficients for \(L^2\)-Lipschitz functions, i.e. for the case \(p=2\). In the case of the circle ([39, Theorem 2.17]) it can be stated as follows:

### Theorem B

*K*is a closed subgroup of a compact Lie group

*G*, the quotient space

*G*/

*K*can be canonically identified with an analytic manifold

*M*. Henceforth

*M*will denote a compact homogeneous manifold. In the case when \(K=\{e\}\) is the identity of the group

*G*, we have \(M=G\) is a compact Lie group itself. To simplify the notation in the sequel we will identify representations of

*G*with their equivalence classes.

We denote by \({\widehat{G}}\) the unitary dual of *G*, and by \({\widehat{G}}_0\) the subset of \({\widehat{G}}\) consisting of representations \(\xi \) of class I with respect to the subgroup *K*: this means that \([\xi ]\in {\widehat{G}}_0\) if \(\xi \) has at least one non-zero invariant vector with respect to *K*, i.e. that for some non-zero *a* in the representation space of \(\xi \) we have \(\xi (h)a=a\) for all \(h\in K\). We will also denote by \(k_{\xi }\) the dimension of the space of invariant vectors of the representation \(\xi \) with respect to *K*.

In particular, if \(K=\{e\}\) so that \(G/K=G\) is a compact Lie group, then \(k_\xi =d_\xi \) is the dimension of the representation \(\xi \in {\widehat{G}}\). If the subgroup *K* is massive, that is if the dimension of the space of invariant vectors of the representation \(\xi \) is one for each class I representation, then we have \(k_\xi =1\) for all \(\xi \in {\widehat{G}}_0\), and \(k_\xi =0\) for all \(\xi \in {\widehat{G}}\backslash {\widehat{G}}_0\).

In order to extend the Definition (1.1) on the circle to a compact homogeneous manifold \(M=G{/}K\) we will employ the geodesic distance on *G*. For the distance from the ‘unit’ element *e* to \(h\in G\) we will write \(|h|=d(h,e)\). This convention is not restrictive since the analysis of our problems is local.

### Definition 1.1

*f*is \((\alpha ;p)\)-Lipschitz. Since

*G*acts on

*G*/

*K*from left, it is natural to consider left shifts \(f(h\cdot )\) with respect to

*h*in Definition 1.1. However, if \(K=\{e\}\) and our homogeneous space is the Lie group

*G*itself, one can also consider right shifts by

*h*, and to define the right-Lipschitz space by taking \(f(\cdot h)\) instead of \(f(h\cdot )\) in the Definition 1.1, namely, by the condition

Some remarks regarding the equivalence between these two notions in the case of groups are included at the end of Sect. 2 in Remark 2.3.

*p*-Lipschitz spaces on

*G*via localisations. Given \(f\in L^p(G{/}K)\), we say that

*G*/

*K*. The converse also follows if we take \(h=\exp (h_j X_j)\), \(j=1,\ldots ,\dim G\), for small real numbers \(h_j\), with \(\exp :{\mathfrak {g}}\rightarrow G\) the exponential mapping near

*e*, and \(\{X_j\}_{j=1}^{\dim G}\) a basis of \({\mathfrak {g}}\), e.g. orthonormalised with respect to the Killing form. Using that the push-forwards \(\chi ^*\) by coordinate chart maps have non-vanishing Jacobians, compactness, and adapting the argument above in the other direction, the equality (1.5) can be extended to any

*p*, namely, we have

*G*/

*K*.

**Theorem A**\('\).

*Let*

*G*/

*K*

*be a compact homogeneous manifold of dimension*

*n*.

*Let*\(0<\alpha \le 1\), \(1<p\le 2\),

*and let*

*q*

*be such that*\(\frac{1}{p}+\frac{1}{q}=1\).

*Let*\(f\in {{\,\mathrm{Lip}\,}}_{G/K}(\alpha ;p)\).

*Then*

*where*\(\mathcal {L}_{G/K}\)

*is the Laplacian on*

*G*/

*K*.

*Consequently, we also have*

*G*/

*K*and many other functional analytic properties. These spaces were analysed in [20, Section 2] in the setting of compact homogeneous manifolds, extending the corresponding definition on the compact Lie groups introduced earlier in [30, Section 10.3.3], in which case we would have \({\widehat{G}}_0={\widehat{G}}\) and \(k_\xi =d_\xi \): see Definition 2.2 for this case. The Fourier coefficients of functions on

*G*/

*K*, although matrices in \(\mathbb {C}^{d_\xi \times d_\xi }\), have a lot of zeros, see (2.13) and (2.14). This explains the factor \(\sqrt{k_\xi }\) appearing in the denominator in (1.7). The spaces \(\ell ^\beta ({\widehat{G}}_0)\) are interpolation spaces, with embeddings into another known family of \(\ell ^\beta \)-spaces defined using Schatten norms, we refer to [20, Section 2] for a description of functional analytic properties of these spaces and to [14, Section 2.1.4] for the embedding properties.

Clearly (see Remark 3.3) Theorem A\('\) implies Theorem A when \(G={{{\mathbb {T}}}^1}\) and \(K=\{0\}\), so that \(G/K={{{\mathbb {T}}}^1}\). Since both Theorem A\('\) and Theorem B\('\) absorb Titchmarsh theorems in the case of the torus they are also sharp.

**Theorem B**\('\).

*Let*\(0<\alpha \le 1\)

*and*\(f\in L^2(G{/}K)\).

*Then*\(f\in {{\,\mathrm{Lip}\,}}_{G/K}(\alpha ;2)\)

*if and only if*

*G*/

*K*: namely, we have \((I-\mathcal {L}_{G/K})^{1/2}\xi _{ij}={\left\langle {\xi }\right\rangle }\xi _{ij}\) for all \(\xi \in {\widehat{G}}_0\), \(1\le i\le d_\xi \) and \(1\le j\le k_\xi \). The choice of the range of the indices

*i*,

*j*happens in this way since these are the only representation coefficients interacting with the Fourier coefficients of functions on

*G*/

*K*, see (2.13) and (2.14). In the case of Lie groups the quantity \({\left\langle {\xi }\right\rangle }\) is also explained in detail in (2.4).

*G*/

*K*takes the form

In view of Lemma 2.4, Theorem A\('\) and Theorem B\('\) follow from their respective versions on compact Lie groups formulated in Theorems 3.2 and 3.4, respectively.

*A*with symbol \(a(\xi )\in \mathbb {C}^{d_{\xi }\times d_{\xi }}\) for each \(\xi \) acts by multiplication by \(a(\xi )\) on the Fourier transform side, i.e. by

*G*(or the Casimir element of the universal enveloping algebra), we denote by \({\left\langle {\xi }\right\rangle }\) the eigenvalue of the elliptic pseudo-differential operator \((I-{\mathcal {L}}_G)^{1/2}\) corresponding to \(\xi \), i.e.

### Corollary 1.2

*A*by symbol

*a*,

For example, the matrix symbol of the Bessel potential \(A=(I-{\mathcal {L}}_G)^{-\gamma /2}\) is \(a(\xi )={\left\langle {\xi }\right\rangle }^{-\gamma } I_{d_\xi }\), where \(I_{d_\xi }\in {\mathbb {C}}^{d_\xi \times d_\xi }\) is the identity matrix, so that \(\Vert a(\xi )\Vert _{op}={\left\langle {\xi }\right\rangle }^{-\gamma }\).

A similar result to Corollary 1.2 is true on homogeneous manifolds *G* / *K*, although in this case, in view of (2.13) and (2.14), the symbol \(a(\xi )\) may be assumed to be zero outside the first \(k_\xi \times k_\xi \) block, i.e. we would take \(a(\xi )=0\) for \(\xi \not \in {\widehat{G}}_0\), and \(a(\xi )_{ij}=0\) for all \(\xi \in {\widehat{G}}_0\) and for \(i>k_\xi \) or \(j>k_\xi \).

The multiplier theorem in Corollary 1.2 complements other known multiplier theorems on compact Lie groups, e.g. the Mihlin multiplier theorem [32, 33], Hörmander multiplier theorem [13], or more general results for the boundedness of pseudo-differential operators in \(L^p\)-spaces [7].

In Sect. 4 we discuss analogues of the obtained results for the Dini–Lipschitz classes. The Dini–Lipschitz classes are a logarithmic extension on Lipschitz classes and these spaces have been analysed in different settings, e.g. in low dimensions [40], or using Helgason transforms on rank 1 symmetric spaces in [10, 11]. Our analysis in this paper is based on a logarithmic extension of the Duren lemma allowing inclusion of the log-terms in the characterisations.

## 2 Preliminaries

In this section we recall some basic facts on the Fourier analysis and the notion of global matrix-symbols for pseudo-differential operators on compact Lie groups and more generally on compact homogeneous manifolds. We refer to [30, 31] for a comprehensive account of such topics.

*G*, we equip it with the normalised Haar measure \(\mu \equiv dx\) on the Borel \(\sigma \)-algebra associated to the topology of the smooth manifold

*G*. The Lie algebra of

*G*will be denoted by \({\mathfrak {g}}\). We also denote by \({\widehat{G}}\) the set of equivalence classes of continuous irreducible unitary representations of

*G*and by \({\mathrm{Rep}}(G)\) the set of all such representations. Since

*G*is compact, the set \({\widehat{G}}\) is discrete. For \([\xi ]\in {\widehat{G}}\), by choosing a basis in the representation space of \(\xi \), we can view \(\xi \) as a matrix-valued function \(\xi :G\rightarrow \mathbb {C}^{d_{\xi }\times d_{\xi }}\), where \(d_{\xi }\) is the dimension of the representation space of \(\xi \). By the Peter–Weyl theorem the collection

*A*from \(C^{\infty }(G)\) to \({\mathcal {D}}'(G) \) we define its

*matrix-valued symbol*\(\sigma (x,\xi )\in \mathbb {C}^{d_{\xi }\times d_{\xi }}\) by

*A*to each component of the matrix \(\xi (x)\). Then one has ([30, 31]) the global quantization

*A*is a linear continuous operator from \(C^{\infty }(G)\) to \(C^{\infty }(G)\), the series (2.8) is absolutely convergent and can be interpreted in the pointwise sense. The symbol \(\sigma \) can be interpreted as a matrix-valued function on \(G\times {\widehat{G}}\). We refer to [30, 31] for the consistent development of this quantization and the corresponding symbolic calculus. If the operator

*A*is left-invariant then its symbol \(\sigma \) does not depend on

*x*. We often simply call such operators invariant.

*x*| denoting the geodesic distance from

*x*to

*e*, see [30, Section 10.6].

We note that Definition 1.1 in the case of compact Lie groups can be stated in the following form by taking the geodesic distance on *G* from the unit element *e* to \(h\in G\):

### Definition 2.1

*G*be a compact Lie group. Let \(0<\alpha \le 1\) and \(1\le p\le \infty \). We define the space \({{\,\mathrm{Lip}\,}}_G(\alpha ;p)\) by

For the analysis of the Fourier transform we will require the following function spaces on the unitary dual \({\widehat{G}}\). We refer the reader to [30, Section 10.3.3] for the basic properties of such spaces.

### Definition 2.2

*G*of dimension

*n*, that will be of use on several occasions: asymptotically as \(\lambda \rightarrow \infty \) we have

### Remark 2.3

*f*is a right-Lipschitz of type \((\alpha ;p)\) function if and only if \(f^{\cdot }\) is left-Lipschitz of type \((\alpha ;p)\).

Our statements will be stated for left-Lipschitz \((\alpha ;p)\). A look at the arguments in the proofs of our results shows that all the statements also hold for right-Lipschitz \((\alpha ;p)\) type. At the level of compact Lie groups both concepts agree when \(p=2\) as a consequence of Theorem 3.5: indeed, the criterion (3.7) is the same for both left- or right-Lipschitz functions.

We shall now explain how the proof of Theorems A\('\) and B\('\) can be reduced to the case of compact Lie groups. For a function \(f\in C^\infty (G{/}K)\), its canonical lifting \({\widetilde{f}}\) is defined by \({\widetilde{f}}(x)=f(xK)\) for all \(x\in G\), so that \({\widetilde{f}}\) is constant on the right cosets.

### Lemma 2.4

We have \({\widetilde{f}}\in {{\,\mathrm{Lip}\,}}_{G}(\alpha ; p)\) if and only if \(f\in {{\,\mathrm{Lip}\,}}_{G/K}(\alpha ; p)\).

### Proof

*K*is equal to one. \(\square \)

## 3 Titchmarsh theorems for Fourier transforms of Lipschitz functions

In this section we prove our results. We start with a lemma on Fourier transforms that we will apply in the proof of the extension of Theorem reftha. The notion of global symbol discussed in Sect. 2 will be useful in the proof of the first main result. Henceforth *G* will denote a compact Lie group of dimension *n*.

### Lemma 3.1

### Proof

Since \(\langle \xi \rangle \ge 1\), we have \(\langle \xi \rangle H(\xi )\in \ell ^{\beta _0}({\widehat{G}})\) implies that \(H\in \ell ^{\beta _0}({\widehat{G}})\), and hence \(H\in \ell ^{\beta }({\widehat{G}})\) for all \(\beta \ge \beta _0\). So we need to prove the case \(\beta <\beta _0\), which we now assume.

We now state our first main result which extends Theorem A to general compact Lie groups and consequently, in view of Lemma 2.4, to compact homogeneous manifolds.

### Theorem 3.2

*G*be a compact Lie group of dimension

*n*. Let \(0<\alpha \le 1\), \(1<p\le 2\), and let

*q*be such that \(\frac{1}{p}+\frac{1}{q}=1\). Let \(f\in {{\,\mathrm{Lip}\,}}_G(\alpha ;p)\). ThenConsequently,

### Proof

*e*of order 1 and

*e*is its isolated zero, i.e., there exist constants \(C_1, C_2>0\) such that

*U*is unitary. Hence

*G*, see [30, (10.25)]. The ellipticity is preserved by the action of an unitary matrix, due to the identities

*V*,

*W*unitary.

*h*|. By (3.1) we have

### Remark 3.3

Consequently Theorem 3.2 with the second conclusion is an extension of Titchmarsh theorem (cf. Titchmarsh [37, Theorem 84]) and Younis [39, Theorem 2.6]) to the setting of compact Lie groups. As it was already mentioned in the introduction the lower bound \(\frac{p}{\alpha p+p-1}\) in Theorem 3.2 is in general sharp as a counterexample can be constructed for \(p=2\) in the case \(G={{{\mathbb {T}}}^1}\).

For the proof of the second Titchmarsh theorem we will be using the following lemma due to Duren (cf. [9, p. 101]):

### Lemma 3.4

We can now state our second main result which extends Theorem B to compact Lie groups and consequently, in view of Lemma 2.4, to compact homogeneous manifolds (Theorem B\('\)).

### Theorem 3.5

### Proof

*W*is discrete countable we can enumerate it as \(W=\{\lambda _k:k\in \mathbb {N}\}\). Of course this enumeration also induces an enumeration of \({\widehat{G}}\) and by the eigenvalue estimate \(\lambda _k\approx k^{\frac{1}{n}}\) (see e.g [1]) we obtain

We now formulate an application of Theorem 3.5 to the regularity of Fourier multipliers on Hölder spaces mentioned in Corollary 1.2. For convenience of the reader we repeat its statement:

### Corollary 3.6

*A*be the Fourier multiplier with symbol

*a*, i.e. \({\widehat{Af}}(\xi )=a(\xi ){\widehat{f}}(\xi )\) for all \(\xi \in {\widehat{G}}\). Then

### Proof

### Corollary 3.7

## 4 Dini–Lipschitz functions

So far we have being dealing with the Hölder–Lipschitz condition on compact homogeneous manifolds in the \(L^p\) setting and a characterisation in the \(L^2\) case. In this section we will consider a different condition, the so-called Dini–Lipschitz condition on \(L^2\) and we will generalise the corresponding Titchmarsh theorems (cf. [37, Theorem 85]).

We shall first establish an extension of Duren’s lemma (cf. [9, p. 101]), Lemma 3.4 in this paper, adapted to the Dini–Lipschitz condition.

### Lemma 4.1

### Proof

We can now establish our first theorem on Dini–Lipschitz functions.

### Theorem 4.2

*G*/

*K*be a compact homogeneous manifold. Let \(\alpha \ge 0\) and \(d\in \mathbb {R}\). Then the conditions

### Proof

To end this work we now give a Dini–Lipschitz version of Theorem 3.2.

### Theorem 4.3

*G*/

*K*be a compact homogeneous manifold of dimension

*n*. Let \(0<\alpha \le 1\), \(d\in \mathbb {R}\), \(1<p\le 2\), and let

*q*be such that \(\frac{1}{p}+\frac{1}{q}=1\). Suppose that

### Proof

## Notes

### Acknowledgements

The authors want to thank the anonymous referee for the valuable corrections.

### Funding

Funding was provided by Leverhulme Trust (Grant No. RPG-2017-151), Engineering and Physical Sciences Research Council (Grant No. EP/R003025/1).

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