Continuous Wavelets on Compact Manifolds

TL;DR

This paper develops a theory of continuous wavelets on compact Riemannian manifolds, demonstrating their localization properties, characterizing Hölder functions, and providing explicit formulas for specific cases like the torus and sphere.

Contribution

It introduces continuous ${ m S}$-wavelets on manifolds, proves their localization, and characterizes Hölder continuity via wavelet transforms, extending wavelet theory to curved spaces.

Findings

Wavelet kernels are well-localized near the diagonal.

Hölder continuous functions are characterized by wavelet transform size.

Explicit formulas for kernels on the torus and sphere are provided.

Abstract

Let $\bf M$ be a smooth compact oriented Riemannian manifold, and let $\Delta_{\bf M}$ be the Laplace-Beltrami operator on ${\bf M}$. Say $0 \neq f \in \mathcal{S}(\RR^+)$, and that $f(0) = 0$. For $t > 0$, let $K_t(x,y)$ denote the kernel of $f(t^2 \Delta_{\bf M})$. We show that $K_t$ is well-localized near the diagonal, in the sense that it satisfies estimates akin to those satisfied by the kernel of the convolution operator $f(t^2\Delta)$ on $\RR^n$. We define continuous ${\cal S}$-wavelets on ${\bf M}$, in such a manner that $K_t(x,y)$ satisfies this definition, because of its localization near the diagonal. Continuous ${\cal S}$-wavelets on ${\bf M}$ are analogous to continuous wavelets on $\RR^n$ in $\mathcal{S}(\RR^n)$. In particular, we are able to characterize the H$\ddot{o}$lder continuous functions on ${\bf M}$ by the size of their continuous ${\mathcal{S}}-$wavelet transforms, for H$\ddot{o}$lder exponents strictly between 0 and 1. If $\bf M$ is the torus $\TT^2$ or the sphere $S^2$, and $f(s)=se^{-s}$ (the ``Mexican hat'' situation), we obtain two explicit approximate formulas for $K_t$, one to be used when $t$ is large, and one to be used when $t$ is small.