Nearly Tight Frames and Space-Frequency Analysis on Compact Manifolds

Abstract

Let $\bf M$ be a smooth compact oriented Riemannian manifold, and let $Δ$ 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 $f(t^2 Δ)$. Suppose $f$ satisfies Daubechies' criterion, and $b > 0$. For each $j$, write ${\bf M}$ as a disjoint union of measurable sets $E_{j,k}$ with diameter at most $ba^j$, and comparable to $ba^j$ if $ba^j$ is sufficiently small. Take $x_{j,k} \in E_{j,k}$. We then show that the functions $ϕ_{j,k}(x)=[μ(E_{j,k})]^{1/2} \bar{K_{a^j}}(x_{j,k},x)$ form a frame for $(I-P)L^2({\bf M})$, for $b$ sufficiently small (here $P$ is the projection onto the constant functions). Moreover, we show that the ratio of the frame bounds approaches 1 nearly quadratically as the dilation parameter approaches 1, so that the frame quickly becomes nearly tight (for $b$ sufficiently small). Moreover, based upon how well-localized a function $F \in (I-P)L^2$ is in space and in frequency, we can describe which terms in the summation $F \sim SF = \sum_j \sum_k < F,ϕ_{j,k} > ϕ_{j,k}$ are so small that they can be neglected. If $n=2$ and $\bf M$ is the torus or the sphere, and $f(s)=se^{-s}$ (the "Mexican hat" situation), we obtain two explicit approximate formulas for the $ϕ_{j,k}$, one to be used when $t$ is large, and one to be used when $t$ is small. Finally we explain in what sense the kernel $K_t(x,y)$ should itself be regarded as a continuous wavelet on ${\bf M}$, and characterize the Hölder continuous functions on ${\bf M}$ by the size of their continuous wavelet transforms, for Hölder exponents strictly between 0 and 1.