Universal admissibility for scattering transforms

Abstract

We resolve the admissibility problem for scattering transforms: every Parseval filter bank on $\mathbb{R}^d$ whose low-pass filter is nonvanishing at the origin induces a norm-preserving scattering transform, without requiring any analyticity, frequency-localization, or geometric covering assumptions. Furthermore, for any Sobolev input $f\in H^s(\mathbb{R}^d)$, we establish a universal decay bound of $\mathcal{O}(N^{-\min\{s,1\}/d})$ on the depth-$N$ residual norm. Finally, we construct a filter bank of band-limited Schwartz functions and a band-limited Schwartz input for which the low-pass multiplier equals one near the origin but the propagated energy decays subexponentially. This demonstrates that no universal exponential rate can hold under these assumptions alone.