Cotlar Properties, Hyperbolicity, and Unconditional Fourier Decompositions

Abstract

Let a discrete group $\Gamma$ act by isometries on a metric space $X$. We show that actions on geodesic hyperbolic spaces satisfy a branch Cotlar property. Under a finite low-orbit-frequency hypothesis, this yields uniform $L^4$-unconditionality for branch subspaces associated with sufficiently separated subsets; when the union of these branches has finite complement, the corresponding signed projections are uniformly bounded on $L^p(\hat{\Gamma })$ for every $1<p<\infty$. Finally, for connected vertex-transitive graphs, we prove that a scale-wise branch Cotlar property characterizes hyperbolicity of $X$.

Added an equivalence result between the branch Cotlar property and the Hyperbolicity (Theorem 4.4)