Transversal Hölder Criteria and Dini--Zygmund Endpoint Regularity for Hyperbolic Harmonic Mappings

Abstract: Let $n\ge3$ and let $u$ be a bounded mapping on the upper half-space that is harmonic for the real hyperbolic Laplacian. For $0<α<1$, uniform $α$-Hölder continuity of $u$ on the vertical lines is shown to be quantitatively equivalent to global $α$-Hölder continuity. For real-valued $u$, the vertical approach of $|u|$ to its boundary modulus already suffices. Both statements fail when $α=1$: a lacunary trace produces a hyperbolic harmonic extension that is vertically Lipschitz but not globally Lipschitz. Endpoint conclusions are recovered under a Dini--Zygmund, equivalently $B_{\infty,1}<sup>{1}$, summability condition. The proofs combine the Fourier--Bessel multiplier of the hyperbolic Poisson kernel with inverse approximation and critical Besov estimates.