Gabriel-Type Estimates for Harmonic Quasiregular Mappings and Stoilow Classes

Abstract

Gabriel's classical theorem bounds the integral of $|f|^p$ over every convex curve in the unit disk by the boundary $H^p$ norm of an analytic function, with sharp constant $2$. Das recently proved a harmonic Hardy-space analogue for $p>1$, with constant $4$ when $p\ge2$. This paper records several quasiregular variants of Gabriel's inequality and separates the genuinely quantitative estimates from the general maximal-function mechanism behind them. The main estimate concerns sense-preserving harmonic $K$-quasiregular mappings. If $f=h+\overline g\in h^2$ and $|g'|\le k|h'|$, where $K=(1+k)/(1-k)$, then every convex curve $Γ\subset\mathbb{D}$ satisfies \[ \int_Γ|f(z)|^2{\rm d}s(z) \le 2\frac{(1+k)^2}{1+k^2} \int_{\mathbb{T}} |f^*(ζ)|^2\,|dζ| = \frac{4K^2}{K^2+1} \int_{\mathbb{T}} |f^*(ζ)|^2\,|dζ|. \] Thus the analytic constant $2$ is recovered for $K=1$, while the bound tends to Das's harmonic constant $4$ as $K\to\infty$. We also include a maximal-function criterion for general quasiregular classes, a log-subharmonic modulus case in which the sharp analytic constant is inherited from the Loziński majorant theorem, and Stoilow-factorization criteria under explicit boundary distortion hypotheses.