Self-improvement of fractional Hardy inequalities in metric measure spaces via hyperbolic fillings

TL;DR

This paper proves a self-improvement property for fractional Hardy inequalities in metric measure spaces, using hyperbolic fillings and trace techniques, broadening the class of weights and domains where these inequalities hold.

Contribution

It introduces a new approach linking fractional Hardy inequalities to classical Hardy inequalities via hyperbolic fillings and develops a theory of regularizable weights for self-improvement.

Findings

Fractional Hardy inequalities are equivalent to classical Hardy inequalities in hyperbolic fillings.

A new weighted self-improvement result for p-Hardy inequalities is established.

Broader classes of weights and domains satisfy fractional Hardy inequalities.

Abstract

In this paper, we prove a self-improvement result for $(\theta,p)$-fractional Hardy inequalities, in both the exponent $1<p<\infty$ and the regularity parameter $0<\theta<1$, for bounded domains in doubling metric measure spaces. The key conceptual tool is a Caffarelli-Silvestre-type argument, which relates fractional Sobolev spaces on $Z$ to Newton-Sobolev spaces in the hyperbolic filling $\overline{X}_{\varepsilon}$ of $Z$ via trace results. Using this insight, it is shown that a fractional Hardy inequality in an open subset of $Z$ is equivalent to a classical Hardy inequality in the filling $\overline{X}_{\varepsilon}$. The main result is then obtained by applying a new weighted self-improvement result for $p$-Hardy inequalities. The exponent $p$ can be self-improved by a classical Koskela-Zhong argument, but a new theory of regularizable weights is developed to obtain the self-improvement in the regularity parameter $\theta$. This generalizes a result of Lehrb\"ack and Koskela on self-improvement of $d_\Omega^\beta$-weighted $p$-Hardy inequalities by allowing a much broader class of weights. Using the equivalence of fractional Hardy inequalities with Hardy inequalities in the fillings, we also give new examples of domains satisfying fractional Hardy inequalities.