Doubling measures and Poincaré inequalities for sphericalizations of metric spaces, with applications to $p$-harmonic functions in unbounded domains

Abstract

The identification between the complex plane and the Riemann sphere preserves holomorphic and harmonic functions and is a classical tool. In this paper we consider a similar mapping from an unbounded metric space $X$ to a bounded space and show how it preserves $p$-harmonic functions and Poincaré inequalities. When $X$ is Ahlfors regular, this was shown in our earlier paper ($\text{J. Math. Anal. Appl. }474$ (2019), 852--875). Here we only require the much weaker (and more natural) doubling property of the measure. Furthermore, we consider a broader class of transformed measures. The sphericalization is then applied to obtain new results for the Dirichlet boundary value problem in unbounded sets and for boundary regularity at infinity for $p$-harmonic functions. Some of these results are new also for unweighted ${\mathbf{R}}^n$, $n\ge 2$ and $p\ne 2$.