On the Poincaré inequality on open sets in ${\mathbb{R}}^n$

Abstract

We show that the Poincaré inequality holds on an open set $D\subset {\mathbb{R}}^n$ if and only if $D$ admits a smooth, bounded function whose Laplacian has a positive lower bound on $D$. Moreover, we prove that the existence of such a bounded, strictly subharmonic function on $D$ is equivalent to the finiteness of the strict inradius of $D$ measured with respect to the Newtonian capacity. We also obtain a sharp upper bound, in terms of this notion of inradius, for the smallest eigenvalue of the Dirichlet--Laplacian.

arXiv commentCorrected some typos. To appear in Computational Methods and Function Theory