Uniqueness of extremals for some sharp Poincar\'e-Sobolev constants

TL;DR

This paper proves the uniqueness of extremal functions for certain sharp Poincaré-Sobolev constants in smooth connected domains when q is close to p, and applies this to unique solutions of the Lane-Emden equation.

Contribution

It establishes the uniqueness of extremals for the embedding constants and solutions to the Lane-Emden equation in a new parameter regime near q=p.

Findings

Uniqueness of extremals for q close to p in smooth connected domains.

Application to unique minimal energy solutions of the Lane-Emden equation.

Extension of linearization techniques to this setting.

Abstract

We study the sharp constant for the embedding of $W^{1,p}_0(\Omega)$ into $L^q(\Omega)$, in the case $2<p<q$. We prove that for smooth connected sets, when $q>p$ and $q$ is sufficiently close to $p$, extremal functions attaining the sharp constant are unique, up to a multiplicative constant. This in turn gives the uniqueness of solutions with minimal energy to the Lane-Emden equation, with super-homogeneous right-hand side. The result is achieved by suitably adapting a linearization argument due to C.-S. Lin. We rely on some fine estimates for solutions of $p-$Laplace--type equations by L. Damascelli and B. Sciunzi.