Concavity and perturbed concavity for $p$-Laplace equations

TL;DR

This paper investigates convexity and concavity properties of solutions to p-Laplace equations with various nonlinearities, extending classical results and including fractional cases, with some findings being novel even for the linear case.

Contribution

It introduces new convexity and concavity results for p-Laplace equations, including fractional and singular cases, generalizing previous work and establishing uniqueness of critical points.

Findings

Solutions inherit concavity properties from the coefficient function a

Concavity up to an error when a is near constant

New results for fractional p-Laplacian and singular cases

Abstract

In this paper we study convexity properties for quasilinear Lane-Emden-Fowler equations of the type $$\begin{cases} -\Delta_p u = a(x) u^q & \quad \hbox{ in $\Omega$},\\ u >0 & \quad \hbox{ in $\Omega$}, \\ u =0 & \quad \hbox{ on $\partial \Omega$}, \end{cases}$$ when $\Omega \subset \mathbb{R}^N$ is a convex domain. In particular, in the subhomogeneous case $q \in [0,p-1]$, the solution $u$ inherits concavity properties from $a$ whenever assumed, while it is proved to be concave up to an error if $a$ is near to a constant. More general problems are also taken into account, including a wider class of nonlinearities. These results generalize some contained in [Kennington, Indiana Univ. Math. J., 1985] and [Sakaguchi, Ann. Sc. Norm. Super. Pisa, 1987]. Additionally, some results for the singular case $q \in [-1,0)$ and the superhomogeneous case $q>p-1$, $q \approx p-1$ are obtained. Some properties for the $p$-fractional Laplacian $(-\Delta)^s_p$, $s\in (0,1)$, $s \approx 1$, are shown as well. We highlight that some results are new even in the semilinear framework $p=2$; in some of these cases, we deduce also uniqueness (and nondegeneracy) of the critical point of $u$.