# A concave-convex problem with a variable operator

## TL;DR

This paper investigates a mixed elliptic problem combining Laplacian and p-Laplacian operators, revealing a concave-convex structure with multiple positive solutions depending on a parameter, using variational and mountain pass methods.

## Contribution

It introduces a novel analysis of a variable operator elliptic problem exhibiting concave-convex behavior, establishing existence, nonexistence, and multiplicity of solutions.

## Key findings

- Existence of a critical parameter ster* where solutions change behavior.
- Presence of at least two positive solutions for subcritical p and certain ster values.
- Identification of a threshold ster* beyond which no positive solutions exist.

## Abstract

We study the following elliptic problem $-A(u) = \lambda u^q$ with Dirichlet boundary conditions, where $A(u) (x) = \Delta u (x) \chi_{D_1} (x)+ \Delta_p u(x) \chi_{D_2}(x)$ is the Laplacian in one part of the domain, $D_1$, and the $p-$Laplacian (with $p>2$) in the rest of the domain, $D_2 $. We show that this problem exhibits a concave-convex nature for $1<q<p-1$. In fact, we prove that there exists a positive value $\lambda^*$ such that the problem has no positive solution for $\lambda > \lambda^*$ and a minimal positive solution for $0<\lambda < \lambda^*$. If in addition we assume that $p$ is subcritical, that is, $p<2N/(N-2)$ then there are at least two positive solutions for almost every $0<\lambda < \lambda^*$, the first one (that exists for all $0<\lambda < \lambda^*$) is obtained minimizing a suitable functional and the second one (that is proven to exist for almost every $0<\lambda < \lambda^*$) comes from an appropriate (and delicate) mountain pass argument.