Existence of solutions on the critical hyperbola for a pure Lane-Emden system with Neumann boundary conditions

Abstract

We study the following Lane-Emden system \[ -Δu=|v|^{q-1}v \quad \text{ in } Ω, \qquad -Δv=|u|^{p-1}u \quad \text{ in } Ω, \qquad u_ν=v_ν=0 \quad \text{ on } \partial Ω, \] with $\Omega$ a bounded regular domain of ${\mathbb{R}}^N$, $N\ge 4$, and exponents $p,q$ belonging to the so-called critical hyperbola $1/(p+1)+1/(q+1)=(N-2)/N$. We show that, under suitable conditions on $p,q$, least-energy (sign-changing) solutions exist, and they are classical. In the proof we exploit a dual variational formulation which allows to deal with the strong indefinite character of the problem. We establish a compactness condition which is based on a new Cherrier type inequality. We then prove such condition by using as test functions the solutions to the system in the whole space and performing delicate asymptotic estimates. If $N\ge 5$, $p=1$, the system above reduces to a biharmonic equation, for which we also prove existence of least-energy solutions. Finally, we prove some partial symmetry and symmetry-breaking results in the case $\Omega$ is a ball or an annulus.

In this revised version, we add a regularity result (Proposition 1.3) which in particular shows that strong solutions are classical. In turn, this allows us to prove some symmetry-breaking results when the domain is a ball (see Theorem 1.9)

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• Topics in elliptic problems: from semilinear equations to shape optimization