The polyharmonic Hénon--Lane--Emden conjecture

Abstract

We study positive classical solutions of the polyharmonic Hénon--Lane--Emden system $(-Δ)^m u=|x|^αv^p$, $(-Δ)^m v=|x|^βu^q$ in $\mathbb{R}^n$, where $m\ge2$, $p,q\ge1$, $pq>1$, and $α,β\ge0$. We prove that there is no such solution whenever $\frac{n+α}{p+1}+\frac{n+β}{q+1}>n-2m$, in every dimension and without any assumption on the behavior of the solutions at infinity. Combined with an existence result of Villavert, this proves the polyharmonic Hénon--Lane--Emden conjecture of Phan for $m\ge2$: for these exponents and weights, a positive classical solution exists if and only if $\frac{n+α}{p+1}+\frac{n+β}{q+1}\le n-2m$. The proof has two parts. We first show that positivity forces $(-Δ)^i u>0$ and $(-Δ)^i v>0$ for $1\le i\le m-1$. For $m$ even this needs $pq>1$, and explicit radial examples show that it fails when $pq<1$ and when $p=q=1$. The system is then equivalent to a weighted integral system with a Riesz kernel of order $2m$. For such integral systems we prove the corresponding Liouville theorem for every Riesz order $μ\in(1,n)$, adapting a recent argument for the second-order case that combines a localized virial identity with an interval comparison along source layers.