On the generalized Fredholm alternative for the $p$-Laplacian with resonant subhomogeneous terms

Abstract: Let $1 \leq q &lt; p$ and let $λ<em>1$ be the first eigenvalue of the $p$-Laplacian in a bounded domain $Ω$. We study the energy functional $$E</em>λ(u)=\frac{1}{p}\left(\int_Ω|\nabla u|^p\,dx -λ\int_Ω|u|^{p\,dx\right)-\mathcal{F}(u),} \quad u\in W_0^{1,p}(Ω),$$ where $\mathcal{F}$ is positively $q$-homogeneous and vanishes along the first eigenspace. Assuming a suitable relation between $\mathcal{F}$ and the $κ$-th power of the principal part of $E_{λ<em>1}$ near this eigenspace, we describe the behavior of $E</em>{λ<em>1}$ according to the relations $pκ&lt;q$, $pκ=q$, or $pκ&gt;q$. In particular, the functional is unbounded from below in the first case, and has a negative infimum in the last case. We then study how sufficiently small $q$-homogeneous perturbations of $\mathcal{F}$ influence the geometry of $E</em>λ$. In this way, we describe assumptions guaranteeing the existence of three critical points in a left neighborhood of $λ_1$ and two critical points in a right neighborhood of $λ_1$, which indicates an $S$-shaped structure of the solution set. The results are applied to double-phase functionals and the nonlinear Fredholm alternative.