Quasilinear Elliptic Cooperative and Competitive Systems

We study the existence and multiplicity of weak solutions for the following quasilinear elliptic system: \[ \begin{cases} -\mathrm{div}(A_1(x,u_1)\nabla u_1) + \displaystyle\frac{1}{2} D_{u_1}A_1(x,u_1)\nabla u_1 \cdot \nabla u_1 = λ_1 u_1 + g_{β,1}(u) & \text{in } Ω, \\[3mm] -\mathrm{div}(A_2(x,u_2)\nabla u_2) + \displaystyle\frac{1}{2} D_{u_2}A_2(x,u_2)\nabla u_2 \cdot \nabla u_2 = λ_2 u_2 + g_{β,2}(u) & \text{in } Ω, \\[2mm] u_1 = u_2 = 0 & \text{on } \partialΩ, \end{cases} \] where $λ_1, λ_2 < μ_1$, the first Dirichlet eigenvalue of the Laplacian, and $Ω$ is a bounded domain. The nonlinearity derives from a potential $G_β$ with subcritical growth. Due to the lack of differentiability of the associated energy functional, we employ nonsmooth critical point theory and variational methods based on the concept of weak slope. We prove the existence of least energy solutions in both the cooperative ($β> 0$) and competitive ($β< 0$) regimes.