Lipschitz continuity of the eigenfunctions on optimal sets for functionals with variable coefficients

Abstract

This paper is dedicated to the spectral optimization problemmin{λ1(Ω)+⋯+λk(Ω)+Λ|Ω| : Ω⊂Dquasi-open}whereD⊂ ℝdis a bounded open set and 0 <λ1(Ω) ≤⋯ ≤λk(Ω) are the firstkeigenvalues onΩof an operator in divergence form with Dirichlet boundary condition and Hölder continuous coefficients. We prove that the firstkeigenfunctions on an optimal set for this problem are locally Lipschtiz continuous inDand, as a consequence, that the optimal sets are open sets. We also prove the Lipschitz continuity of vector-valued functions that are almost-minimizers of a two-phase functional with variable coefficients.