Optimal regularity for a two-phase obstacle-like problem with logarithmic singularity

Abstract: We consider the semilinear problem [ \Delta u = \lambda_+ \left(-\log u^+\right) 1_{{u > 0}} - \lambda_- \left(-\log u^- \right) 1_{{u < 0}} \qquad \hbox{ in } B_1, ] where $B_1$ is the unit ball in $\mathbb{R}^n$ and assume $\lambda_+, \lambda_- > 0$. Using a monotonicity formula argument, we prove an optimal regularity result for solutions: $\nabla u$ is a log-Lipschitz function. This problem introduces two main difficulties. The first is the lack of invariance in the scaling and blow-up of the problem. The other (more serious) issue is a term in the Weiss energy which is potentially non-integrable unless one already knows the optimal regularity of the solution: this puts us in a catch-22 situation.