The free boundary for semilinear problems with highly oscillating singular terms

Abstract We investigate general semilinear (obstacle‐like) problems of the form , where has a singularity/jump at giving rise to a free boundary. Unlike many works on such equations where is approximately homogeneous near , we work under assumptions allowing for highly oscillatory behavior. We establish the regularity of the free boundary at flat points. Our approach is to first establish that flat free boundaries are Lipschitz, using a comparison argument with the Kelvin transform. For higher regularity, we study the highly degenerate partial differential equations (PDE) satisfied by ratios of derivatives of , using changes of variable and then the hodograph transform. Along the way, we prove and make use of new Caffarelli–Peral type estimates for such degenerate equations. Much of our approach appears new even in the case of Alt–Phillips and classical obstacle problems.