Regularity of a free boundary in parabolic problem without sign restriction

Abstract

A parabolic obstacle-type problem without sigh restriction on a solution is considered. An exact representation of the global solutions (i.e., solutions in the entire half-space \{(x,t)\in\mathbb{R}^{n+1}:x_{1}<0\}) is found. It is proved, without any additional assumptions on a free boundary, that near the fixed boundary where the homogeneous Dirichlet condition is fulfilled, the boundary of the ';noncoincidence set'; is the graph of a Lipschitz function.