The Singular Set of Higher Dimensional Unstable Obstacle Type Problems
  In this paper we will investigate the singular points of the following unstable free boundary problem: {equation}\label{Eq} \Delta u= -\chi_{\{u>0\}} \quad\quad\textrm{in} B_1(0) {equation} where $\chi_{\{u>0\}}$ is the characteristic function of the set $\{u>0\}$. 
John Andersson, Henrik Shahgholian, Georg Weiss
math.AP