On the behavior of free boundaries near the boundary of the domain

Let $u$ be a solution of the obstacle problem $u\ge0$, $-\Delta u+f\ge0$, $u(-\Delta u+f)=0$ in a domain $\Omega\subset\mathbb R^n$. In this paper, the behaviour of the free boundary in a neighborhood of $\partial\Omega$ is studied. It is proved that under some conditions the free boundary touches $\partial\Omega$ at contact points. Bibliography: 4 titles.