Optimal Regularity for The Signorini Problem and its Free Boundary

Abstract: We will show optimal regularity for minimizers of the Signorini problem for the Lame system. In particular if $\u=(u^1,u^2,u^3)\in W^{1,2}(B_1^+:\R^3)$ minimizes $$ J(\u)=\int_{B_1^+}|\nabla \u+\nabla^\bot \u|^2+\lambda\div(\u)^2 $$ in the convex set $$ K=\big{\u=(u^1,u^2,u^3)\in W^{1,2}(B_1^+:\R^3);\; u^3\ge 0 \textrm{on}\Pi, $$ $$ \u=f\in C^\infty(\partial B_1) \textrm{on}(\partial B_1)^+ \big}, $$ where $\lambda\ge 0$ say. Then $\u\in C^{1,1/2}(B_{1/2}^+)$. Moreover the free boundary, given by $$ \Gamma_\u=\partial {x;\;u^3(x)=0,\; x_3=0}\cap B_{1}, $$ will be a $C^{1,\alpha}$ graph close to points where $\u$ is not degenerate. Similar results have been know before for scalar partial differential equations (see for instance \cite{AC} and \cite{ACS}). The novelty of this approach is that it does not rely on maximum principle methods and is therefore applicable to systems of equations.