The two-phase fractional obstacle problem
  We study minimizers of the functional $$ \int_{B_1^+}|\nabla u|^2 x_n^a\,d x +2\int_{B_1'} (\lambda_+ u^++\lambda_- u^-)\,d x', $$ for $a\in(-1,1)$. The problem arises in connection with heat flow with control on the boundary. It can also be seen as a non-local analogue of the, by now well studied, two-phase obstacle problem. Moreover, when $u$ does not change signs this is equivalent to the fractional obstacle problem. Our main results are the optimal regularity of the minimizer and the separation of the two free boundaries $\Gamma^+=\partial'\{u(\cdot,0)>0\}$ and $\Gamma^-=\partial'\{u(\cdot,0)<0\}$ when $a\geq 0$. 
Mark Allen, Erik Lindgren, Arshak Petrosyan
math.AP