Two-phase almost minimizers for a fractional free boundary problem

Abstract: In this paper, we study almost minimizers to a fractional Alt-Caffarelli-Friedman type functional. Our main results concern the optimal $C^{0,s}$ regularity of almost minimizers as well as the structure of the free boundary. We first prove that the two free boundaries $F^+(u)=\partial{u(\cdot,0)>0}$ and $F^-(u)=\partial{u(\cdot,0)<0}$ cannot touch, that is, $F^+(u)\cap F^-(u)=\emptyset$. Lastly, we prove a flatness implies $C^{1,\gamma}$ result for the free boundary.