Interior Schauder estimates for fractional elliptic equations in nondivergence form

Abstract: We obtain sharp interior Schauder estimates for solutions to nonlocal Poisson problems driven by fractional powers of nondivergence form elliptic operators $(-a^{ij}(x) \partial_{ij})^s$, for $0<s<1$, in bounded domains under minimal regularity assumptions on the coefficients $a^{ij}(x)$. Solutions to the fractional problem are characterized by a local degenerate/singular extension problem. We introduce a novel notion of viscosity solutions for the extension problem and implement Caffarelli's perturbation methodology in the corresponding degenerate/singular Monge--Amp`ere geometry to prove Schauder estimates in the extension. This in turn implies interior Schauder estimates for solutions to the fractional nonlocal equation. Furthermore, we prove a new Hopf lemma, the interior Harnack inequality and H\"older regularity in the Monge--Amp`ere geometry for viscosity solutions to the extension problem.