Schauder Estimates for Flat Solutions to a Class of Fully Nonlinear Elliptic PDEs with Dini Continuous Data: A Geometric Tangential Approach

Abstract In this manuscript, we establish local Schauder estimates for flat viscosity solutions, that is, solutions with sufficiently small norms, to a class of fully nonlinear elliptic partial differential equations of the form F (D^2 u, x) + B (x), D u = f (x) in B₁ R^n, F (D 2 u, x) + ⟨ B (x), D u ⟩ = f (x) in B 1 ⊂ R n, where the operator F F is differentiable, though not necessarily convex or concave. In addition, we impose suitable Dini-type continuity assumptions on the data. Our methodology is based on geometric tangential techniques, combined with compactness and perturbative arguments. This approach is strongly motivated by recent advances in the theory of nonlinear elliptic equations and free boundary problems. As a byproduct of our analysis, we also obtain an Evans–Krylov type estimate. Our results can be viewed as an extension of the work by dos Prazeres and Teixeira (Ann Sc Norm Super Pisa Cl Sci (5) 15: 485–500, 2016, Theorem 2. 2), now within the framework of linear drift terms and Dini continuity assumptions. Finally, we apply our results to characterize the nodal sets of flat viscosity solutions of non-convex, fully nonlinear, uniformly elliptic PDEs.