Geometric regularity estimates for quasi-linear elliptic models in non-divergence form with strong absorption

Abstract: In this manuscript, we investigate geometric regularity estimates for problems governed by quasi-linear elliptic models in non-divergence form, which may exhibit either degenerate or singular behavior when the gradient vanishes, under strong absorption conditions of the form: [ |\nabla u(x)|^{\gamma} \Delta_p^{\mathrm{N}} u(x) = f(x, u) \quad \text{in} \quad B_1, ] where $\gamma > -1$, $p \in (1, \infty)$, and the mapping $u \mapsto f(x, u) \lesssim \mathfrak{a}(x) u_{+}^m$ (with $m \in [0, \gamma + 1)$) does not decay sufficiently fast at the origin. This condition allows for the emergence of plateau regions, i.e., a priori unknown subsets where the non-negative solution vanishes identically. We establish improved geometric $\mathrm{C}^\kappa_{\text{loc}}$ regularity along the set $\mathscr{F}_0 = \partial {u > 0} \cap B_1$ (the free boundary of the model) for a sharp value of $\kappa \gg 1$, which is explicitly determined in terms of the structural parameters. Additionally, we derive non-degeneracy results and other measure-theoretic properties. Furthermore, we prove a sharp Liouville theorem for entire solutions exhibiting controlled growth at infinity.