Sharp regularity estimates for quasi-linear elliptic dead core problems and applications
João Vítor da Silva, Ariel Salort
TL;DR
This work analyzes dead-core free boundary problems for quasi-linear elliptic equations of $p$-Laplace type with strong absorption, proving a sharp $C^{\gamma}$ regularity at free boundary points with $\gamma = \frac{p}{p-1-q}$. By leveraging scaling-invariant arguments, Harnack-type inequalities, and barrier constructions, it establishes non-degeneracy, positive density, and porosity of the free boundary, along with Liouville-type results and precise $(N-1)$-Hausdorff measure estimates. The results apply to a broad class of operators with $p$-growth and $p$-ellipticity, extending beyond the standard $p$-Laplace operator and including both variational and non-variational frameworks. Together, these findings yield quantitative control over dead-core interfaces, with potential applications to reaction-diffusion, combustion, and related diffusion processes.
Abstract
In this manuscript we study geometric regularity estimates for quasi-linear elliptic equations of $p$-Laplace type ($1 < p< \infty$) with strong absorption condition: $$ -\text{div}\,(Φ(x, u, \nabla u)) + λ_0(x) u_{+}^q(x) = 0 \quad \text{in} \quad Ω\subset \mathbb{R}^N, $$ where $Φ: Ω\times \mathbb{R}_{+} \times \mathbb{R}^N \to \mathbb{R}^N$ is a vector field with an appropriate $p$-structure, $λ_0$ is a non-negative and bounded function and $0\leq q<p-1$. Such a model is mathematically relevant because permits existence of solutions with dead core zones, i.e, \textit{a priori} unknown regions where non-negative solutions vanish identically. We establish sharp and improved $C^γ$ regularity estimates along free boundary points, namely $\mathfrak{F}_0(u, Ω) = \partial \{u>0\} \cap Ω$, where the regularity exponent is given explicitly by $γ= \frac{p}{p-1-q} \gg 1$. Some weak geometric and measure theoretical properties as non-degeneracy, uniform positive density and porosity of free boundary are proved. As an application, a Liouville-type result for entire solutions is established provided that their growth at infinity can be controlled in an appropriate manner. Finally, we obtain finiteness of $(N-1)$-Hausdorff measure of free boundary for a particular class of dead core problems. The approach employed in this article is novel even to dead core problems governed by the $p$-Laplace operator $-Δ_p u + λ_0 u^qχ_{\{u>0\}} = 0$ for any $λ_0>0$. \newline \newline \noindent \textbf{Keywords:} Quasi-linear elliptic operators of $p$-Laplace type, improved regularity estimates, Free boundary problems of dead core type, Liouville type results, Hausdorff measure estimates.