Conditions for Eliminating Cusps in One-Phase Free Boundary Problems with Degeneracy

TL;DR

This paper proves that in certain degenerate free boundary problems, cusps do not form on the boundary when the domain is two-dimensional and the degeneracy occurs along a line, extending previous work to a broader physical context.

Contribution

It demonstrates the absence of cusps in the free boundary for a specific degenerate functional in two dimensions, generalizing prior results and applying to physical models like Stokes' wave.

Findings

Cusps do not exist on the free boundary when n=2 and Γ is a line.

The results extend the understanding of free boundary regularity in degenerate problems.

Application to variational models of Stokes' wave.

Abstract

In this paper, we continue the study local minimizers of a degenerate version of the Alt-Caffarelli functional. Specifically, we consider local minimizers of the functional $J_{Q}(u, \Omega):= \int_{\Omega} |\nabla u|^2 + Q(x)^2\chi_{\{u>0\}}dx$ where $Q(x) = dist(x, \Gamma)^{\gamma}$ for $\gamma>0$ and $\Gamma$ a submanifold of dimension $0 \le k \le n-1$. Previously, it was shown that on $\Gamma$, the free boundary $\partial \{u>0\}$ may be decomposed into a rectifiable set $\mathcal{S}$, which satisfies effective estimates, and a cusp set $\Sigma$. In this note, we prove that under mild assumptions, in the case $n = 2$ and $\Gamma$ a line, the cusp set $\Sigma$ does not exist. Building upon the work of Arama and Leoni \cite{AramaLeoni12}, our results apply to the physical case of a variational formulation of the Stokes' wave and provide a complete characterization of the singular portion of the free boundary in complete generality in this context.