Higher Regularity of the Free Boundary in a Semilinear System

TL;DR

This paper proves higher regularity and analyticity of the free boundary and related functions in a semilinear elliptic system with a degenerate structure, using advanced transformations and the implicit function theorem.

Contribution

It establishes the analyticity of the free boundary and associated functions for a semilinear elliptic system with $0 \\leq q < 1$, advancing understanding of free boundary regularity.

Findings

Analyticity of the regular part of the free boundary.

Analyticity of $|rac{u}{|u|}$ and $|u|^{\frac{1-q}{2}}$ up to the free boundary.

Local existence of solutions with analytic free boundary data.

Abstract

In this paper we are concerned with higher regularity properties of the elliptic system \[ \Delta\mathbf{u}= |\mathbf{u}|^{q-1}\mathbf{u}\chi_{\{|\mathbf{u}|>0\}},\qquad\mathbf{u}=(u^1,\dots,u^m) \] for $0\leq q<1$. We show analyticity of the regular part of the free boundary $\partial\{|\mathbf{u}|>0\}$, analyticity of $|\mathbf{u}|^{\frac{1-q}2} $ and $ \frac{\mathbf{u}}{|\mathbf{u}|}$ up to the regular part of the free boundary. Applying a variant of the partial hodograph-Legendre transformation and the implicit function theorem, we arrive at a degenerate equation, which introduces substantial challenges to be dealt with. Along the lines of our study, we also establish a Cauchy-Kowalevski type statement to show the local existence of solution when the free boundary and the restriction of $ \frac{\mathbf{u}}{|\mathbf{u}|} $ from both sides to the free boundary are given as analytic data.