Higher Regularity of the Free Boundary in a Semilinear System

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.