Constraint maps with free boundaries: the Bernoulli case

In this manuscript, we delve into the study of maps $u\in W^{1,2}(\Omega;\overline M)$ that minimize the Alt-Caffarelli energy functional $$\int_\Omega (|Du|^2 + q^2 \chi_{u^{-1}(M)})\,dx,$$ under the condition that the image $u(\Omega)$ is confined within $\overline M$. Here, $\Omega$ denotes a bounded domain in the ambient space $\mathbb{R}^n$ (with $n\geq 1$), and $M$ represents a smooth domain in the target space $\mathbb{R}^m$ (where $m\geq 2$). Since our minimizing constraint maps coincide with harmonic maps in the interior of the coincidence set, ${\rm int}(u^{-1}(\partial M))$, such maps are prone to developing discontinuities due to their inherent nature. This research marks the commencement of an in-depth analysis of potential singularities that might arise within and around the free boundary. Our first significant contribution is the validity of a $\varepsilon$-regularity theorem. This theorem is founded on a novel method of Lipschitz approximation near points exhibiting low energy. Utilizing this approximation and extending the analysis through a bootstrapping approach, we show Lipschitz continuity of our maps whenever the energy is small energy. Our subsequent key finding reveals that, whenever the complement of $M$ is uniformly convex and of class $C^3$, the maps minimizing the Alt-Caffarelli energy with a positive parameter $q$ exhibit Lipschitz continuity within a universally defined neighborhood of the non-coincidence set $u^{-1}(M)$. In particular, this Lipschitz continuity extends to the free boundary. A noteworthy consequence of our findings is the smoothness of flat free boundaries and of the resulting image maps.