Stability of Minimising Harmonic Maps under $W^{1,p}$ Perturbations of Boundary Data: $p\geq 2$

TL;DR

This paper proves that harmonic maps minimizing energy are stable under small $W^{1,p}$ boundary perturbations for $p extgreater= 2$, with stability failing for $p<2$, highlighting the critical nature of $p=2$.

Contribution

The work establishes a stability result for harmonic minimizers under $W^{1,p}$ boundary data perturbations for $p extgreater= 2$, and shows this does not hold for $p<2$, clarifying the boundary regularity threshold.

Findings

Stability of harmonic minimizers for $p extgreater= 2$

Failure of stability for $p<2$ due to counterexamples

Quantitative closeness in Hölder norm modulo bi-Lipschitz maps

Abstract

Let $\Omega \subset \mathbb{R}^3$ be a Lipschitz domain, and consider a harmonic map $v: \Omega \rightarrow \mathbb{S}^2$ with boundary data $v|\partial\Omega = \varphi$ which minimises the Dirichlet energy. For $p\geq 2$, we show that any energy minimiser $u$ whose boundary map $\psi$ has a small $W^{1,p}$-distance to $\varphi$ is close to $v$ in H\"{o}lder norm modulo bi-Lipschitz homeomorphisms, provided that $v$ is the unique minimiser attaining the boundary data. The index $p=2$ is sharp: the above stability result fails for $p<2$ due to the constructions by Almgren--Lieb \cite{al} and Mazowiecka--Strzelecki \cite{ms}.