Minimisers and Kellogg's theorem

Abstract: We extend the celebrated theorem of Kellogg for conformal mappings to the minimizers of Dirichlet energy. Namely we prove that a diffeomorphic minimiser of Dirichlet energy of Sobolev mappings between double connected domains $D$ and $\Omega$ having $\mathscr{C}^{n,\alpha}$ boundary is $\mathscr{C}^{n,\alpha}$ up to the boundary, provided $\text{Mod}(D)\ge \text{Mod}(\Omega)$. If $\text{Mod}(D)< \text{Mod}(\Omega)$ and $n=1$ we obtain that the diffeomorphic minimiser has $\mathscr{C}^{1,\alpha'}$ extension up to the boundary, for $\alpha'=\alpha/(2+\alpha)$. It is crucial that, every diffeomorphic minimizer of Dirichlet energy has a very special Hopf differential and this fact is used to prove that every diffeomorphic minimizer of Dirichlet energy can be locally lifted to a certain minimal surface near an arbitrary point inside and at the boundary.