Hölder continuity of bounded, weak solutions of a variational system in the critical case

Abstract

Let $\Omega \subset {\mathbb{R}}^2$ be a bounded, Lipschitz domain. We consider bounded, weak solutions ($u\in W^{1,2}\cap L^{\infty }(\Omega ;{\mathbb{R}}^N)$) of the vector-valued, Euler-Lagrange system: \text{div } \big( A(x, u)Du\big)=g(x, u, Du)\quad\text{in }Ω. Under natural growth conditions on the principal part and the inhomogeneity, but without any further restriction on the growth of the inhomogeneity (for example, via a smallness condition), we use a blow-up argument to prove that every bounded, weak solution of the system is Hölder continuous. Since the dimension of $\Omega$ is $2$ and $u\in W^{1,2}(\Omega ;{\mathbb{R}}^N)$, we are in the critical setting, and hence, cannot use the Sobolev embedding theorem to deduce Hölder continuity. Our results are connected to a particular case of the open problem of whether all solutions (and not just extremals) of variational systems are Hölder continuous in the critical setting.

Keywords: Hölder, regularity, variational, elliptic, quasi-linear, smallness, one-sided, critical, blow-up