A uniqueness criterion and a counterexample to regularity in an incompressible variational problem

Abstract

In this paper we consider the problem of minimizing functionals of the form $E(u)={\int }_{B}f(x,\nabla u)dx$ in a suitably prepared class of incompressible, planar maps $u:B\to {\mathbb{R}}^2$. Here, $B$ is the unit disk and $f(x,\xi )$ is quadratic and convex in $\xi$. It is shown that if $u$ is a stationary point of $E$ in a sense that is made clear in the paper, then $u$ is a unique global minimizer of $E(u)$ provided the gradient of the corresponding pressure satisfies a suitable smallness condition. We apply this result to construct a non-autonomous, uniformly convex functional $f(x,\xi )$, depending smoothly on $\xi$ but discontinuously on $x$, whose unique global minimizer is the so-called $N-$covering map, which is Lipschitz but not $C^1$.