# Existence, Uniqueness and Structure of Second Order absolute minimisers

## TL;DR

This paper proves the existence and uniqueness of a second order absolute minimiser for a supremal functional involving the Laplacian, showing partial smoothness and a relation to harmonic functions under natural conditions.

## Contribution

It establishes the first existence and uniqueness results for second order absolute minimisers of supremal functionals with prescribed boundary data.

## Key findings

- Existence and uniqueness of the minimiser under natural assumptions.
- Partial smoothness of the minimiser.
- Existence of a harmonic function related to the minimiser.

## Abstract

Let $\Omega \subseteq \mathbb{R}^n$ be a bounded open $C^{1,1}$ set. In this paper we prove the existence of a unique second order absolute minimiser $u_\infty$ of the functional \[ \mathrm{E}_\infty (u,\mathcal{O})\, :=\, \| \mathrm{F}(\cdot, \Delta u) \|_{L^\infty( \mathcal{O} )}, \ \ \ \mathcal{O} \subseteq \Omega \text{ measurable}, \] with prescribed boundary conditions for $u$ and $\mathrm{D} u$ on $\partial \Omega$ and under natural assumptions on $\mathrm{F}$. We also show that $u_\infty$ is partially smooth and there exists a harmonic function $f_\infty \in L^1(\Omega)$ such that \[ \mathrm{F}(x, \Delta u_\infty(x)) \, =\, e_\infty\, \mathrm{sgn}\big(f_\infty(x)\big) \] for all $x \in \{f_\infty \neq 0\}$, where $e_\infty$ is the infimum of the global energy.