Approximation of Lipschitz functions preserving boundary values

Abstract

Given an open subset $\Omega$ of a Banach space and a Lipschitz function $u_0:\overline{\Omega }\to \mathbb{R},$ we study whether it is possible to approximate $u_0$ uniformly on $\Omega$ by $C^k$-smooth Lipschitz functions which coincide with $u_0$ on the boundary $\partial \Omega$ of $\Omega$ and have the same Lipschitz constant as $u_0.$ As a consequence, we show that every $1$-Lipschitz function $u_0:\overline{\Omega }\to \mathbb{R},$ defined on the closure $\overline{\Omega }$ of an open subset $\Omega$ of a finite dimensional normed space of dimension $n\ge 2$, and such that the Lipschitz constant of the restriction of $u_0$ to the boundary of $\Omega$ is less than $1$, can be uniformly approximated by differentiable $1$-Lipschitz functions $w$ which coincide with $u_0$ on $\partial \Omega$ and satisfy the equation $\Vert Dw{\Vert }_{\ast }=1$ almost everywhere on $\Omega .$ This result does not hold in general without assumption on the restriction of $u_0$ to the boundary of $\Omega$.

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