On smooth interior approximation of Sets of Finite Perimeter

TL;DR

This paper establishes a precise relationship between the smooth interior approximation of sets of finite perimeter and the measure of their measure-theoretic boundary, providing conditions for when perimeter can be approximated.

Contribution

It introduces bounds linking perimeter limits of smooth approximations to the measure-theoretic boundary, characterizing when perimeter approximation is possible.

Findings

Smooth sets can approximate finite perimeter sets with controlled perimeter increase.

A necessary and sufficient condition for perimeter approximation involves the measure-theoretic boundary.

The measure of the measure-theoretic boundary determines the feasibility of perimeter approximation.

Abstract

In this paper, we prove that for any bounded set of finite perimeter $\Omega \subset \mathbb{R}^n$, we can choose smooth sets $E_k \Subset \Omega$ such that $E_k \rightarrow \Omega$ in $L^1$ and \begin{align} \label{moregeneralapproximation} \limsup_{i \rightarrow \infty} P(E_i) \le P(\Omega)+C_1(n) \mathscr{H}^{n-1}(\partial \Omega \cap \Omega^1). \end{align}In the above $\Omega^1$ is the measure-theoretic interior of $\Omega$, $P(\cdot)$ denotes the perimeter functional on sets, and $C_1(n)$ is a dimensional constant. Conversely, we prove that for any sets $E_k \Subset \Omega$ satisfying $E_k \rightarrow \Omega$ in $L^1$, there exists a dimensional constant $C_2(n)$ such that the following inequality holds: \begin{align} \label{gap} \liminf_{k \rightarrow \infty} P(E_k) \ge P(\Omega)+ C_2(n) \mathscr{H}^{n-1}(\partial \Omega \cap \Omega^1). \end{align} In particular, these results imply that for a bounded set $\Omega$ of finite perimeter,\begin{align} \label{char*} \mathscr{H}^{n-1}(\partial \Omega \cap \Omega^1)=0 \end{align} holds if and only if there exists a sequence of smooth sets $E_k$ such that $E_k \Subset \Omega$, $E_k \rightarrow \Omega$ in $L^1$ and $P(E_k) \rightarrow P(\Omega)$.