On the approximation of finite perimeter sets

Abstract: We prove that if $Ω\subseteq\mathbb{R}<sup>N$ is a set with finite perimeter with $$\mathscr{H}^{{N-1}(\partial} Ω\setminus\partial^* Ω)=0$$, then any set of finite perimeter $E\subseteq\mathbb{R}<sup>N$ can be approximated by a polyhedral or smooth bounded set $F$ in such a way that both the total perimeter of $E$ and the perimeter of $E$ inside $Ω$ are approximated by those of $F$, and the boundary of $F$ has negligible intersection with the boundary of $Ω$. In addition, we address the approximation for perimeter and volume with densities, and we present counterexamples illustrating the sharpness of our assumptions. Our constructions rely on a technical result that replaces $E$ with a set $F$ which agrees with $E$ and has the same boundary inside $Ω$, while sharing no common boundary with $Ω$, and does so without substantially altering the perimeter or the volume of the original set.