On indecomposable sets with applications

Abstract

In this note we show the characteristic function of every indecomposable set $F$ in the plane is $BV$ equivalent to the characteristic function a closed set $\mathbb{F}$, i.e. $||\mathbb{1}_{F}-\mathbb{1}_{\mathbb{F}}||_{BV(\mathbb{R}^2)}=0$. We show by example this is false in dimension three and above. As a corollary to this result we show that for every $ε>0$ a set of finite perimeter $S$ can be approximated by a closed subset $\mathbb{S}_ε$ with finitely many indecomposable components and with the property that $H^1(\partial^M \mathbb{S}_ε\backslash \partial^M S)=0$ and $||\mathbb{1}_{S}-\mathbb{1}_{\mathbb{S}_ε}||_{BV(\mathbb{R}^2)}<ε$. We apply this corollary to give a short proof that locally quasiminimizing sets in the plane are $BV_l$ extension domains.