Upper bounds for the relaxed area of $\mathbb S^1$-valued Sobolev maps and its countably subadditive interior envelope

Abstract

Given a bounded open connected Lipschitz set $Ω\subset \mathbb R^2$, we show that the relaxed Cartesian area functional $\overline{\mathcal A}(u,Ω)$ of a map $u\in W^{1,1}(Ω;\mathbb S^1)$ is finite, and provide a useful upper bound for its value. Using this estimate, we prove a modified version of a De Giorgi conjecture [17] adapted to $W^{1,1}(Ω;\mathbb S^1)$, on the largest countably subadditive set function $\overline {\overline{\mathcal A}}(u, \cdot)$ smaller than or equal to $\overline{\mathcal A}(u,\cdot)$.