An optimal class of domains permitting inner boundaries in a divergence theorem for rough integrands (2506.09978v1)

Abstract: The divergence theorem on an open domain $U \subset \mathbb{R}^m$ of finite measure is investigated for the rough class $\mathcal{DM}\infty(U)$ of bounded vector fields $u \, \colon \, U \to \mathbb{R}^m$ with a finite Radon measure divergence. Let $\mathcal{DB}(U)$ denote the closure of $\mathcal{DM}\infty(U)$ in the Lebesgue space $L_\infty(U)$. Equivalent conditions are proved for the domain $U$ to admit the divergence theorem \begin{equation} \exists ! \ell \in \mathcal{DB}'(U) \, \colon \, \int_U \, \mathrm{d} \, \mathrm{div} u = \ell(u) \quad \forall u \in \mathcal{DM}\infty(U), \end{equation} where the functional $\ell$ generalizes classical surface measure by being an equivalence class of finitely additive measures concentrated on the domain boundary. Namely, the following are equivalent to this divergence theorem: (a) the divergence integral $$ \mathcal{DM}\infty(U) \to \mathbb{R} \, \colon \, u \mapsto \int_U \, \mathrm{d} \, \mathrm{div} u $$ is continuous in the uniform topology, (b) the non-exterior boundary $\partial U \setminus \mathrm{ext}* U$ has finite 1-codimensional Hausdorff measure, where $\partial U$ is the topological boundary, and $\mathrm{ext}* U$ is the measure theoretic exterior, (c) the set indicator function $\chi_U$ is a (sequential) limit from below of smooth functions with gradient uniformly bounded in $L_1$, (d) the set $U$ is a (sequential) limit from inside of smooth sets with uniformly bounded perimeter. This provides an optimal regularity class of domains on which the divergence theorem for $\mathcal{DM}_\infty$-fields holds with a surface functional that is continuous in the uniform topology. Unlike the classical formulation, the new formulation can take discontinuities of the integrand along 1-codimensional inner boundaries into account.