Weak-type characterizations of Sobolev and bounded variation spaces on metric measure spaces

Abstract

Given a complete doubling metric measure space $(X,ρ,μ)$ supporting a Poincaré inequality, we prove weak-type characterizations of the Sobolev space $\dot{W}^{1,p}(μ)$ and the space of functions of bounded variation, achieving a full analogy in general Poincaré spaces with the Euclidean results of Brezis et al. [Anal. PDE 17 (2024), 943-979]. The main novelty is that the finiteness of a weak-type norm, which only refers to differences or mean oscillations of $f$ without assuming any smoothness a priori, already guarantees the membership of $f$ in the relevant Sobolev or BV space. This distinguishes our contribution from the recent work of F. Dai et al. [Adv. Math. 502 (2026), Paper No. 111153], where the related norm-equivalence was obtained under the a priori Lipschitz assumption on $f$. A key intermediate step in our approach is a new localized Bourgain-Brezis-Mironescu type characterization. More precisely, we prove that, if $p\in(1,\infty)$ and $γ\in\mathbb R\setminus\{0\}$, then, for any $f\in L^1_{\mathrm{loc}}(μ)$, \begin{equation*}\tag{$*$} \|f\|_{\dot W^{1,p}(μ)} \sim \|ρ^{-1}ϕ^{-γ}F\|_{L^{p,\infty}(ϕ^{γp}V^{-1})}, \qquad F\in\{Δf,m_f\},\quad ϕ\in\{ρ,V\}, \end{equation*} where the homogeneous Sobolev space $\dot{W}^{1,p}(μ)$ is defined by the minimal $p$-weak upper gradient and, for any $x,y\in X$, we denote $V(x,y):=μ(B(x,ρ(x,y)))$ and $Δf(x,y):=|f(x) - f(y)|$, and $m_f(x,y)$ is the mean oscillation of $f$ on the ball $B(x,ρ(x,y))$. For $p=1$, the equivalence $(*)$ holds after replacing $\|f\|_{\dot W^{1,1}(μ)}$ by a bounded variation norm and restricting the parameters to the optimal ranges $γ\in(-\infty,-1)\cup(0,\infty)$ for $ϕ=ρ$ or $γ\in (-\infty,-\frac1d)\cup(0,\infty)$ for $ϕ=V$, where $d\in(0,\infty)$ is the lower dimension of $X$.