Quantitative Alberti representations in spaces of bounded geometry

Abstract

A metric measure space $(X,d,\mu)$ is said to be $A_{\infty}$ on curves if there exist constants $\tau < 1$ and $\theta > 0$ with the following property. For every $x \in X$, $0 < r \leq \mathrm{diam}(X)$, and a Borel set $S \subset B(x,r)$ with $\mu(S) > \tau \mu(B(x,r))$, there exists a continuum $\gamma \subset X$ of length $\leq r$ satisfying $\mathcal{H}^{1}_{\infty}(\gamma \cap S) \geq \theta r$. I first observe that spaces of $Q$-bounded geometry, $Q > 1$, are $A_{\infty}$ on curves. Then, I show that any complete, doubling, and quasiconvex space $(X,d,\mu)$ which is $A_{\infty}$ on curves has Alberti representations with $L^{p}$-densities for some $p > 1$, depending only on the doubling and $A_{\infty}$-constants. More precisely, any normalised restriction of $\mu$ to a ball $B \subset X$ can be written as $\mu_{B} = f_{B} \, d\nu_{B}$, where $\nu_{B}$ is a convex combination of measures of linear growth supported on continua of length $\le \mathrm{diam}(B)$, and $\|f_{B}\|_{L^{p}(\nu_{B})} \leq C$ for some constant $C \geq 1$ independent of $B$.