Connections between metric differentiability and rectifiability

We combine Kirchheim’s metric differentials with Cheeger charts in order to establish a non-embeddability principle for any collection $\mathcal{C}$ of Banach (or metric) spaces: if a metric measure space $X$ bi-Lipschitz embeds in some element in $\mathcal{C}$, and if every Lipschitz map $X\to Y\in \mathcal{C}$ is differentiable, then $X$ is rectifiable. This gives a simple proof of the rectifiability of Lipschitz differentiability spaces that are bi-Lipschitz embeddable in Euclidean space, due to Kell–Mondino. Our principle also implies a converse to Kirchheim’s theorem: if all Lipschitz maps from a domain space to arbitrary targets are metrically differentiable, the domain is rectifiable. We moreover establish the compatibility of metric and w${}^{\ast }$-differentials of maps from metric spaces in the spirit of Ambrosio–Kirchheim.