On various Carleson-type geometric lemmas and uniform rectifiability in metric spaces: Part 2

We characterize uniform $k$-rectifiability in Euclidean spaces in terms of a Carleson-type geometric lemma for a new notion of flatness coefficients, which we call $\iota$-numbers. The characterization follows from an abstract statement about approximation by generalized planes in metric spaces, which also applies to the study of low-dimensional sets in Heisenberg groups. A key aspect is that the $\iota$-coefficients are in general not pointwise comparable to the usual squared $\beta$-numbers for dyadic cubes on $k$-regular sets in ${\mathbb{R}}^n$, however our result implies that they are still equivalent in terms of a Carleson-type geometric lemma.