Stratified $\beta$-numbers and traveling salesman in Carnot groups

Abstract

We introduce a modified version of P. Jones's $\beta$-numbers for Carnot groups which we call {\it stratified $\beta$-numbers}. We show that an analogue of Jones's traveling salesman theorem on 1-rectifiability of sets holds for any Carnot group if we replace previous notions of $\beta$-numbers in Carnot groups with stratified $\beta$-numbers. As we generalize both directions of the traveling salesman theorem, we get a characterization of subsets of Carnot groups that lie on finite length rectifiable curves. Our proof expands upon previous analysis of the Hebisch-Sikora norm for Carnot groups. In particular, we find new estimates on the drift between almost parallel line segments that take advantage of the stratified $\beta$'s and also develop a Taylor expansion technique of the norm. We also give an example of a Carnot group for which a traveling salesman theorem based on the unmodified $\beta$-numbers must exhibit a gap between the necessary and sufficient directions.

Proved new statements about insufficiency of non-stratified beta numbers (Prop 1.4) as well as different characterizations of stratified betas (Prop 1.6). Implemented referee suggested simplifications of many proofs. Fixed many typos