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- Title
Embedding the Heisenberg group into a bounded-dimensional Euclidean space with optimal distortion.
- Authors
Tao, Terence
- Abstract
Let H:=... denote the Heisenberg group with the usual Carnot-Caratheodory metric d. It is known (since the work of Pansu and Semmes) that the metric space (H, d) cannot be embedded in a bilipschitz fashion into a Hilbert space; however, from a general theorem of Assouad, for any 0 < ε ≤ 1/2, the snowflaked metric space (H, d1-ε) embeds into an infinite-dimensional Hilbert space with distortion O(e-1/2). This distortion bound was shown by Austin, Naor, and Tessera to be sharp for the Heisenberg group H. Assouad's argument allows I² to be replaced by RD(ε) for some dimension D(ε) dependent on ε. Naor and Neiman showed that D could be taken independent of ε, at the cost of worsening the bound on the distortion to O(ε-1-cD ), where cD → as D → ∞. In this paper we show that one can in fact retain the optimal distortion bound O(ε-1/2) and still embed into a bounded-dimensional space RD, answering a question of Naor and Neiman. As a corollary, the discrete ball of radius R ≥ 2 in r :=... can be embedded into a bounded-dimensional space RD with the optimal distortion bound of O(log1/2 R). The construction is iterative, and is inspired by the Nash-Moser iteration scheme as used in the isometric embedding problem; this scheme is needed in order to counteract a certain "loss of derivatives" problem in the iteration.
- Subjects
METRIC spaces; HILBERT space; ARGUMENT; ISOMETRICS (Mathematics)
- Publication
Revista Mathematica Iberoamericana, 2021, Vol 37, Issue 2, p1
- ISSN
0213-2230
- Publication type
Article
- DOI
10.4171/rmi/1200