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Intrinsic rectifiability via flat cones in the Heisenberg group. (English) Zbl 07558535

Given a constant \(\alpha > 0\), a flat cone \(\mathtt{fC}(\alpha)\) in \(\mathbb{R}^3\) is a set of the form \[ \{ (x,y,0) : |y| < \alpha |x| \}. \] A set \(S\) in the first sub-Riemannian Heisenberg group \(\mathbb{H}\) has the flat cone property if there is some \(\alpha > 0\) such that \[ \mathtt{fC}(\alpha) \cap p^{-1} S = \emptyset \] for all \(p \in \mathbb{H}\). Intrinsic Lipschitz graphs may be defined as those sets in \(\mathbb{H}\) such that, for some open cone \(C \subset \mathbb{H}\) that intersects the \(xy\)-plane nontrivially and satisfies \(C = -C\), we have \[ C \cap p^{-1} S = \emptyset \] for all \(p \in \mathbb{H}\). Since any such open cone contains a flat cone as a subset, it follows that every intrinsic Lipschitz graph in \(\mathbb{H}\) satisfies the flat cone property. In the paper under review, the authors prove the reverse implication for topological surfaces. That is, any topological surface satisfying the flat cone property must locally be an intrinsic Lipschitz graph. This is obtained by noticing the following crucial fact: translating the \(xy\)-plane by a point \((x,y,0)\) results in an affine plane, and the family of planes created by fixing \(y\) and allowing \(x\) to be arbitrary all contain a common line in the \(xz\)-plane.

MSC:

53C17 Sub-Riemannian geometry
28A75 Length, area, volume, other geometric measure theory
22E25 Nilpotent and solvable Lie groups

References:

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[12] Orsay, France antoine.julia@universite-paris-saclay.fr Department of Mathematics and Statistics 40014 University of Jyväskylä, Finland sebastiano2.72@gmail.com
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