We consider the topological and geometric reconstruction of a geodesic subspace of [Formula: see text] both from the Čech and VietorisRips filtrations on a finite, Hausdorffclose, Euclidean sample. Our reconstruction technique leverages the intrinsic length metric induced by the geodesics on the subspace. We consider the distortion and convexity radius as our sampling parameters for the reconstruction problem. For a geodesic subspace with finite distortion and positive convexity radius, we guarantee a correct computation of its homotopy and homology groups from the sample. This technique provides alternative sampling conditions to the existing and commonly used conditions based on weak feature size and [Formula: see text]–reach, and performs better under certain types of perturbations of the geodesic subspace. For geodesic subspaces of [Formula: see text], we also devise an algorithm to output a homotopy equivalent geometric complex that has a very small Hausdorff distance to the unknown underlying space.
Metric Thickenings and Group Actions
Let [Formula: see text] be a group acting properly and by isometries on a metric space [Formula: see text]; it follows that the quotient or orbit space [Formula: see text] is also a metric space. We study the Vietoris–Rips and Čech complexes of [Formula: see text]. Whereas (co)homology theories for metric spaces let the scale parameter of a Vietoris–Rips or Čech complex go to zero, and whereas geometric group theory requires the scale parameter to be sufficiently large, we instead consider intermediate scale parameters (neither tending to zero nor to infinity). As a particular case, we study the Vietoris–Rips and Čech thickenings of projective spaces at the first scale parameter where the homotopy type changes.
 Publication Date:
 NSFPAR ID:
 10207695
 Journal Name:
 Journal of Topology and Analysis
 Page Range or eLocationID:
 1 to 27
 ISSN:
 17935253
 Sponsoring Org:
 National Science Foundation
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We derive conditions under which the reconstruction of a target space is topologically correct via the Čech complex or the VietorisRips complex obtained from possibly noisy point cloud data. We provide two novel theoretical results. First, we describe sufficient conditions under which any nonempty intersection of finitely many Euclidean balls intersected with a positive reach set is contractible, so that the Nerve theorem applies for the restricted Čech complex. Second, we demonstrate the homotopy equivalence of a positive μreach set and its offsets. Applying these results to the restricted Čech complex and using the interleaving relations with the Čech complex (or the VietorisRips complex), we formulate conditions guaranteeing that the target space is homotopy equivalent to the Čech complex (or the VietorisRips complex), in terms of the μreach. Our results sharpen existing results.

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