Searcharxiv⌕ Search

arXiv · 0709.2512

Quantifying Homology Classes II: Localization and Stability

Abstract

In the companion paper, we measured homology classes and computed the optimal homology basis. This paper addresses two related problems, namely, localization and stability. We localize a class with the cycle minimizing a certain objective function. We explore three different objective functions, namely, volume, diameter and radius. We show that it is NP-hard to compute the smallest cycle using the former two. We also prove that the measurement defined in the companion paper is stable with regard to small changes of the geometry of the concerned space.

Explore related subjects

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Chao Chen, Daniel Freedman. 2007-09-21. Quantifying Homology Classes II: Localization and Stability. https://arxiv.org/abs/0709.2512

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

Spherical Geometrical Bases of Spherical Origami

This paper establishes a rigorous geometrical framework for spherical origami (origami using spherical sheets based on spherical geometry) in two settings: origami restricted to the unit sphere $\mathbb{S}^2$, and three-dimensional folding of spherical sheets in space. Three main results are proved. First, all seven Huzita--Justin axioms admit explicit constructions on $\mathbb{S}^2$, so spherical origami inherits the same one-fold axiom structure as Euclidean origami. Second, the Kawasaki--Justin theorem on flat-foldability extends to $\mathbb{S}^2$ by a local-tangent-plane argument, and the resulting vertex condition is equivalent to the angle-sum condition that characterizes Robertson's spherical $f$-tilings. Third, taking equidistant curves (small arcs) as fold curves yields a one-parameter family of isometric 3D folds, parametrized by the dihedral angle whose closed-form expression is derived, and recovers geodesic folds as the flat limit. The framework is illustrated by constructing computer graphics of spherical origami birds as a concrete example.

cs.CG↗

A Bicriteria Approximation Scheme for the Minimum Well-Separated Pair Decomposition

A well-separated pair decomposition (WSPD) of a set $P$ of $n$ points covers all $\binom{n}{2}$ pairs of points by a family of dumbbells $\{X,Y\}$, each covering the pairs of $X\times Y$, such that the diameters of $X$ and $Y$ are at most an $\varepsilon$-fraction of their distance. In $\mathbb{R}^d$, WSPDs with $O(n/\varepsilon^d)$ dumbbells always exist, but many inputs admit much smaller ones. However, computing a WSPD of minimum size is NP-hard in the plane. For $d\ge2$, the only known approximation that preserves the separation has a factor of $2^{O(d)}$, and Buchin et al. (ESA 2026) asked whether a $(1+δ)$-approximation exists. We give a bicriteria approximation scheme: for fixed $d$ and $δ\in(0,1]$, we compute in $f(d,δ)\cdot n^{6}$ time a $(1+δ)\varepsilon$-WSPD whose size is at most $(1+δ)$ times the minimum size of an $\varepsilon$-WSPD. The same holds in metric spaces of bounded doubling dimension. The algorithm combines shifting over scales with the local-growth technique of Nieberg, Hurink, and Kern, applied to a graph whose vertices are the pairs of points. In contrast, we show that in general metric spaces, relaxing the separation by a factor below $2$ keeps the problem hard: for every fixed rational $\varepsilon\le1/4$, it is NP-hard to compute such a relaxed WSPD whose size is within a factor $n^c$ of the minimum, for every $c<2/3$.

cs.CG↗

Tiling 3D by Translates of a Single Polycube is Undecidable

We prove co-RE-completeness, and thus undecidability, of the following problem: given a single (connected) polycube, decide whether it tiles 3D Euclidean space by translations. We reduce from Wang tiling using the decorated two-prime Sudoku construction of Greenfeld and Tao and a cyclic encoding adapted from OpenAI's 3D aperiodic tile, and apply a reduction of Kim to make the prototile connected (via faces). Dimension three is optimal: translational monotiling is known to be decidable in $\mathbb{Z}^2$ and for a single (possibly disconnected) polyomino in $\mathbb{R}^2$.

cs.CG↗