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Convex Hulls of Random Order Types

Published: 16 January 2023 Publication History

Abstract

We establish the following two main results on order types of points in general position in the plane (realizable simple planar order types, realizable uniform acyclic oriented matroids of rank 3):
(a)
The number of extreme points in an n-point order type, chosen uniformly at random from all such order types, is on average 4+o(1). For labeled order types, this number has average \(4- \mbox{$\frac{8}{n^2 - n +2}$}\) and variance at most 3.
(b)
The (labeled) order types read off a set of n points sampled independently from the uniform measure on a convex planar domain, smooth or polygonal, or from a Gaussian distribution are concentrated, i.e., such sampling typically encounters only a vanishingly small fraction of all order types of the given size.
Result (a) generalizes to arbitrary dimension d for labeled order types with the average number of extreme points 2d+o (1) and constant variance. We also discuss to what extent our methods generalize to the abstract setting of uniform acyclic oriented matroids. Moreover, our methods show the following relative of the Erdős-Szekeres theorem: for any fixed k, as n → ∞, a proportion 1 - O(1/n) of the n-point simple order types contain a triangle enclosing a convex k-chain over an edge.
For the unlabeled case in (a), we prove that for any antipodal, finite subset of the two-dimensional sphere, the group of orientation preserving bijections is cyclic, dihedral, or one of A4, S4, or A5 (and each case is possible). These are the finite subgroups of SO(3) and our proof follows the lines of their characterization by Felix Klein.

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  1. Convex Hulls of Random Order Types

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    Published In

    cover image Journal of the ACM
    Journal of the ACM  Volume 70, Issue 1
    February 2023
    405 pages
    ISSN:0004-5411
    EISSN:1557-735X
    DOI:10.1145/3572730
    Issue’s Table of Contents

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    Association for Computing Machinery

    New York, NY, United States

    Publication History

    Published: 16 January 2023
    Online AM: 02 December 2022
    Accepted: 07 October 2022
    Revised: 31 May 2022
    Received: 15 September 2020
    Published in JACM Volume 70, Issue 1

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    Author Tags

    1. Order type
    2. oriented matroid
    3. Sylvester’s Four-Point Problem
    4. random polytope
    5. sampling random order types
    6. projective plane
    7. excluded pattern
    8. Hadwiger’s transversal theorem
    9. hairy ball theorem
    10. finite subgroups of SO(3)

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    • Research-article
    • Refereed

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    • ASPAG
    • Agence Nationale de la Recherche
    • Institut Universitaire de France
    • Swiss National Science Foundation
    • Arrangements and Drawings as SNSF

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