{"title":"On the Maximal Distance Between the Centers of Mass of a Planar Convex Body and Its Boundary","authors":"Fedor Nazarov, Dmitry Ryabogin, Vladyslav Yaskin","doi":"10.1007/s00454-024-00650-0","DOIUrl":"https://doi.org/10.1007/s00454-024-00650-0","url":null,"abstract":"<p>We prove that the length of the projection of the vector joining the centers of mass of a convex body on the plane and of its boundary to an arbitrary direction does not exceed <span>(frac{1}{6})</span> of the body width in this direction. It follows that the distance between these centers of mass does not exceed <span>(frac{1}{6})</span> of the diameter of the body and <span>(frac{1}{12})</span> of its boundary length. None of those constants can be improved.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"30 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2024-05-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140889499","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Convex Bodies of Constant Width with Exponential Illumination Number","authors":"Andrii Arman, Andrii Bondarenko, Andriy Prymak","doi":"10.1007/s00454-024-00647-9","DOIUrl":"https://doi.org/10.1007/s00454-024-00647-9","url":null,"abstract":"<p>We show that there exist convex bodies of constant width in <span>({mathbb {E}}^n)</span> with illumination number at least <span>((cos (pi /14)+o(1))^{-n})</span>, answering a question by Kalai. Furthermore, we prove the existence of finite sets of diameter 1 in <span>({mathbb {E}}^n)</span> which cannot be covered by <span>((2/sqrt{3}-o(1))^{n})</span> balls of diameter 1, improving a result of Bourgain and Lindenstrauss.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"118 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2024-05-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140888833","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Finitary Affine Oriented Matroids","authors":"Emanuele Delucchi, Kolja Knauer","doi":"10.1007/s00454-024-00651-z","DOIUrl":"https://doi.org/10.1007/s00454-024-00651-z","url":null,"abstract":"<p>We initiate the axiomatic study of affine oriented matroids (AOMs) on arbitrary ground sets, obtaining fundamental notions such as minors, reorientations and a natural embedding into the frame work of Complexes of Oriented Matroids. The restriction to the finitary case (FAOMs) allows us to study tope graphs and covector posets, as well as to view FAOMs as oriented finitary semimatroids. We show shellability of FAOMs and single out the FAOMs that are affinely homeomorphic to <span>(mathbb {R}^n)</span>. Finally, we study group actions on AOMs, whose quotients in the case of FAOMs are a stepping stone towards a general theory of affine and toric pseudoarrangements. Our results include applications of the multiplicity Tutte polynomial of group actions of semimatroids, generalizing enumerative properties of toric arrangements to a combinatorially defined class of arrangements of submanifolds. This answers partially a question by Ehrenborg and Readdy.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"42 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2024-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140826676","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Niklas Christoph Affolter, Béatrice de Tilière, Paul Melotti
{"title":"The Schwarzian Octahedron Recurrence (dSKP Equation) II: Geometric Systems","authors":"Niklas Christoph Affolter, Béatrice de Tilière, Paul Melotti","doi":"10.1007/s00454-024-00640-2","DOIUrl":"https://doi.org/10.1007/s00454-024-00640-2","url":null,"abstract":"<p>We consider nine geometric systems: Miquel dynamics, P-nets, integrable cross-ratio maps, discrete holomorphic functions, orthogonal circle patterns, polygon recutting, circle intersection dynamics, (corrugated) pentagram maps and the short diagonal hyperplane map. Using a unified framework, for each system we prove an explicit expression for the solution as a function of the initial data; more precisely, we show that the solution is equal to the ratio of two partition functions of an oriented dimer model on an Aztec diamond whose face weights are constructed from the initial data. Then, we study the Devron property (Glick in J Geom Phys 87:161–189, 2015), which states the following: if the system starts from initial data that is singular for the backwards dynamics, this singularity is expected to reoccur after a finite number of steps of the forwards dynamics. Again, using a unified framework, we prove this Devron property for all of the above geometric systems, for different kinds of singular initial data. In doing so, we obtain new singularity results and also known ones (Glick in J Geom Phys 87:161–189, 2015; Yao in Glick’s conjecture on the point of collapse of axis-aligned polygons under the pentagram maps (2014). Preprint arXiv:1410.7806). Our general method consists in proving that these nine geometric systems are all related to the Schwarzian octahedron recurrence (dSKP equation), and then to rely on the companion paper (Affolter et al. in Comb Theory 3(2), 2023), where we study this recurrence in general, prove explicit expressions and singularity results.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"1 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2024-04-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140806042","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Igor Araujo, Bryce Frederickson, Robert A. Krueger, Bernard Lidický, Tyrrell B. McAllister, Florian Pfender, Sam Spiro, Eric Nathan Stucky
{"title":"Triangle Percolation on the Grid","authors":"Igor Araujo, Bryce Frederickson, Robert A. Krueger, Bernard Lidický, Tyrrell B. McAllister, Florian Pfender, Sam Spiro, Eric Nathan Stucky","doi":"10.1007/s00454-024-00645-x","DOIUrl":"https://doi.org/10.1007/s00454-024-00645-x","url":null,"abstract":"<p>We consider a geometric percolation process partially motivated by recent work of Hejda and Kala. Specifically, we start with an initial set <span>(X subseteq {mathbb {Z}}^2)</span>, and then iteratively check whether there exists a triangle <span>(T subseteq {mathbb {R}}^2)</span> with its vertices in <span>({mathbb {Z}}^2)</span> such that <i>T</i> contains exactly four points of <span>({mathbb {Z}}^2)</span> and exactly three points of <i>X</i>. In this case, we add the missing lattice point of <i>T</i> to <i>X</i>, and we repeat until no such triangle exists. We study the limit sets <i>S</i>, the sets stable under this process, including determining their possible densities and some of their structure.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"65 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2024-04-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140805990","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Curvature Sets Over Persistence Diagrams","authors":"Mario Gómez, Facundo Mémoli","doi":"10.1007/s00454-024-00634-0","DOIUrl":"https://doi.org/10.1007/s00454-024-00634-0","url":null,"abstract":"<p>We study a family of invariants of compact metric spaces that combines the Curvature Sets defined by Gromov in the 1980 s with Vietoris–Rips Persistent Homology. For given integers <span>(kge 0)</span> and <span>(nge 1)</span> we consider the dimension <i>k</i> Vietoris–Rips persistence diagrams of <i>all</i> subsets of a given metric space with cardinality at most <i>n</i>. We call these invariants <i>persistence sets</i> and denote them as <span>({textbf{D}}_{n,k}^{textrm{VR}})</span>. We first point out that this family encompasses the usual Vietoris–Rips diagrams. We then establish that (1) for certain range of values of the parameters <i>n</i> and <i>k</i>, computing these invariants is significantly more efficient than computing the usual Vietoris–Rips persistence diagrams, (2) these invariants have very good discriminating power and, in many cases, capture information that is imperceptible through standard Vietoris–Rips persistence diagrams, and (3) they enjoy stability properties analogous to those of the usual Vietoris–Rips persistence diagrams. We precisely characterize some of them in the case of spheres and surfaces with constant curvature using a generalization of Ptolemy’s inequality. We also identify a rich family of metric graphs for which <span>({textbf{D}}_{4,1}^{textrm{VR}})</span> fully recovers their homotopy type by studying split-metric decompositions. Along the way we prove some useful properties of Vietoris–Rips persistence diagrams using Mayer–Vietoris sequences. These yield a geometric algorithm for computing the Vietoris–Rips persistence diagram of a space <i>X</i> with cardinality <span>(2k+2)</span> with quadratic time complexity as opposed to the much higher cost incurred by the usual algebraic algorithms relying on matrix reduction.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"17 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2024-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140634134","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Inductive Freeness of Ziegler’s Canonical Multiderivations","authors":"Torsten Hoge, Gerhard Röhrle","doi":"10.1007/s00454-024-00644-y","DOIUrl":"https://doi.org/10.1007/s00454-024-00644-y","url":null,"abstract":"<p>Let <span>({{mathscr {A}}})</span> be a free hyperplane arrangement. In 1989, Ziegler showed that the restriction <span>({{mathscr {A}}}'')</span> of <span>({{mathscr {A}}})</span> to any hyperplane endowed with the natural multiplicity <span>(kappa )</span> is then a free multiarrangement <span>(({{mathscr {A}}}'',kappa ))</span>. The aim of this paper is to prove an analogue of Ziegler’s theorem for the stronger notion of inductive freeness: if <span>({{mathscr {A}}})</span> is inductively free, then so is the multiarrangement <span>(({{mathscr {A}}}'',kappa ))</span>. In a related result we derive that if a deletion <span>({{mathscr {A}}}')</span> of <span>({{mathscr {A}}})</span> is free and the corresponding restriction <span>({{mathscr {A}}}'')</span> is inductively free, then so is <span>(({{mathscr {A}}}'',kappa ))</span>—irrespective of the freeness of <span>({{mathscr {A}}})</span>. In addition, we show counterparts of the latter kind for additive and recursive freeness.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"1 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2024-04-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140634127","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Numerical Semigroups via Projections and via Quotients","authors":"Tristram Bogart, Christopher O’Neill, Kevin Woods","doi":"10.1007/s00454-024-00643-z","DOIUrl":"https://doi.org/10.1007/s00454-024-00643-z","url":null,"abstract":"<p>We examine two natural operations to create numerical semigroups. We say that a numerical semigroup <span>({mathcal {S}})</span> is <i>k</i>-normalescent if it is the projection of the set of integer points in a <i>k</i>-dimensional polyhedral cone, and we say that <span>({mathcal {S}})</span> is a <i>k</i>-quotient if it is the quotient of a numerical semigroup with <i>k</i> generators. We prove that all <i>k</i>-quotients are <i>k</i>-normalescent, and although the converse is false in general, we prove that the projection of the set of integer points in a cone with <i>k</i> extreme rays (possibly lying in a dimension smaller than <i>k</i>) is a <i>k</i>-quotient. The discrete geometric perspective of studying cones is useful for studying <i>k</i>-quotients: in particular, we use it to prove that the sum of a <span>(k_1)</span>-quotient and a <span>(k_2)</span>-quotient is a <span>((k_1+k_2))</span>-quotient. In addition, we prove several results about when a numerical semigroup is <i>not</i> <i>k</i>-normalescent.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"10 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2024-04-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140574271","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"On the Banach–Mazur Distance in Small Dimensions","authors":"Tomasz Kobos, Marin Varivoda","doi":"10.1007/s00454-024-00641-1","DOIUrl":"https://doi.org/10.1007/s00454-024-00641-1","url":null,"abstract":"<p>We establish some results on the Banach–Mazur distance in small dimensions. Specifically, we determine the Banach–Mazur distance between the cube and its dual (the cross-polytope) in <span>(mathbb {R}^3)</span> and <span>(mathbb {R}^4)</span>. In dimension three this distance is equal to <span>(frac{9}{5})</span>, and in dimension four, it is equal to 2. These findings confirm well-known conjectures, which were based on numerical data. Additionally, in dimension two, we use the asymmetry constant to provide a geometric construction of a family of convex bodies that are equidistant to all symmetric convex bodies.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"17 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2024-04-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140574198","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Affine Stresses: The Partition of Unity and Kalai’s Reconstruction Conjectures","authors":"Isabella Novik, Hailun Zheng","doi":"10.1007/s00454-024-00642-0","DOIUrl":"https://doi.org/10.1007/s00454-024-00642-0","url":null,"abstract":"<p>Kalai conjectured that if <i>P</i> is a simplicial <i>d</i>-polytope that has no missing faces of dimension <span>(d-1)</span>, then the graph of <i>P</i> and the space of affine 2-stresses of <i>P</i> determine <i>P</i> up to affine equivalence. We propose a higher-dimensional generalization of this conjecture: if <span>(2le ile d/2)</span> and <i>P</i> is a simplicial <i>d</i>-polytope that has no missing faces of dimension <span>(ge d-i+1)</span>, then the space of affine <i>i</i>-stresses of <i>P</i> determines the space of affine 1-stresses of <i>P</i>. We prove this conjecture for (1) <i>k</i>-stacked <i>d</i>-polytopes with <span>(2le ile kle d/2-1)</span>, (2) <i>d</i>-polytopes that have no missing faces of dimension <span>(ge d-2i+2)</span>, and (3) flag PL <span>((d-1))</span>-spheres with generic embeddings (for all <span>(2le ile d/2)</span>). We also discuss several related results and conjectures. For instance, we show that if <i>P</i> is a simplicial <i>d</i>-polytope that has no missing faces of dimension <span>(ge d-2i+2)</span>, then the <span>((i-1))</span>-skeleton of <i>P</i> and the set of sign vectors of affine <i>i</i>-stresses of <i>P</i> determine the combinatorial type of <i>P</i>. Along the way, we establish the partition of unity of affine stresses: for any <span>(1le ile (d-1)/2)</span>, the space of affine <i>i</i>-stresses of a simplicial <i>d</i>-polytope as well as the space of affine <i>i</i>-stresses of a simplicial <span>((d-1))</span>-sphere (with a generic embedding) can be expressed as the sum of affine <i>i</i>-stress spaces of vertex stars. This is analogous to Adiprasito’s partition of unity of linear stresses for Cohen–Macaulay complexes.</p>","PeriodicalId":50574,"journal":{"name":"Discrete & Computational Geometry","volume":"107 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2024-04-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140574193","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}