Geometric Unity Constructions
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Self-Dual Polygons and Self-Dual Curves, with D. Fuchs
Funct. Anal. Other Math. (2009) 2: 203–220 DOI 10.1007/s11853-008-0020-5 FAOM Self-dual polygons and self-dual curves Dmitry Fuchs · Serge Tabachnikov Received: 23 July 2007 / Revised: 1 September 2007 / Published online: 20 November 2008 © PHASIS GmbH and Springer-Verlag 2008 Abstract We study projectively self-dual polygons and curves in the projective plane. Our results provide a partial answer to problem No 1994-17 in the book of Arnold’s problems (2004). Keywords Projective plane · Projective duality · Polygons · Legendrian curves · Radon curves Mathematics Subject Classification (2000) 51A99 · 53A20 1 Introduction The projective plane P is the projectivization of 3-dimensional space V (we consider the cases of two ground fields, R and C); the points and the lines of P are 1- and 2-dimensional subspaces of V . The dual projective plane P ∗ is the projectivization of the dual space V ∗. Assign to a subspace in V its annihilator in V ∗. This gives a correspondence between the points in P and the lines in P ∗, and between the lines in P and the points in P ∗, called the projective duality. Projective duality preserves the incidence relation: if a point A belongs to a line B in P then the dual point B∗ belongs to the dual line A∗ in P ∗. Projective duality is an involution: (A∗)∗ = A. Projective duality extends to polygons in P .Ann-gon is a cyclically ordered collection of n points and n lines satisfying the incidences: two consecutive vertices lie on the respec- tive side, and two consecutive sides pass through the respective vertex. -
An FPT Algorithm for the Embeddability of Graphs Into Two-Dimensional Simplicial Complexes∗
An FPT Algorithm for the Embeddability of Graphs into Two-Dimensional Simplicial Complexes∗ Éric Colin de Verdière† Thomas Magnard‡ Abstract We consider the embeddability problem of a graph G into a two-dimensional simplicial com- plex C: Given G and C, decide whether G admits a topological embedding into C. The problem is NP-hard, even in the restricted case where C is homeomorphic to a surface. It is known that the problem admits an algorithm with running time f(c)nO(c), where n is the size of the graph G and c is the size of the two-dimensional complex C. In other words, that algorithm is polynomial when C is fixed, but the degree of the polynomial depends on C. We prove that the problem is fixed-parameter tractable in the size of the two-dimensional complex, by providing a deterministic f(c)n3-time algorithm. We also provide a randomized algorithm with O(1) expected running time 2c nO(1). Our approach is to reduce to the case where G has bounded branchwidth via an irrelevant vertex method, and to apply dynamic programming. We do not rely on any component of the existing linear-time algorithms for embedding graphs on a fixed surface; the only elaborated tool that we use is an algorithm to compute grid minors. 1 Introduction An embedding of a graph G into a host topological space X is a crossing-free topological drawing of G into X. The use and computation of graph embeddings is central in the communities of computational topology, topological graph theory, and graph drawing. -
The Octagonal Pets by Richard Evan Schwartz
The Octagonal PETs by Richard Evan Schwartz 1 Contents 1 Introduction 11 1.1 WhatisaPET?.......................... 11 1.2 SomeExamples .......................... 12 1.3 GoalsoftheMonograph . 13 1.4 TheOctagonalPETs . 14 1.5 TheMainTheorem: Renormalization . 15 1.6 CorollariesofTheMainTheorem . 17 1.6.1 StructureoftheTiling . 17 1.6.2 StructureoftheLimitSet . 18 1.6.3 HyperbolicSymmetry . 21 1.7 PolygonalOuterBilliards. 22 1.8 TheAlternatingGridSystem . 24 1.9 ComputerAssists ......................... 26 1.10Organization ........................... 28 2 Background 29 2.1 LatticesandFundamentalDomains . 29 2.2 Hyperplanes............................ 30 2.3 ThePETCategory ........................ 31 2.4 PeriodicTilesforPETs. 32 2.5 TheLimitSet........................... 34 2.6 SomeHyperbolicGeometry . 35 2.7 ContinuedFractions . 37 2.8 SomeAnalysis........................... 39 I FriendsoftheOctagonalPETs 40 3 Multigraph PETs 41 3.1 TheAbstractConstruction. 41 3.2 TheReflectionLemma . 43 3.3 ConstructingMultigraphPETs . 44 3.4 PlanarExamples ......................... 45 3.5 ThreeDimensionalExamples . 46 3.6 HigherDimensionalGeneralizations . 47 2 4 TheAlternatingGridSystem 48 4.1 BasicDefinitions ......................... 48 4.2 CompactifyingtheGenerators . 50 4.3 ThePETStructure........................ 52 4.4 CharacterizingthePET . 54 4.5 AMoreSymmetricPicture. 55 4.5.1 CanonicalCoordinates . 55 4.5.2 TheDoubleFoliation . 56 4.5.3 TheOctagonalPETs . 57 4.6 UnboundedOrbits ........................ 57 4.7 TheComplexOctagonalPETs. 58 4.7.1 ComplexCoordinates. -
Arxiv:1812.02806V3 [Math.GT] 27 Jun 2019 Sphere Formed by the Triangles Bounds a Ball and the Vertex in the Identification Space Looks Like the Centre of a Ball
TRAVERSING THREE-MANIFOLD TRIANGULATIONS AND SPINES J. HYAM RUBINSTEIN, HENRY SEGERMAN AND STEPHAN TILLMANN Abstract. A celebrated result concerning triangulations of a given closed 3–manifold is that any two triangulations with the same number of vertices are connected by a sequence of so-called 2-3 and 3-2 moves. A similar result is known for ideal triangulations of topologically finite non-compact 3–manifolds. These results build on classical work that goes back to Alexander, Newman, Moise, and Pachner. The key special case of 1–vertex triangulations of closed 3–manifolds was independently proven by Matveev and Piergallini. The general result for closed 3–manifolds can be found in work of Benedetti and Petronio, and Amendola gives a proof for topologically finite non-compact 3–manifolds. These results (and their proofs) are phrased in the dual language of spines. The purpose of this note is threefold. We wish to popularise Amendola’s result; we give a combined proof for both closed and non-compact manifolds that emphasises the dual viewpoints of triangulations and spines; and we give a proof replacing a key general position argument due to Matveev with a more combinatorial argument inspired by the theory of subdivisions. 1. Introduction Suppose you have a finite number of triangles. If you identify edges in pairs such that no edge remains unglued, then the resulting identification space looks locally like a plane and one obtains a closed surface, a two-dimensional manifold without boundary. The classification theorem for surfaces, which has its roots in work of Camille Jordan and August Möbius in the 1860s, states that each closed surface is topologically equivalent to a sphere with some number of handles or crosscaps. -
Volume 2 Shape and Space
Volume 2 Shape and Space Colin Foster Introduction Teachers are busy people, so I’ll be brief. Let me tell you what this book isn’t. • It isn’t a book you have to make time to read; it’s a book that will save you time. Take it into the classroom and use ideas from it straight away. Anything requiring preparation or equipment (e.g., photocopies, scissors, an overhead projector, etc.) begins with the word “NEED” in bold followed by the details. • It isn’t a scheme of work, and it isn’t even arranged by age or pupil “level”. Many of the ideas can be used equally well with pupils at different ages and stages. Instead the items are simply arranged by topic. (There is, however, an index at the back linking the “key objectives” from the Key Stage 3 Framework to the sections in these three volumes.) The three volumes cover Number and Algebra (1), Shape and Space (2) and Probability, Statistics, Numeracy and ICT (3). • It isn’t a book of exercises or worksheets. Although you’re welcome to photocopy anything you wish, photocopying is expensive and very little here needs to be photocopied for pupils. Most of the material is intended to be presented by the teacher orally or on the board. Answers and comments are given on the right side of most of the pages or sometimes on separate pages as explained. This is a book to make notes in. Cross out anything you don’t like or would never use. Add in your own ideas or references to other resources. -
Fun Problems in Geometry and Beyond
Symmetry, Integrability and Geometry: Methods and Applications SIGMA 15 (2019), 097, 21 pages Fun Problems in Geometry and Beyond Boris KHESIN †y and Serge TABACHNIKOV ‡z †y Department of Mathematics, University of Toronto, Toronto, ON M5S 2E4, Canada E-mail: [email protected] URL: http://www.math.toronto.edu/khesin/ ‡z Department of Mathematics, Pennsylvania State University, University Park, PA 16802, USA E-mail: [email protected] URL: http://www.personal.psu.edu/sot2/ Received November 17, 2019; Published online December 11, 2019 https://doi.org/10.3842/SIGMA.2019.097 Abstract. We discuss fun problems, vaguely related to notions and theorems of a course in differential geometry. This paper can be regarded as a weekend “treasure\treasure chest”chest" supplemen- ting the course weekday lecture notes. The problems and solutions are not original, while their relation to the course might be so. Key words: clocks; spot it!; hunters; parking; frames; tangents; algebra; geometry 2010 Mathematics Subject Classification: 00A08; 00A09 To Dmitry Borisovich Fuchs on the occasion of his 80th anniversary Áîëüøå çàäà÷, õîðîøèõ è ðàçíûõ! attributed to Winston Churchill (1918) This is a belated birthday present to Dmitry Borisovich Fuchs, a mathematician young at 1 heart.1 Many of these problems belong to the mathematical folklore. We did not attempt to trace them back to their origins (which might be impossible anyway), and the given references are not necessarily the first mentioning of these problems. 1 Problems 1. AA wall wall clock clock has has the the minute minute and and hour hour hands hands of of equal equal length. -
Arxiv:Math/0405482V2 [Math.CO] 12 Sep 2016 (A) Lozenge Tiling (B) Lozenge Dual (C) Rhombus Tiling (D) Rhombus Dual
FROM DOMINOES TO HEXAGONS DYLAN P. THURSTON Abstract. There is a natural generalization of domino tilings to tilings of a polygon by hexagons, or, dually, configurations of oriented curves that meet in triples. We show exactly when two such tilings can be connected by a series of moves analogous to the domino flip move. The triple diagrams that result have connections to Legendrian knots, cluster algebras, and planar algebras. 1. Introduction The study of tilings of a planar region by lozenges, as in Figure 1(a), or more generally by rhombuses, as in Figure 1(c), has a long history in combinatorics. Interesting questions include deciding when a region can be tiled, connectivity of the space of tilings under the basic move $ , counting the number of tilings, and behavior of a random tiling. One useful tool for studying these tilings is the dual picture, as in Figures 1(b). It consists of replacing each rhombus in the tiling by a cross of two strands connecting opposite, parallel sides. If we trace a strand through the tiling it comes out on a parallel face on the opposite side of the region, independently of the tiling of the region. Lozenge tilings give patterns of strands with three families of parallel (non-intersecting strands). If we drop the restriction to three families of parallel strands, as in Figure 1(d), and take any set of strands connecting boundary points which are both generic (only two strands intersect at a time) and minimal (no strand intersects itself and no two strands intersect more than once), then there is a corresponding dual tiling by rhombuses, as in Figure 1(c). -
Annales Sciennifiques Supérieuke D L ÉCOLE Hormale
ISSN 0012-9593 ASENAH quatrième série - tome 46 fascicule 5 septembre-octobre 2013 aNNALES SCIENnIFIQUES d L ÉCOLE hORMALE SUPÉRIEUkE Alexander B. GONCHAROV & Richard KENYON Dimers and cluster integrable systems SOCIÉTÉ MATHÉMATIQUE DE FRANCE Ann. Scient. Éc. Norm. Sup. 4 e série, t. 46, 2013, p. 747 à 813 DIMERS AND CLUSTER INTEGRABLE SYSTEMS ʙʏ Aʟɴʀ B. GONCHAROV ɴ Rɪʜʀ KENYON Aʙʀ. – We show that the dimer model on a bipartite graph Γ on a torus gives rise to a quantum integrable system of special type, which we call a cluster integrable system. The phase space of the classical system contains, as an open dense subset, the moduli space LΓ of line bundles with connections on the graph Γ. The sum of Hamiltonians is essentially the partition function of the dimer model. We say that two such graphs Γ1 and Γ2 are equivalent if the Newton polygons of the correspond- ing partition functions coincide up to translation. We define elementary transformations of bipartite surface graphs, and show that two equivalent minimal bipartite graphs are related by a sequence of elementary transformations. For each elementary transformation we define a birational Poisson iso- morphism LΓ LΓ providing an equivalence of the integrable systems. We show that it is a cluster 1 → 2 Poisson transformation, as defined in [10]. We show that for any convex integral polygon N there is a non-empty finite set of minimal graphs Γ for which N is the Newton polygon of the partition function related to Γ. Gluing the varieties LΓ for graphs Γ related by elementary transformations via the corresponding cluster Poisson transformations, we get a Poisson space X N . -
Tightening Curves on Surfaces Monotonically with Applications
Tightening Curves on Surfaces Monotonically with Applications † Hsien-Chih Chang∗ Arnaud de Mesmay March 3, 2020 Abstract We prove the first polynomial bound on the number of monotonic homotopy moves required to tighten a collection of closed curves on any compact orientable surface, where the number of crossings in the curve is not allowed to increase at any time during the process. The best known upper bound before was exponential, which can be obtained by combining the algorithm of de Graaf and Schrijver [J. Comb. Theory Ser. B, 1997] together with an exponential upper bound on the number of possible surface maps. To obtain the new upper bound we apply tools from hyperbolic geometry, as well as operations in graph drawing algorithms—the cluster and pipe expansions—to the study of curves on surfaces. As corollaries, we present two efficient algorithms for curves and graphs on surfaces. First, we provide a polynomial-time algorithm to convert any given multicurve on a surface into minimal position. Such an algorithm only existed for single closed curves, and it is known that previous techniques do not generalize to the multicurve case. Second, we provide a polynomial-time algorithm to reduce any k-terminal plane graph (and more generally, surface graph) using degree-1 reductions, series-parallel reductions, and ∆Y -transformations for arbitrary integer k. Previous algorithms only existed in the planar setting when k 4, and all of them rely on extensive case-by-case analysis based on different values of k. Our algorithm≤ makes use of the connection between electrical transformations and homotopy moves, and thus solves the problem in a unified fashion. -
Coloured Quivers for Rigid Objects and Partial Triangulations: The
COLOURED QUIVERS FOR RIGID OBJECTS AND PARTIAL TRIANGULATIONS: THE UNPUNCTURED CASE ROBERT J. MARSH AND YANN PALU Abstract. We associate a coloured quiver to a rigid object in a Hom-finite 2-Calabi–Yau triangulated category and to a partial triangulation on a marked (unpunctured) Riemann surface. We show that, in the case where the category is the generalised cluster category associated to a surface, the coloured quivers coincide. We also show that compatible notions of mutation can be defined and give an explicit description in the case of a disk. A partial description is given in the general 2-Calabi–Yau case. We show further that Iyama-Yoshino reduction can be interpreted as cutting along an arc in the surface. Introduction Let (S,M) be a pair consisting of an oriented Riemann surface S with non-empty boundary and a set M of marked points on the boundary of S, with at least one marked point on each component of the boundary. We further assume that (S,M) has no component homeomorphic to a monogon, digon, or triangle. A partial triangulation R of (S,M) is a set of noncrossing simple arcs between the points in M. We define a mutation of such triangulations, involving replacing an arc α of R with a new arc depending on the surface and the rest of the partial triangulation. This allows us to associate a coloured quiver to each partial triangulation of M in a natural way. The coloured quiver is a directed graph in which each edge has an associated colour which, in general, can be any integer. -
A Discrete Representation of Einstein's Geometric Theory of Gravitation: the Fundamental Role of Dual Tessellations in Regge Calculus
A Discrete Representation of Einstein’s Geometric Theory of Gravitation: The Fundamental Role of Dual Tessellations in Regge Calculus Jonathan R. McDonald and Warner A. Miller Department of Physics, Florida Atlantic University, Boca Raton, FL 33431, USA [email protected] In 1961 Tullio Regge provided us with a beautiful lattice representation of Einstein’s geometric theory of gravity. This Regge Calculus (RC) is strikingly different from the more usual finite difference and finite element discretizations of gravity. In RC the fundamental principles of General Relativity are applied directly to a tessellated spacetime geometry. In this manuscript, and in the spirit of this conference, we reexamine the foundations of RC and emphasize the central role that the Voronoi and Delaunay lattices play in this discrete theory. In particular we describe, for the first time, a geometric construction of the scalar curvature invariant at a vertex. This derivation makes use of a new fundamental lattice cell built from elements inherited from both the simplicial (Delaunay) spacetime and its circumcentric dual (Voronoi) lattice. The orthogonality properties between these two lattices yield an expression for the vertex-based scalar curvature which is strikingly similar to the corre- sponding and more familiar hinge-based expression in RC (deficit angle per unit Voronoi dual area). In particular, we show that the scalar curvature is simply a vertex-based weighted average of deficits per weighted average of dual areas. What is most striking to us is how naturally spacetime is represented by Voronoi and Delaunay structures and that the laws of gravity appear to be encoded locally on the lattice spacetime with less complexity than in the continuum, yet the continuum is recovered by convergence in mean. -
Arxiv:1808.05573V2 [Math.GT] 13 May 2020 A.K.A
THE MAXIMAL INJECTIVITY RADIUS OF HYPERBOLIC SURFACES WITH GEODESIC BOUNDARY JASON DEBLOIS AND KIM ROMANELLI Abstract. We give sharp upper bounds on the injectivity radii of complete hyperbolic surfaces of finite area with some geodesic boundary components. The given bounds are over all such surfaces with any fixed topology; in particular, boundary lengths are not fixed. This extends the first author's earlier result to the with-boundary setting. In the second part of the paper we comment on another direction for extending this result, via the systole of loops function. The main results of this paper relate to maximal injectivity radius among hyperbolic surfaces with geodesic boundary. For a point p in the interior of a hyperbolic surface F , by the injectivity radius of F at p we mean the supremum injrad p(F ) of all r > 0 such that there is a locally isometric embedding of an open metric neighborhood { a disk { of radius r into F that takes 1 the disk's center to p. If F is complete and without boundary then injrad p(F ) = 2 sysp(F ) at p, where sysp(F ) is the systole of loops at p, the minimal length of a non-constant geodesic arc in F with both endpoints at p. But if F has boundary then injrad p(F ) is bounded above by the distance from p to the boundary, so it approaches 0 as p approaches @F (and we extend it continuously to @F as 0). On the other hand, sysp(F ) does not approach 0 as p @F .