DOCSLIB.ORG

On K-Level Matroids: Geometry and Combinatorics Francesco Grande

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
On K-Level Matroids: Geometry and Combinatorics Francesco Grande On k-level matroids: geometry and combinatorics ⊕ ⊕ Dissertation zur Erlangung des akademisches Grades eines Doktors der Naturwissenschaften am Fachbereich Mathematik und Informatik der Freien Universität Berlin vorgelegt von Francesco Grande Berlin 2015 Advisor and first reviewer: Professor Dr. Raman Sanyal Second reviewer: Professor Rekha Thomas, Ph.D. Date of the defense: October 12, 2015 Summary The Theta rank of a finite point configuration V is the maximal degree nec- essary for a sum-of-squares representation of a non-negative linear function on V . This is an important invariant for polynomial optimization that is in general hard to determine. We study the Theta rank of point configu- rations via levelness, that is a discrete-geometric invariant, and completely classify the 2-level (equivalently Theta-1) configurations whose convex hull is a simple or a simplicial polytope. We consider configurations associated to the collection of bases of matroids and show that the class of matroids with bounded Theta rank or levelness is closed under taking minors. This allows us to find a characterization of matroids with bounded Theta rank or levelness in terms of forbidden minors. We give the complete (finite) list of excluded minors for Theta-1 matroids which generalize the well-known series-parallel graphs. Moreover, we char- acterize the class of Theta-1 matroids in terms of the degree of generation of the vanishing ideal and in terms of the psd rank for the associated matroid base polytope. We analyze in full detail Theta-1 matroids from a constructive perspective and discover that they are sort-closed, which allows us to determine a uni- modular triangulation of every matroid base polytope and to characterize its volume by means of permutations. A closed formula for the enumeration of Theta-1 matroids on a ground set of size n seems out of reach, but we exploit the constructive properties to provide asymptotic estimates. As a consequence, we obtain an exponential lower bound on the number of 2-level polytopes of any fixed dimension. As for the k-level matroids with k > 2, we prove that the list of excluded minors is finite for every k and we describe the excluded minors for k-level graphs. We also investigate the excluded minors for graphs of Theta rank 2. For the case of hypersimplices, that is, matroid base polytopes of uniform matroids, we present results about the non-negative rank and the Gröbner fan together with conjectures about possible generalizations to the class of Theta-1 matroids. v Acknowledgements The day I started my Ph.D., I was new to Discrete Geometry and to mathe- matical research. About a thousand days later, I’m at the end of this amazing academical journey and my biggest and most sincere thanks go to my ad- visor Raman Sanyal. For always trying to find time to discuss maths, even during the overwhelming weeks, for teaching me how to do research and how to present the results, for helping me out whenever I was stuck. I want to thank my coauthor Juanjo Rué for uncountably many (even for him) hours spent together at improving our paper and Arnau Padrol for the huge amount of time dedicated to answering my geometric questions. I’d like to express my gratitude towards Rekha Thomas, Francisco Santos, Bernd Sturmfels, and Federico Ardila for their challenging and helpful input. Besides useful discussions and several borrowed books, I’d like to thank Gün- ter Ziegler for directing the research group at the Villa. It has been a pleasure to work in a welcoming and challenging environment with my office mates To- bias Friedl, Katy Beeler, Albert Haase, and people from “upstairs” Katherina Jochemko, Nevena Palic, and Philip Brinkmann. I want to thank BMS and MDS for providing plenty of opportunities for young mathematicians, my mentor Tibor Szabó for his precious advise, Flo- rian Frick and Giulia Battiston for the enjoyable time spent together chat- ting about math and beyond, Bruno Benedetti for offering me the chance of tutoring a course, Lorenzo Prelli, Elena Lavanda, and David Rolnick for reading parts of my thesis. My last thanks go to my family and to Francesca for supporting me every single day from a distance. I’ve been abroad for three years and I haven’t felt alone one minute. vii Contents Summary v Acknowledgements vii Introduction xiii 1 Basics 1 1.1 Point configurations and polytopes . .1 1.1.1 Basic definitions and properties . .1 1.1.2 Gale duality . .7 1.2 Matroids . 11 1.2.1 Basic definitions and properties . 11 1.2.2 Matroid base configurations . 13 1.2.3 Matroid operations . 17 2 Configurations and levelness 19 2.1 Configurations of 0/1-points . 19 2.2 An overview of 2-level configurations . 21 2.2.1 Levelness and 2-levelness . 21 2.2.2 Polytopal constructions and levelness . 24 2.2.3 A catalog of 2-level polytopes . 24 2.3 Simple and simplicial . 27 3 Matroid base configurations 33 3.1 Face-hereditary properties . 33 3.2 2-level matroids . 35 3.3 k-level graphs . 40 3.3.1 Excluded minors for k-level graphs . 40 ix x CONTENTS 3.3.2 The class of 3-level graphs . 42 3.3.3 4-level and Theta-2 graphs . 44 3.4 Excluded minors for k-level matroids . 47 4 The constructive approach 55 4.1 Structural properties . 55 4.1.1 Tree decompositions of matroids . 55 4.1.2 The family of UMR-trees . 59 4.2 Base polytopes and 2-sum . 62 4.2.1 Sort-closed matroids and triangulations . 62 4.2.2 Alcoved polytopes and volumes . 67 5 Enumeration of 2-level matroids 71 5.1 The generating function T (x) .................. 71 5.1.1 Preliminaries . 71 5.1.2 The combinatorial class of UMR-trees . 72 5.1.3 Counting pointed UMR-trees . 73 5.1.4 The Dissymmetry Theorem . 76 5.2 Asymptotic analysis of T (x) ................... 79 5.2.1 Preliminaries . 79 5.2.2 Asymptotic analysis . 82 5.2.3 Self-duality . 86 5.2.4 Many 2-level polytopes from matroids . 89 6 Matroid ideals and cone-ranks 91 6.1 Degree of base configurations . 91 6.2 Psd rank . 94 6.3 Extension complexity of hypersimplices . 97 6.4 Vanishing ideal of uniform matroids . 101 6.4.1 Gröbner bases of In;k ................... 101 6.4.2 Gröbner fan and state polyhedron . 105 CONTENTS xi Bibliography 111 List of Figures 119 Zusammenfassung 121 Eidesstattliche Erklärung 123 Introduction Let V Rd be a configuration of finitely many points and c Rd a vector. ⊂ 2 If we are asked to maximize the linear function c; x = c1x1 + ::: + cdxd on V , we are dealing with a simple task, namely, toh evaluatei c; v for all v V and record the maximum value. h i 2 x2 x2 (1, 1) 1/2 ( 1 , 1) − 2 13 3 c, v 7/10 ( 5 , 5 ) h i − x1 = x1 ( 1, 0) (3, 0) ⇒ 1/2 3/2 − − ( 13 , 3 ) 19/10 5 − 5 − (1, 1) 3/2 − − Figure 1: Linear optimization over a configuration V . It is tempting to claim that linear optimization over a finite point configura- tion is computationally easy and, in particular, that the computational cost is linear in the number of points of V . These considerations rely on one key assumption, namely that V is provided as a finite list of points. For instance, a linear optimization over the configuration V leads to the same outcome if solved on the polytope P = conv(V ). x2 x +2x 1 1 2 ≥ − x1 x 2x 1 1− 2 ≥ − Figure 2: Linear inequalities defining conv(V ). xiii xiv CONTENTS Moreover, the polytope P = conv(V ) can be described by a system of linear inequalities Cx δ, where C Rm×d and δ Rm (see Figure 2). It is known that the linear≤ programming2 problem 2 max c; p s.t. Cp δ p2Rd h i ≤ can be solved in polynomial time [Sch03a, Ch. 5]. On the other hand, V could be described as the set of solutions to a system of non-linear polynomial equations (see Figure 3) in which case the direct approach to the optimization requires us to solve the system as a first step and therefore is rather unpractical. In general, performing linear optimiza- tion over a finite configuration of points defined by non-linear polynomial constraints is NP-hard [Lau09, Sect. 1]. x2+4x2 2x 3=0 1 2− 1− (x 2x +1)(x 2x 1.4)(x 2x 3)(x 2x 3.8)=0 1− 2 1− 2− 1− 2− 1− 2− (x1+2x2+1)(x1+2x2 1.4)(x1+2x2 3)(x1+2x2 3.8)=0 − − − x2 x1 Figure 3: A polynomial description of V . An alternative way of tackling the problem is to optimize over a relaxation of conv(V ), that is, a set containing conv(V ), and this yields an approximate so- lution in polynomial time. The key observation is that the polytope conv(V ) is determined by the set of all linear inequalities of the form `(x) 0, where `(x) is a non-negative linear function on V . ≥ A linear function `(x) = δ c; x which is non-negative on V is called k-sos −h i (sum of squares) with respect to V if there exist polynomials h ; : : : ; hs 1 2 R[x1; : : : ; xd] such that deg(hi) k and ≤ `(v) = h2(v) + h2(v) + + h2(v) (1) 1 2 ··· s for all v V . 2 CONTENTS xv Among the non-negative linear functions defining conv(V ) we consider the ones that are k-sos.
Load more

Recommended publications
  • Topics in Algorithmic, Enumerative and Geometric Combinatorics
    Thesis for the Degree of Doctor of Philosophy Topics in algorithmic, enumerative and geometric combinatorics Ragnar Freij Division of Mathematics Department of Mathematical Sciences Chalmers University of Technology and University of Gothenburg G¨oteborg, Sweden 2012 Topics in algorithmic, enumerative and geometric combinatorics Ragnar Freij ISBN 978-91-7385-668-3 c Ragnar Freij, 2012. Doktorsavhandlingar vid Chalmers Tekniska H¨ogskola Ny serie Nr 3349 ISSN 0346-718X Department of Mathematical Sciences Chalmers University of Technology and University of Gothenburg SE-412 96 GOTEBORG,¨ Sweden Phone: +46 (0)31-772 10 00 [email protected] Printed in G¨oteborg, Sweden, 2012 Topics in algorithmic, enumerative and geometric com- binatorics Ragnar Freij ABSTRACT This thesis presents five papers, studying enumerative and extremal problems on combinatorial structures. The first paper studies Forman’s discrete Morse theory in the case where a group acts on the underlying complex. We generalize the notion of a Morse matching, and obtain a theory that can be used to simplify the description of the G-homotopy type of a simplicial complex. As an application, we determine the S2 × Sn−2-homotopy type of the complex of non-connected graphs on n nodes. In the introduction, connections are drawn between the first paper and the evasiveness conjecture for monotone graph properties. In the second paper, we investigate Hansen polytopes of split graphs. By applying a partitioning technique, the number of nonempty faces is counted, and in particular we confirm Kalai’s 3d-conjecture for such polytopes. Further- more, a characterization of exactly which Hansen polytopes are also Hanner polytopes is given.
    [Show full text]
  • Plenary Lectures
    Plenary lectures A. D. Alexandrov and the Birth of the Theory of Tight Surfaces Thomas Banchoff Brown University, Providence, Rhode Island 02912, USA e-mail: Thomas [email protected] Alexander Danilovich Alexandrov was born in 1912, and just over 25 years later in 1938 (the year of my birth), he proved a rigidity theorem for real analytic surfaces of type T with minimal total absolute curvature. He showed in particular that that two real analytic tori of Type T in three-space that are iso- metric must be congruent. In 1963, twenty-five years after that original paper, Louis Nirenberg wrote the first generalization of that result, for five times differentiable surfaces satisfying some technical hypothe- ses necessary for applying techniques of ordinary and partial differential equations. Prof. Shiing-Shen Chern, my thesis advisor at the University of California, Berkeley, gave me this paper to present to his graduate seminar and challenged me to find a proof without these technical hypotheses. I read Konvexe Polyeder by A. D. Alexandrov and the works of Pogoreloff on rigidity for convex surfaces using polyhedral methods and I decided to try to find a rigidity theorem for Type T surfaces using similar techniques. I found a condition equivalent to minimal total absolute curvature for smooth surfaces, that also applied to polyhedral surfaces, namely the Two-Piece Property or TPP. An object in three- space has the TPP if every plane separates the object into at most two connected pieces. I conjectured a rigidity result similar to those of Alexandroff and Nirenberg, namely that two polyhedral surfaces in three-space with the TPP that are isometric would have to be congruent.
    [Show full text]
  • Minimal Volume Product of Three Dimensional Convex Bodies with Various Discrete Symmetries
    Minimal volume product of three dimensional convex bodies with various discrete symmetries Hiroshi Iriyeh ∗ Masataka Shibata † August 12, 2021 Abstract We give the sharp lower bound of the volume product of three dimensional convex bodies which are invariant under a discrete subgroup of O(3) in several cases. We also characterize the convex bodies with the minimal volume product in each case. In particular, this provides a new partial result of the non-symmetric version of Mahler’s conjecture in the three dimensional case. 1 Introduction and main results 1.1 Mahler’s conjecture and its generalization Let K be a convex body in Rn, i.e, K is a compact convex set in Rn with nonempty interior. Then K ⊂ Rn is said to be centrally symmetric if it satisfies that K = −K. Denote by Kn the set of all convex bodies in Rn equipped with the Hausdorff metric and n n by K0 the set of all K ∈ K which are centrally symmetric. The interior of K ∈ Kn is denoted by int K. For a point z ∈ int K, the polar body of K with respect to z is defined by z n K = {y ∈ R ;(y − z) · (x − z) ≤ 1 for any x ∈ K} , where · denotes the standard inner product on Rn. Then an affine invariant P(K) := min |K| |Kz| (1) z∈int K is called volume product of K, where |K| denotes the n-dimensional volume of K in Rn. arXiv:2007.08736v2 [math.MG] 8 Oct 2020 It is known that for each K ∈ Kn the minimum of (1) is attained at the unique point z on K, which is called Santaló point of K (see, e.g., [13]).
    [Show full text]
  • Incidence Graphs and Unneighborly Polytopes
    Incidence Graphs and Unneighborly Polytopes vorgelegt von Diplom-Mathematiker Ronald Frank Wotzlaw aus G¨ottingen Von der Fakult¨at II – Mathematik und Naturwissenschaften der Technischen Universit¨at Berlin zur Erlangung des akademischen Grades Doktor der Naturwissenschaften – Dr. rer. nat. – genehmigte Dissertation Promotionsausschuss: Vorsitzender: Prof. Fredi Tr¨oltzsch Berichter: Prof. G¨unter M. Ziegler Prof. Gil Kalai Tag der wissenschaftlichen Aussprache: 6. Februar 2009 Berlin 2009 D 83 . for though it cannot hope to be useful or informative on all matters, it does at least make the reassuring claim, that where it is inaccurate it is at least definitively inaccurate. In cases of major discrepancy it’s always reality that’s got it wrong. The Restaurant at the End of the Universe Douglas Adams For Finja and Nora Preface This thesis is about polytopes and “their life in high dimensions.” Nearly all questions considered here are trivial or simple exercises when restricted to dimensions that we can “see.” More often than not, the interesting examples start to appear in dimensions d = 6, 7, 8 or even much higher. Nevertheless, I have tried to illustrate ideas with pictures throughout. The reader should be warned however that they rarely show the real nature of the mathematical problem at hand. Some of the results are generalized to objects that are more abstract than polytopes. I have taken a pragmatical point of view with respect to the level of generality that theorems are stated and proved: If a generalization was asked for in the existing literature, it is provided (if possible). The following is a list of the main results in this thesis in order of appearance: The connectivity of (k,ℓ)-incidence graphs is determined for the case • ℓ 2k + 1 in Chapter 3.
    [Show full text]
  • On Some Problems Related to 2-Level Polytopes
    On some problems related to 2-level polytopes Thèse n. 9025 2018 présenté le 22 Octobre 2018 à la Faculté des Sciences de Base chaire d’optimisation discrète programme doctoral en mathématiques École Polytechnique Fédérale de Lausanne pour l’obtention du grade de Docteur ès Sciences par Manuel Francesco Aprile acceptée sur proposition du jury: Prof Hess Bellwald Kathryn, président du jury Prof Eisenbrand Friedrich, directeur de thèse Prof Faenza Yuri, codirecteur de thèse Prof Svensson Ola, rapporteur Prof Fiorini Samuel, rapporteur Prof Weltge Stefan, rapporteur Lausanne, EPFL, 2018 "You must go from wish to wish. What you don’t wish for will always be beyond your reach." — Michael Ende, The Neverending Story. To my brother Stefano, to whom I dedicated my Bachelor thesis too. He is now six years old and he has not read it yet. Maybe one day? Acknowledgements My PhD has been an incredible journey of discovery and personal growth. There were mo- ments of frustration and doubt, when research just wouldn’t work, even depression, but there was also excitement, enjoyment, and satisfaction for my progresses. Oh, there was hard work too. A whole lot of people contributed to making this experience so amazing. First, I would like to thank my advisor Fritz Eisenbrand for his support, the trust he put in me, and for letting me do research in total freedom. He allowed me to go to quite some conferences and workshops, where I could expand my knowledge and professional network, as well as enjoying some traveling. A big “grazie” goes to Yuri Faenza, my co-advisor: in a moment when I was not sure about the direction of my research, he gave a talk about 2-level polytopes, immediately catching my interest, and, a few days after, he was introducing me to extended formulations with an improvised crash course.
    [Show full text]
  • DUALITY PHENOMENA and VOLUME INEQUALITIES in CONVEX GEOMETRY a Dissertation Submitted to Kent State University in Partial Fulfil
    DUALITY PHENOMENA AND VOLUME INEQUALITIES IN CONVEX GEOMETRY A dissertation submitted to Kent State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy by Jaegil Kim August, 2013 Dissertation written by Jaegil Kim B.S., Pohang University of Science and Technology, 2005 M.S., Pohang University of Science and Technology, 2008 Ph.D., Kent State University, 2013 Approved by Chair, Doctoral Dissertation Committee Dr. Artem Zvavitch Members, Doctoral Dissertation Committee Dr. Richard M. Aron Members, Doctoral Dissertation Committee Dr. Fedor Nazarov Members, Doctoral Dissertation Committee Dr. Dmitry Ryabogin Members, Outside Discipline Dr. Fedor F. Dragan Members, Graduate Faculty Representative Dr. David W. Allender Accepted by Chair, Department of Mathematical Sciences Dr. Andrew Tonge Associate Dean Raymond Craig Dean, College of Arts and Sciences ii TABLE OF CONTENTS ACKNOWLEDGEMENTS . v Introduction and Preliminaries 1 0.1 Basic concepts in Convex Geometry . 2 0.2 Summary and Notation . 5 0.3 Bibliographic Note . 7 I Mahler conjecture 8 1 Introduction to the volume product . 9 1.1 The volume product and its invariant properties . 9 1.2 Mahler conjecture and related results . 13 2 More on Hanner polytopes . 16 2.1 Connection with perfect graphs . 16 2.2 Direct sums of polytopes . 20 3 Local minimality in the symmetric case . 31 3.1 Construction of Polytopes . 31 3.2 Computation of volumes of polytopes . 41 3.3 A gap between K and a polytope . 60 4 Local minimality in the nonsymmetric case . 79 4.1 Continuity of the Santal´omap . 80 iii 4.2 Construction of polytopes for the simplex case .
    [Show full text]
  • Lecture Notes Parts 1-11
    Discrete Geometry I http://wikis.fu-berlin.de/display/discgeom/Discrete+Geometry+I — Preliminary Lecture Notes (without any guarantees) — Prof. Günter M. Ziegler Fachbereich Mathematik und Informatik, FU Berlin, 14195 Berlin [email protected] FU Berlin, Winter term 2013/2014 This is the first in a series of three courses on Discrete Geometry. We will get to know fascinating geo- metric structures such as configurations of points and lines, hyperplane arrangements, and in particular polytopes and polyhedra, and learn how to handle them using modern methods for computation and vi- sualization and current analysis and proof techniques. A lot of this looks quite simple and concrete at first sight (and some of it is), but it also very quickly touches topics of current research. For students with an interest in discrete mathematics and geometry, this is the starting point to specialize in discrete geometry. The topics addressed in the course supplement and deepen the understanding of discrete-geometric structures appearing in differential geometry, optimization, combinatorics, topology, and algebraic geometry. To follow the course, a solid background in linear algebra is necessary. Some knowledge of combinatorics and geometry is helpful. Basic Literature [1] Satyan L. Devadoss and Joseph O’Rourke. Discrete and Computational Geometry. Princeton Uni- versity Press, Princeton NJ, 2011. [2] Jacob E. Goodman and Joseph O’Rourke, editors. Handbook of Discrete and Computational Geom- etry. Chapman & Hall/CRC Press, Boca Raton, second edition, 2004. [3] Peter M. Gruber. Convex and Discrete Geometry, volume 336 of Grundlehren Series. Springer, 2007. [4] Peter M. Gruber and Jörg Wills, editors.
    [Show full text]
  • Equality Cases in Viterbo's Conjecture and Isoperimetric Billiard Inequalities
    EQUALITY CASES IN VITERBO'S CONJECTURE AND ISOPERIMETRIC BILLIARD INEQUALITIES ALEXEY BALITSKIY Abstract. We apply the billiard technique to deduce some results on Viterbo's con- jectured inequality between the volume of a convex body and its symplectic capacity. We show that the product of a permutohedron and a simplex (properly related to each other) delivers equality in Viterbo's conjecture. Using this result as well as previously known equality cases, we prove some special cases of Viterbo's conjecture and interpret them as isoperimetric-like inequalities for billiard trajectories. 1. Introduction Claude Viterbo [16] conjectured an isoperimetric inequality for any (normalized) sym- plectic capacity and the volume of a convex body X ⊂ R2n: c(X)n (1.1) vol(X) ≥ : n! The minimum is supposed to be attained (perhaps, not uniquely) on the symplectic images of the Euclidean ball. This inequality is proven up to a constant factor to the power of n in [4]. We investigate the case when c is the Hofer{Zehnder symplectic capacity cHZ . The computation of symplectic capacities is considered difficult, so we restrict to the case of convex bodies that are Lagrangian products of a component in V = Rn with q-coordinates and a component in V ∗ = Rn with p-coordinates. For such bodies there is a simple geometric interpretation, established in [5], of the Hofer{Zehnder capacity. Namely, cHZ (K × T ) = ξT (K); where ξT (K) denotes the length of the shortest closed billiard trajectory in a convex body K ⊂ V with geometry of lengths determined by another convex body T ⊂ V ∗ by the formula kqkT = maxhp; qi; q 2 V: p2T Here we assume that T contains the origin in the interior, but we do not assume that T is symmetric, so our \norm" function k · kT is not symmetric in general.
    [Show full text]
  • Prof. Günter M. Ziegler Fachbereich Mathematik Und Informatik, FU Berlin, 14195 Berlin [email protected] FU Berlin, Winter Term 2013/2014
    Discrete Geometry I http://wikis.fu-berlin.de/display/discgeom/Discrete+Geometry+I — Preliminary Lecture Notes (without any guarantees) — Prof. Günter M. Ziegler Fachbereich Mathematik und Informatik, FU Berlin, 14195 Berlin [email protected] FU Berlin, Winter term 2013/2014 This is the first in a series of three courses on Discrete Geometry. We will get to know fascinating geo- metric structures such as configurations of points and lines, hyperplane arrangements, and in particular polytopes and polyhedra, and learn how to handle them using modern methods for computation and vi- sualization and current analysis and proof techniques. A lot of this looks quite simple and concrete at first sight (and some of it is), but it also very quickly touches topics of current research. For students with an interest in discrete mathematics and geometry, this is the starting point to specialize in discrete geometry. The topics addressed in the course supplement and deepen the understanding of discrete-geometric structures appearing in differential geometry, optimization, combinatorics, topology, and algebraic geometry. To follow the course, a solid background in linear algebra is necessary. Some knowledge of combinatorics and geometry is helpful. Basic Literature [1] Satyan L. Devadoss and Joseph O’Rourke. Discrete and Computational Geometry. Princeton Uni- versity Press, Princeton NJ, 2011. [2] Jacob E. Goodman and Joseph O’Rourke, editors. Handbook of Discrete and Computational Geom- etry. Chapman & Hall/CRC Press, Boca Raton, second edition, 2004. [3] Peter M. Gruber. Convex and Discrete Geometry, volume 336 of Grundlehren Series. Springer, 2007. [4] Peter M. Gruber and Jörg Wills, editors.
    [Show full text]
  • Enumeration on Words, Complexes and Polytopes
    Thesis for the Degree of Licentiate of Philosophy Enumeration on words, complexes and polytopes Ragnar Freij Division of Mathematics Department of Mathematical Sciences Chalmers University of Technology and University of Gothenburg G¨oteborg, Sweden 2010 Enumeration on words, complexes and polytopes Ragnar Freij c Ragnar Freij, 2010. NO 2010:12 ISSN 1652-9715 Department of Mathematical Sciences Chalmers University of Technology and University of Gothenburg SE-412 96 GOTEBORG,¨ Sweden Phone: +46 (0)31-772 10 00 Author e-mail: [email protected] Printed in G¨oteborg, Sweden, 2010 Enumeration on words, complexes and polytopes Ragnar Freij ABSTRACT This thesis presents four papers, studying enumerative problems on combina- torial structures. The first paper studies Forman’s discrete Morse theory in the case where a group acts on the underlying complex. We generalize the notion of a Morse matching, and obtain a theory that can be used to simplify the description of the G-homotopy type of a simplicial complex. The main motivation is the case where some group acts transitively on the vertex set of the complex, and G is some large subgroup of this group. In particular we are interested in complexes of graph properties. As an application, we determine the C2 × Sn−2-homotopy type of the complex of non-connected graphs on n nodes. The motivation behind the second paper is Gil Kalai’s conjecture from 1989, that a centrally symmetric d-polytope must have at least 3d non-empty faces. Looking for examples that are close to achieving the lower bound, we study the centrally symmetric Hansen polytopes, associated to perfect graphs.
    [Show full text]
  • Minimal Volume Product Near Hanner Polytopes
    Minimal volume product near Hanner polytopes Jaegil Kim∗ July 30, 2018 Abstract Mahler's conjecture asks whether the cube is a minimizer for the volume product of a body and its polar in the class of symmetric convex bodies in a fixed dimension. It is known that every Hanner polytope has the same volume product as the cube or the cross-polytope. In this paper we prove that every Hanner polytope is a strict local minimizer for the volume product in the class of symmetric convex bodies endowed with the Banach-Mazur distance. 1 Introduction and Notation n A body is a compact set which is the closure of its interior and, in particular, a convex body in R n is a compact convex set with nonempty interior. Let K be a convex body in R containing the origin. Then the polar of K is defined by ◦ n K = y R x; y 1 for all x K 2 h i ≤ 2 n where ; is the usual scalar product in R . The volume product of a convex body K is defined by (Kh·)·i = min K (K z)◦ where denotes the volume. In particular, if a convex body K is P z2K j j j − j j · j symmetric, that is, K = K, then the volume product of K is given by − (K) = K K◦ P j j j j because the minimum in the definition of the volume product is attained at the origin in this case. n It turns out that the volume product is invariant under any invertible affine transformation on R , n n and also invariant under the polarity, that is, for any invertible affine transformation T : R R , ! (TK) = (K); and (K◦) = (K): (1) arXiv:1212.2544v1 [math.FA] 11 Dec 2012 P P P P Moreover, the volume product of the `p-sum of two symmetric convex bodies is the same as that of the `q-sum for p; q [1; ] with 1=p + 1=q = 1 (see [28], [15]), that is, 2 1 (K p L) = (K q L): (2) P ⊕ P ⊕ We provide the definition and other details for the `p-sum in Section 3.
    [Show full text]
  • Two-Level Polytopes with a Prescribed Facet
    Two-Level Polytopes with a Prescribed Facet Samuel Fiorini, Vissarion Fisikopoulos, and Marco Macchia(B) D´epartement de Math´ematique,Universit´elibre de Bruxelles, ULB CP 216, Boulevard du Triomphe, 1050 Brussels, Belgium sfiorini,vfisikop,mmacchia @ulb.ac.be { } Abstract. A (convex) polytope is said to be 2-level if for every facet- defining direction of hyperplanes, its vertices can be covered with two hyperplanes of that direction. These polytopes are motivated by ques- tions, e.g., in combinatorial optimization and communication complexity. We study 2-level polytopes with one prescribed facet. Based on new gen- eralfindings about the structure of 2-level polytopes, we give a complete characterization of the 2-level polytopes with some facet isomorphic to a sequentially Hanner polytope, and improve the enumeration algorithm of Bohn et al. (ESA 2015). We obtain, for thefirst time, the complete list ofd-dimensional 2-level polytopes up to affine equivalence for dimension d = 7. As it turns out, geometric constructions that we call suspensions play a prominent role in both our theoretical and experimental results. This yields exciting new research questions on 2-level polytopes, which we state in the paper. 1 Introduction We start by giving a formal definition of 2-level polytopes and reasons why we find that they are interesting objects. Definition 1 (2-level polytope). A polytopeP is said to be 2-level if each hyperplaneΠ defining a facet ofP has a parallelΠ � such that every vertex ofP is on eitherΠ orΠ �. Motivation. First of all, many famous polytopes are 2-level. To name a few, stable set polytopes of perfect graphs [3], twisted prisms over those — also known as Hansen polytopes [15] — Birkhoff polytopes, and order polytopes [21] are all 2-level polytopes.
    [Show full text]