DOCSLIB.ORG

INCREASING CHROMATIC NUMBER and GIRTH Contents 1

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
INCREASING CHROMATIC NUMBER and GIRTH Contents 1 INCREASING CHROMATIC NUMBER AND GIRTH YO JOONG CHOE Abstract. This expository paper deals with some interesting concepts related to the problem of increasing the chromatic number and the girth of a graph. Specifically, the proof of Kneser's conjecture and also related concepts such as Gale's distribution of points on the sphere and the Borsuk graph will be studied in order to show various ways of constructing a graph with infinite chromatic number and girth. Contents 1. Introduction 1 2. Preliminaries 3 3. General position and moment curve kissing lemma 3 4. Gale's distribution of points on the r-sphere 5 5. The Borsuk graph 7 6. The Kneser graph 10 Acknowledgments 13 References 13 1. Introduction In graph theory, many intriguing problems revolve around graph coloring. One common type of question here is about how to reduce the chromatic number of a certain graph. For example, the famous Four (used to be Five) Color Theorem states that you can always color a map with no more than four different colors. Another common type, on the opposite side, is about how to increase the chromatic number. There is an interesting question of this type that we will deal with in this paper: the problem of finding a graph with infinite chromatic number and girth. Definition 1.1 (chromatic number). A k-coloring of a graph G is a labeling of the vertices of G with k colors such that adjacent vertices have different colors. When this is the case, we say that G is k-colorable. The chromatic number χ of a graph G is the minimum value of k such that G is k-colorable. Definition 1.2 (girth). A walk of length k in a graph G is a sequence of vertices v0; v1; : : : ; vk of G such that vi−1 and vi are adjacent for 1 ≤ i ≤ k. A walk is closed if v0 = vk.A cycle is a closed walk without repeated vertices except for v0 = vk. The girth γ of a graph G is the length of its shortest cycle. A graph without a cycle has infinite girth. 1 2 YO JOONG CHOE It is not at all a difficult problem to find a graph satisfying one of the two conditions. For some large n, any complete graph with n vertices has chromatic number of n. Any n-gon has girth of n. But it is not as easy to find a graph satisfying both conditions. A triangle has girth of 3 and requires at least 3 colors. From here, one can increase the girth by simply adding two vertices to make a pentagon. (In fact, any (2k + 1)-gon has chromatic number of 3.) Now, here is a more challenging question: find a graph without a triangle - that is, with girth greater than 3 - and with chromatic number of 4. (Hint: the graph requires at least 11 vertices and has 5-fold rotational symmetry. This graph is referred to as ’Gr¨otzsch graph'.) These mini experiments already hint the existence of a graph with large girth and chromatic number. The proof was given by Paul Erd¨osusing the classic probabilistic method. Theorem 1.3 (Erd¨os,1959). For any positive integers γ and χ, there exists a graph G with girth γ and chromatic number χ. We will not show the proof of this theorem, however, since this paper will focus on introducing significant examples of this result. Mainly, we will study a specific kind of graph with high girth and chromatic number, called the Kneser graph, which was introduced by Martin Kneser. The chromatic number of the Kneser graph, which was conjectured in 1955 and proved much later in 1978, is a great example of the Erd¨ostheorem. Here is a formal definition of the graph: Definition 1.4 (the Kneser graph). For n ≥ 2m + 1, the Kneser graph K(n; m) is the graph with the vertex set as the set of m-subsets of f1; : : : ; ng. Two vertices are adjacent if their corresponding m-subsets are disjoint. Figure 1.1. The Kneser graph K(5,2) and the Petersen graph The most common example is K(5; 2), which is isomorphic (i.e. each vertex in one graph can be mapped to a unique vertex in another graph) to the Petersen graph (Figure 1.1). Ask a seven-year-old kid, and after struggling for a few minutes, the child will tell you that you need at least 3 colors to color this graph properly, and that the shortest cycle is 5. That part is easy, but the proof of the generalized version requires some difficult steps. We will state the theorem here just to clarify that proving this theorem is the goal of this paper. INCREASING CHROMATIC NUMBER AND GIRTH 3 Theorem 1.5 (Kneser's conjecture, 1955) (Lov´asz,1978). The chromatic number of the Kneser graph K(2m + r; m) is r + 2. 2. Preliminaries One important fact about this paper is that many parts of the proofs heavily rely on concepts in linear algebra and related geometry. These may seem totally unrelated at first sight, but they all give important insights to the proof of Kneser's conjecture. Before we move on, there are some preliminary concepts to note about through- out this paper. For statements related to graph theory, all graphs are simple graphs, meaning that we will not be concerned with loops or multiple edges. (This is be- cause such concepts are not important in dealing with chromatic numbers.) Also, all variables (e.g. n, m, r, k, i), unless specified, are nonnegative integers. For statements related to linear algebra, all vectors, unless specified, are column vec- tors. Lastly, this paper contains diagrams containing various different colors in order to illustrate graph coloring more effectively, so please print in color in case you are printing out this document on paper. 3. General position and moment curve kissing lemma We will first introduce the moment curve kissing lemma. This is necessary in proving the next theorem. Definition 3.1 (general position). Let S ⊆ W be a set of points, where W is the linear space of dimension n. S is in general position if any n of the points in S are linearly independent. For example, the vectors (1; 1); (1; 2); (1; 3) are in general position, as any 2 of them are linearly independent. Informally speaking, the concept of general position is the opposite to the Murphy's Law: elements coincide only when necessary. Now, there is a particularly useful instance of a set in general position. Definition 3.2 (the moment curve). The moment curve in Rn is the set 2 n−1 Mn = fmn(α) = (1; α; α ; : : : ; α ) j α 2 Rg: To be clear, the points of the moment curve will be row vectors in this paper. Proposition 3.3. The points of the moment curve are in general position. Proof. Let α1; α2; : : : ; αn be n distinct elements in R. Consider the matrix (mn(α1); mn(α2); : : : ; mn(αn)). This matrix is a Vandermonde matrix, so the determinant Q is 1≤i<j≤n(αj − αi). Because all αi are distinct, the determinant is nonzero. Therefore, the row vectors mn(ai) are linearly independent. With this property established, we can now move on to the actual lemma that uses the concept of the moment curve. 4 YO JOONG CHOE Definition 3.4 (hyperplane and normal vector). In an n-dimensional space, a hyperplane is an (n − 1)-dimensional flat subset. In other words, a hyperplane P in Rn is n T P = fx 2 R j c x = βg where c 2 Rn, c 6= 0, and β 2 R. Here, c is the normal vector of the hyperplane P . A linear hyperplane is a hyperplane crossing the origin. This is precisely when β = 0. d Lemma 3.5 (moment curve kissing lemma). Let d ≥ 1. Let 0 ≤ k ≤ 2 and α1; α2; : : : ; αk 2 R. Then, there exists a linear hyperplane P such that the mo- d+1 ment curve Md+1 ⊂ R lies entirely on one side of P . Here, P \ Md+1 = fmd+1(α1); md+1(α2); : : : ; md+1(αk)g. Proof. A linear hyperplane P is defined precisely by a linear equation cx = 0 where d+1 c = (γ0; γ1; : : : ; γd) 2 R and c 6= 0. (c is a row vector.) We want to find c such that T (1) c · md+1(z) > 0 where z 2 R and z2 = fα1; α2; : : : ; αkg, and T (2) c · md+1(ai) = 0 where i = 1; 2; : : : ; k. Let f be a polynomial of degree k with roots α1; α2; : : : ; αk. Thus, f(z) = Qk i=1(z − αi). 2 We know that the degree of f does not exceed d + 1, so let γi (the components of c) be the coefficients of f 2. That is, d 2 γ0 + γ1z + ::: + γdz = (f(z)) : T 2 In other words, we have c · md+1(z) = (f(z)) for any z 2 R. Thus, if z2 = T 2 T fα1; α2; : : : ; αkg, then c · md+1(z) = (f(z)) > 0, and otherwise, c · md+1(αi) = 2 (f(αi)) = 0. Here is a nice restatement of the lemma in terms of linear algebra that will be used in the next section. Corollary 3.6 (moment curve kissing lemma restated). Let 1 ≤ d ≤ n−1 and 0 ≤ d n×(d+1) k ≤ 2 . Then, there exists a matrix A 2 R such that the n rows are in general position.
Load more

Recommended publications
  • The Determining Number of Kneser Graphs José Cáceres, Delia Garijo, Antonio González, Alberto Márquez, Marıa Luz Puertas
    The determining number of Kneser graphs José Cáceres, Delia Garijo, Antonio González, Alberto Márquez, Marıa Luz Puertas To cite this version: José Cáceres, Delia Garijo, Antonio González, Alberto Márquez, Marıa Luz Puertas. The determining number of Kneser graphs. Discrete Mathematics and Theoretical Computer Science, DMTCS, 2013, Vol. 15 no. 1 (1), pp.1–14. hal-00990602 HAL Id: hal-00990602 https://hal.inria.fr/hal-00990602 Submitted on 13 May 2014 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Discrete Mathematics and Theoretical Computer Science DMTCS vol. 15:1, 2013, 1–14 The determining number of Kneser graphsy Jose´ Caceres´ 1z Delia Garijo2x Antonio Gonzalez´ 2{ Alberto Marquez´ 2k Mar´ıa Luz Puertas1∗∗ 1 Department of Statistics and Applied Mathematics, University of Almeria, Spain. 2 Department of Applied Mathematics I, University of Seville, Spain. received 21st December 2011, revised 19th December 2012, accepted 19th December 2012. A set of vertices S is a determining set of a graph G if every automorphism of G is uniquely determined by its action on S. The determining number of G is the minimum cardinality of a determining set of G.
    [Show full text]
  • Strongly Regular and Pseudo Geometric Graphs
    Strongly regular and pseudo geometric graphs M. Mitjana 1;2 Departament de Matem`atica Aplicada I Universitat Polit`ecnica de Catalunya Barcelona, Spain E. Bendito, A.´ Carmona, A. M. Encinas 1;3 Departament de Matem`atica Aplicada III Universitat Polit`ecnica de Catalunya Barcelona, Spain Abstract Very often, strongly regular graphs appear associated with partial geometries. The point graph of a partial geometry is the graph whose vertices are the points of the geometry and adjacency is defined by collinearity. It is well known that the point graph associated to a partial geometry is a strongly regular graph and, in this case, the strongly regular graph is named geometric. When the parameters of a strongly regular graph, Γ, satisfy the relations of a geometric graph, then Γ is named a pseudo geometric graph. Moreover, it is known that not every pseudo geometric graph is geometric. In this work, we characterize strongly regular graphs that are pseudo geometric and we analyze when the complement of a pseudo geometric graph is also pseudo geometric. Keywords: Strongly regular graph, partial geometry, pseudo geometric graph. 1 Strongly regular graphs A strongly regular graph with parameters (n; k; λ, µ) is a graph on n vertices which is regular of degree k, any two adjacent vertices have exactly λ common neighbours and two non{adjacent vertices have exactly µ common neighbours. We recall that antipodal strongly regular graphs are characterized by sat- isfying µ = k, or equivalently λ = 2k − n, which in particular implies that 2k ≥ n. In addition, any bipartite strongly regular graph is antipodal and it is characterized by satisfying µ = k and n = 2k.
    [Show full text]
  • Strongly Regular Graphs
    Chapter 4 Strongly Regular Graphs 4.1 Parameters and Properties Recall that a (simple, undirected) graph is regular if all of its vertices have the same degree. This is a strong property for a graph to have, and it can be recognized easily from the adjacency matrix, since it means that all row sums are equal, and that1 is an eigenvector. If a graphG of ordern is regular of degreek, it means that kn must be even, since this is twice the number of edges inG. Ifk�n− 1 and kn is even, then there does exist a graph of ordern that is regular of degreek (showing that this is true is an exercise worth thinking about). Regularity is a strong property for a graph to have, and it implies a kind of symmetry, but there are examples of regular graphs that are not particularly “symmetric”, such as the disjoint union of two cycles of different lengths, or the connected example below. Various properties of graphs that are stronger than regularity can be considered, one of the most interesting of which is strong regularity. Definition 4.1.1. A graphG of ordern is called strongly regular with parameters(n,k,λ,µ) if every vertex ofG has degreek; • ifu andv are adjacent vertices ofG, then the number of common neighbours ofu andv isλ (every • edge belongs toλ triangles); ifu andv are non-adjacent vertices ofG, then the number of common neighbours ofu andv isµ; • 1 k<n−1 (so the complete graph and the null graph ofn vertices are not considered to be • � strongly regular).
    [Show full text]
  • The Vertex Isoperimetric Problem on Kneser Graphs
    The Vertex Isoperimetric Problem on Kneser Graphs UROP+ Final Paper, Summer 2015 Simon Zheng Mentor: Brandon Tran Project suggested by: Brandon Tran September 1, 2015 Abstract For a simple graph G = (V; E), the vertex boundary of a subset A ⊆ V consists of all vertices not in A that are adjacent to some vertex in A. The goal of the vertex isoperimetric problem is to determine the minimum boundary size of all vertex subsets of a given size. In particular, define µG(r) as the minimum boundary size of all vertex subsets of G of size r. Meanwhile, the vertex set of the Kneser graph KGn;k is the set of all k-element subsets of f1; 2; : : : ; ng, and two vertices are adjacent if their correspond- ing sets are disjoint. The main results of this paper are to compute µG(r) for small and n 1 n−12 large values of r, and to prove the general lower bound µG(r) ≥ k − r k−1 −r when G = KGn;k. We will also survey the vertex isoperimetric problem on a closely related class of graphs called Johnson graphs and survey cross-intersecting families, which are closely related to the vertex isoperimetric problem on Kneser graphs. 1 1 Introduction The classical isoperimetric problem on the plane asks for the minimum perimeter of all closed curves with a fixed area. The ancient Greeks conjectured that a circle achieves the minimum boundary, but this was not rigorously proven until the 19th century using tools from analysis. There are two discrete versions of this problem in graph theory: the vertex isoperimetric problem and the edge isoperimetric problem.
    [Show full text]
  • The Graphs of Häggkvist and Hell
    The Graphs of H¨aggkvist and Hell by David E. Roberson A thesis presented to the University of Waterloo in fulfillment of the thesis requirement for the degree of Master of Mathematics in Combinatorics & Optimization Waterloo, Ontario, Canada, 2008 c David E. Roberson 2008 Author’s Declaration I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis, including any required final revisions, as accepted by my examiners. I understand that my thesis may be made electronically available to the public. iii Abstract This thesis investigates H¨aggkvist & Hell graphs. These graphs are an ex- tension of the idea of Kneser graphs, and as such share many attributes with them. A variety of original results on many different properties of these graphs are given. We begin with an examination of the transitivity and structural properties of H¨aggkvist & Hell graphs. Capitalizing on the known results for Kneser graphs, the exact values of girth, odd girth, and diameter are derived. We also discuss subgraphs of H¨aggkvist & Hell graphs that are isomorphic to subgraphs of Kneser graphs. We then give some background on graph homomorphisms before giving some explicit homomorphisms of H¨aggkvist & Hell graphs that motivate many of our results. Using the theory of equitable partitions we compute some eigenvalues of these graphs. Moving on to independent sets we give several bounds including the ratio bound, which is computed using the least eigenvalue. A bound for the chromatic number is given using the homomorphism to the Kneser graphs, as well as a recursive bound.
    [Show full text]
  • Treewidth of the Kneser Graph and the Erd˝Os-Ko-Rado Theorem
    Treewidth of the Kneser Graph and the Erd}os-Ko-Rado Theorem Daniel J. Harvey ∗ David R. Wood y Department of Mathematics and Statistics School of Mathematical Sciences The University of Melbourne Monash University Melbourne, Australia Melbourne, Australia [email protected] [email protected] Submitted: Dec 17, 2013; Accepted: Feb 20, 2014; Published: Feb 28, 2014 Mathematics Subject Classifications: 05C75, 05D05 Abstract Treewidth is an important and well-known graph parameter that measures the complexity of a graph. The Kneser graph Kneser(n; k) is the graph with vertex set [n] k , such that two vertices are adjacent if they are disjoint. We determine, for large values of n with respect to k, the exact treewidth of the Kneser graph. In the process of doing so, we also prove a strengthening of the Erd}os-Ko-RadoTheorem (for large n with respect to k) when a number of disjoint pairs of k-sets are allowed. Keywords: graph theory; Kneser graph; treewidth; separators; Erd}os-Ko-Rado 1 Introduction A tree decomposition of a graph G is a pair (T; (Bx ⊂ V (G): x 2 V (T ))) where T is a tree and (Bx ⊆ V (G): x 2 V (T )) is a collection of sets, called bags, indexed by the nodes of T . The following properties must also hold: • for each v 2 V (G), the nodes of T that index the bags containing v induce a non- empty connected subtree of T , • for each vw 2 E(G), there exists some bag containing both v and w.
    [Show full text]
  • Discrete Applied Mathematics 242 (2018) 118–129
    Discrete Applied Mathematics 242 (2018) 118–129 Contents lists available at ScienceDirect Discrete Applied Mathematics journal homepage: www.elsevier.com/locate/dam Decomposing clique search problems into smaller instances based on node and edge colorings Sándor Szabó, Bogdan Zavalnij * Institute of Mathematics and Informatics, University of Pecs, H-7624, Pecs, Ifjusag utja 6, Hungary article info a b s t r a c t Article history: To carry out a clique search in a given graph in a parallel fashion, one divides the problem Received 30 November 2016 into a very large number of smaller instances. To sort out as many resulted smaller Received in revised form 17 November 2017 problems as possible, one can rely on upper estimates of the clique sizes. Legal coloring Accepted 8 January 2018 of the nodes of the graphs is a commonly used tool to establish upper bound of the clique Available online 2 February 2018 size. We will point out that coloring of the nodes can also be used to divide the clique search problem into smaller ones. We will introduce a non-conventional coloring of the edges of Keywords: k-clique the given graph. We will gather theoretical and computational evidence that the proposed Maximum clique edge coloring provides better estimates for the clique size than the node coloring and can Clique search algorithm be used to divide the original problem into subproblems. Independent set ' 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC Branch and Bound BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
    [Show full text]
  • Bipartite Kneser Graphs Are Hamiltonian1
    Bipartite Kneser graphs are Hamiltonian1 Torsten Mütze2, Pascal Su Department of Computer Science ETH Zürich, 8092 Zürich, Switzerland [email protected], [email protected] Abstract. For integers k 1 and n 2k +1 the Kneser graph K(n; k) has as vertices all k-element subsets of [n≥] := 1; 2≥; : : : ; n and an edge between any two vertices (=sets) that are disjoint. The bipartitef Kneserg graph H(n; k) has as vertices all k- element and (n k)-element subsets of [n] and an edge between any two vertices where one is a subset− of the other. It has long been conjectured that all Kneser graphs and bipartite Kneser graphs except the Petersen graph K(5; 2) have a Hamilton cycle. The main contribution of this paper is proving this conjecture for bipartite Kneser graphs H(n; k). We also establish the existence of cycles that visit almost all vertices in Kneser graphs K(n; k) when n = 2k + o(k), generalizing and improving upon previous results on this problem. Keywords: Hamilton cycle, Kneser graph, hypercube, vertex-transitive graph 1. Introduction The question whether a graph has a Hamilton cycle — a cycle that visits every vertex exactly once — is a fundamental graph theoretical problem with a wide range of practical applications, shown to be NP-complete already in Karp’s landmark paper [Kar72]. As a consequence, recent years have seen an increasing interest in Hamiltonicity problems in various different flavors and the solution of several long-standing open problems (the survey [KO] gives an excellent overview of these developments).
    [Show full text]
  • Multi-Coloring the Mycielskian of Graphs
    Multi-Coloring the Mycielskian of Graphs Wensong Lin,1 Daphne Der-Fen Liu,2 and Xuding Zhu3,4 1DEPARTMENT OF MATHEMATICS SOUTHEAST UNIVERSITY NANJING 210096, PEOPLE’S REPUBLIC OF CHINA E-mail: [email protected] 2DEPARTMENT OF MATHEMATICS CALIFORNIA STATE UNIVERSITY LOS ANGELES, CALIFORNIA 90032 E-mail: [email protected] 3DEPARTMENT OF APPLIED MATHEMATICS NATIONAL SUN YAT-SEN UNIVERSITY KAOHSIUNG 80424, TAIWAN E-mail: [email protected] 4NATIONAL CENTER FOR THEORETICAL SCIENCES TAIWAN Received June 1, 2007; Revised March 24, 2009 Published online 11 May 2009 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jgt.20429 Abstract: A k-fold coloring of a graph is a function that assigns to each vertex a set of k colors, so that the color sets assigned to adjacent Contract grant sponsor: NSFC; Contract grant number: 10671033 (to W.L.); Contract grant sponsor: National Science Foundation; Contract grant number: DMS 0302456 (to D.D.L.); Contract grant sponsor: National Science Council; Contract grant number: NSC95-2115-M-110-013-MY3 (to X.Z.). Journal of Graph Theory ᭧ 2009 Wiley Periodicals, Inc. 311 312 JOURNAL OF GRAPH THEORY vertices are disjoint. The kth chromatic number of a graph G, denoted by k (G), is the minimum total number of colors used in a k-fold coloring of G. Let (G) denote the Mycielskian of G. For any positive integer k,it + ≤ ≤ + holds that k (G) 1 k( (G)) k(G) k (W. Lin, Disc. Math., 308 (2008), 3565–3573). Although both bounds are attainable, it was proved in (Z. Pan, X.
    [Show full text]
  • On the Chromatic Number of Matching Kneser Graphs
    Downloaded from orbit.dtu.dk on: Oct 02, 2021 On the Chromatic Number of Matching Kneser Graphs Alishahi, Meysam; Hossein, Hajiabolhassan Published in: Combinatorics Probability and Computing Link to article, DOI: 10.1017/S0963548319000178 Publication date: 2019 Document Version Early version, also known as pre-print Link back to DTU Orbit Citation (APA): Alishahi, M., & Hossein, H. (2019). On the Chromatic Number of Matching Kneser Graphs. Combinatorics Probability and Computing, 29(1), 1-21. https://doi.org/10.1017/S0963548319000178 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. On The Chromatic Number of Matching Graphs Meysam Alishahi† and Hossein Hajiabolhassan∗ † Department of Mathematics University of Shahrood, Iran meysam [email protected] ∗ Department of Mathematical Sciences Shahid Beheshti University, G.C. P.O. Box 19839-63113, Tehran, Iran [email protected] Abstract In an earlier paper, the present authors (2013) [1] introduced the altermatic number of graphs and used Tucker’s Lemma, an equivalent combinatorial ver- sion of the Borsuk-Ulam Theorem, to show that the altermatic number is a lower bound for the chromatic number.
    [Show full text]
  • Independence Number of Products of Kneser Graphs
    Independence number of products of Kneser graphs BoˇstjanBreˇsara;b Mario Valencia-Pabonc November 19, 2018 a Faculty of Natural Sciences and Mathematics, University of Maribor, Slovenia b Institute of Mathematics, Physics and Mechanics, Ljubljana, Slovenia c Universit´eParis-13, Sorbonne Paris Cit´eLIPN, CNRS UMR7030, Villataneuse, France Abstract We study the independence number of a product of Kneser graph K(n; k) with it- self, where we consider all four standard graph products. The cases of the direct, the lexicographic and the strong product of Kneser graphs are not difficult (the formula for α(K(n; k) K(n; k)) is presented in this paper), while the case of the Cartesian prod- uct of Kneser graphs is much more involved. We establish a lower bound and an upper bound for the independence number of K(n; 2)2K(n; 2), which are asymptotically tend- ing to n3=3 and 3n3=8, respectively. The former is obtained by a construction, which differs from the standard diagonalization procedure, while for the upper bound the `-independence number of Kneser graphs can be applied. We also establish some con- structions in odd graphs K(2k + 1; k), which give a lower bound for the 2-independence number of these graphs, and prove that two such constructions give the same lower bound as a previously known one. Finally, we consider the s-stable Kneser graphs K(ks + 1; k)s−stab, derive a formula for their `-independence number, and give the exact value of the independence number of the Cartesian square of K(ks + 1; k)s−stab.
    [Show full text]
  • Kneser Graphs: Bounds and Applications
    The (Generalized) Orthogonality Dimension of (Generalized) Kneser Graphs: Bounds and Applications Alexander Golovnev∗ Ishay Haviv† Abstract The orthogonality dimension of a graph G = (V, E) over a field F is the smallest integer t t for which there exists an assignment of a vector uv 2 F with huv, uvi 6= 0 to every vertex 0 v 2 V, such that huv, uv0 i = 0 whenever v and v are adjacent vertices in G. The study of the orthogonality dimension of graphs is motivated by various application in information theory and in theoretical computer science. The contribution of the present work is two-folded. First, we prove that there exists a constant c such that for every sufficiently large integer t, it is NP-hard to decide whether the orthogonality dimension of an input graph over R is at most t or at least 3t/2 − c. At the heart of the proof lies a geometric result, which might be of inde- pendent interest, on a generalization of the orthogonality dimension parameter for the family of Kneser graphs, analogously to a long-standing conjecture of Stahl (J. Comb. Theo. Ser. B, 1976). Second, we study the smallest possible orthogonality dimension over finite fields of the complement of graphs that do not contain certain fixed subgraphs. In particular, we provide an explicit construction of triangle-free n-vertex graphs whose complement has orthogonality dimension over the binary field at most n1−d for some constant d > 0. Our results involve constructions from the family of generalized Kneser graphs and they are motivated by the rigidity approach to circuit lower bounds.
    [Show full text]