On the Complexity of Recognizing Hamming Graphs and Related Classes of Graphs
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Helly Property and Satisfiability of Boolean Formulas Defined on Set
Helly Property and Satisfiability of Boolean Formulas Defined on Set Systems Victor Chepoi Nadia Creignou LIF (CNRS, UMR 6166) LIF (CNRS, UMR 6166) Universit´ede la M´editerran´ee Universit´ede la M´editerran´ee 13288 Marseille cedex 9, France 13288 Marseille cedex 9, France Miki Hermann Gernot Salzer LIX (CNRS, UMR 7161) Technische Universit¨atWien Ecole´ Polytechnique Favoritenstraße 9-11 91128 Palaiseau, France 1040 Wien, Austria March 7, 2008 Abstract We study the problem of satisfiability of Boolean formulas ' in conjunctive normal form whose literals have the form v 2 S and express the membership of values to sets S of a given set system S. We establish the following dichotomy result. We show that checking the satisfiability of such formulas (called S-formulas) with three or more literals per clause is NP-complete except the trivial case when the intersection of all sets in S is nonempty. On the other hand, the satisfiability of S-formulas ' containing at most two literals per clause is decidable in polynomial time if S satisfies the Helly property, and is NP-complete otherwise (in the first case, we present an O(j'j · jSj · jDj)-time algorithm for deciding if ' is satisfiable). Deciding whether a given set family S satisfies the Helly property can be done in polynomial time. We also overview several well-known examples of Helly families and discuss the consequences of our result to such set systems and its relationship with the previous work on the satisfiability of signed formulas in multiple-valued logic. 1 Introduction The satisfiability of Boolean formulas in conjunctive normal form (SAT problem) is a fundamental problem in theoretical computer science and discrete mathematics. -
ISOMETRIC SUBGRAPHS of HAMMING GRAPHS and D-CONVEXITY
4. K.V. Rudakov, "Completeness and universal constraints in the problem of correction of heuristic classification algorithms," Kibernetika, No, 3, 106-109 (1987). 5. K.V. Rudakov, "Symmetry and function constraints in the problem of correction of heur- istic classification algorithms," Kibernetika, No. 4, 74-77 (1987). 6. K.V. Rudakov, On Some Classes of Recognition Algorithms (General Results) [in Russian], VTs AN SSSR, Moscow (1980). ISOMETRIC SUBGRAPHS OF HAMMING GRAPHS AND d-CONVEXITY V. D. Chepoi UDC 519.176 In this study, we provide criteria of isometric embeddability of graphs in Hamming graphs. We consider ordinary connected graphs with a finite vertex set endowed with the natural metric d(x, y), equal to the number of edges in the shortest chain between the ver- tices x and y. Let al,...,a n be natural numbers. The Hamming graph Hal...a n is the graph with the ver- tex set X = {x = (x l..... xn):l~xi~a~, ~ = l,...n} in which two vertices are joined by an edge if and only if the corresponding vectors differ precisely in one coordinate [i, 2]. In other words, the graph Hal...a n is the Cartesian product of the graphs Hal,...,Han, where Hal is the ai-vertex complete graph. It is easy to show that in the Hamming graph the distance d(x, y) between the vertices x, y is equal to the number of different pairs of coordinates in the tuples corresponding to these vertices, i.e., it is equal to the Hamming distance between these tuples. It is also easy to show that the graph of the n-dimensional cube Qn may be treated as the Hamming graph H2.. -
Towards a Classification of Distance-Transitive Graphs
数理解析研究所講究録 1063 巻 1998 年 72-83 72 Towards a classification of distance-transitive graphs John van Bon Abstract We outline the programme of classifying all finite distance-transitive graphs. We men- tion the most important classification results obtained so far and give special attention to the so called affine graphs. 1. Introduction The graphs in this paper will be always assumed to be finite, connected, undirected and without loops or multiple edges. The edge set of a graph can thus be identified with a subset of the set of unoidered pairs of vertices. Let $\Gamma=(V\Gamma, E\Gamma)$ be a graph and $x,$ $y\in V\Gamma$ . With $d(x, y)$ we will denote the usual distance $\Gamma$ in between the vertices $x$ and $y$ (i.e., the length of the shortest path connecting $x$ and $y$ ) and with $d$ we will denote the diameter of $\Gamma$ , the maximum of all possible values of $d(x, y)$ . Let $\Gamma_{i}(x)=\{y|y\in V\Gamma, d(x, y)=i\}$ be the set of all vertices at distance $i$ of $x$ . An $aut_{omo}rph7,sm$ of a graph is a permutation of the vertex set that maps edges to edges. Let $G$ be a group acting on a graph $\Gamma$ (i.e. we are given a morphism $Garrow Aut(\Gamma)$ ). For a $x\in V\Gamma$ $x^{g}$ vertex and $g\in G$ the image of $x$ under $g$ will be denoted by . The set $\{g\in G|x^{g}=x\}$ is a subgroup of $G$ , called that stabilizer in $G$ of $x$ and will be denoted by $G_{x}$ . -
On the Classification of Locally Hamming Distance-Regular Graphs
数理解析研究所講究録 第 76850巻 1991 年 50-61 On the Classification of Locally Hamming Distance-Regular Graphs BY MAKOTO MATSUMOTO Abstract. A distance-regular graph is locally Hamming if it is locally isomorphic to a Hamming scheme $H(r, 2)$ . This paper rediscovers the connection among locally Ham- ming distance-regular graphs, designs, and multiply transitive permutation groups, through which we classify some of locally Hamming distance-transitive graphs. \S 1. Introduction. By a graph we shall mean a finite undirected graph with no loops and no multiple edges. For a graph $G,$ $V(G)$ denotes the vertex set and $E(G)$ denotes the edge set of $G$ . For a vertex $v$ of a graph $G,$ $N(v)$ denotes the set of adjacent vertices with $v$ . By $\ovalbox{\tt\small REJECT}_{2}$ we denote the two-element field, and by $H(r)$ we denote the r-dimensional Hamming scheme for $r\geq 1$ ; that is, $H(r)$ is such a graph that its vertex set is the $\#_{2}^{=r}$ vector space a,nd $\tau\iota,$ $v\in\#_{2}^{=r}$ are adjacent if and only if the Hamming distance $d(u, v)=1$ ; i.e., $\#\{i u_{i}\neq v_{i}\}=1$ where $u=(u_{1}, \ldots u_{r})$ and $v=(v_{1}, \ldots , v_{r})$ . The graph obtained from $H(r)$ by identifying antipodal points is called a folded Hamming sheme(or a folded Hamming cube in [3, p.140]). The graph $H(3)$ is called a cube, and the induced subgraph obtained by removing one vertex tog$e$ ther with the three incident edges from a cube is called a tulip. -
Solution-Graphs of Boolean Formulas and Isomorphism∗
Journal on Satisfiability, Boolean Modeling, and Computation 10 (2019) 37-58 Solution-Graphs of Boolean Formulas and Isomorphism∗ Patrick Scharpfeneckery [email protected] Jacobo Tor´an [email protected] University of Ulm Institute of Theoretical Computer Science Ulm, Germany Abstract The solution-graph of a Boolean formula on n variables is the subgraph of the hypercube Hn induced by the satisfying assignments of the formula. The structure of solution-graphs has been the object of much research in recent years since it is important for the performance of SAT-solving procedures based on local search. Several authors have studied connectivity problems in such graphs focusing on how the structure of the original formula might affect the complexity of the connectivity problems in the solution-graph. In this paper we study the complexity of the isomorphism problem of solution-graphs of Boolean formulas. We consider the classes of formulas that arise in the CSP-setting and investigate how the complexity of the isomorphism problem depends on the formula type. We observe that for general formulas the solution-graph isomorphism problem can be solved in exponential time while in the cases of 2CNF formulas, as well as for CPSS formulas, the problem is in the counting complexity class C=P, a subclass of PSPACE. We also prove a strong property on the structure of solution-graphs of Horn formulas showing that they are just unions of partial cubes. In addition, we give a PSPACE lower bound for the problem on general Boolean functions. We prove that for 2CNF, as well as for CPSS formulas the solution-graph isomorphism problem is hard for C=P under polynomial time many-one reductions, thus matching the given upper bound. -
A Convexity Lemma and Expansion Procedures for Bipartite Graphs
Europ. J. Combinatorics (1998) 19, 677±685 Article No. ej980229 A Convexity Lemma and Expansion Procedures for Bipartite Graphs WILFRIED IMRICH AND SANDI KLAVZARÏ ² A hierarchy of classes of graphs is proposed which includes hypercubes, acyclic cubical com- plexes, median graphs, almost-median graphs, semi-median graphs and partial cubes. Structural properties of these classes are derived and used for the characterization of these classes by expan- sion procedures, for a characterization of semi-median graphs by metrically de®ned relations on the edge set of a graph and for a characterization of median graphs by forbidden subgraphs. Moreover, a convexity lemma is proved and used to derive a simple algorithm of complexity O(mn) for recognizing median graphs. c 1998 Academic Press 1. INTRODUCTION Hamming graphs and related classes of graphs have been of continued interest for many years as can be seen from the list of references. As the subject unfolded, many interesting problems arose which have not been solved yet. In particular, it is still open whether the known algorithms for recognizing partial cubes or median graphs, which comprise an important subclass of partial cubes, are optimal. The best known algorithms for recognizing whether a graph G is a member of the class of partial cubes have complexity O(mn), where m and n denote, respectively, the numbersPC of edges and vertices of G, see [1, 13]. As the recognition process involves a coloring of the edges of G, which is a special case of sorting, one might be tempted to look for an algorithm of complexity O(m log n). -
Steps in the Representation of Concept Lattices and Median Graphs Alain Gély, Miguel Couceiro, Laurent Miclet, Amedeo Napoli
Steps in the Representation of Concept Lattices and Median Graphs Alain Gély, Miguel Couceiro, Laurent Miclet, Amedeo Napoli To cite this version: Alain Gély, Miguel Couceiro, Laurent Miclet, Amedeo Napoli. Steps in the Representation of Concept Lattices and Median Graphs. CLA 2020 - 15th International Conference on Concept Lattices and Their Applications, Sadok Ben Yahia; Francisco José Valverde Albacete; Martin Trnecka, Jun 2020, Tallinn, Estonia. pp.1-11. hal-02912312 HAL Id: hal-02912312 https://hal.inria.fr/hal-02912312 Submitted on 5 Aug 2020 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. Steps in the Representation of Concept Lattices and Median Graphs Alain Gély1, Miguel Couceiro2, Laurent Miclet3, and Amedeo Napoli2 1 Université de Lorraine, CNRS, LORIA, F-57000 Metz, France 2 Université de Lorraine, CNRS, Inria, LORIA, F-54000 Nancy, France 3 Univ Rennes, CNRS, IRISA, Rue de Kérampont, 22300 Lannion, France {alain.gely,miguel.couceiro,amedeo.napoli}@loria.fr Abstract. Median semilattices have been shown to be useful for deal- ing with phylogenetic classication problems since they subsume me- dian graphs, distributive lattices as well as other tree based classica- tion structures. Median semilattices can be thought of as distributive _-semilattices that satisfy the following property (TRI): for every triple x; y; z, if x ^ y, y ^ z and x ^ z exist, then x ^ y ^ z also exists. -
Graphs of Acyclic Cubical Complexes
Europ . J . Combinatorics (1996) 17 , 113 – 120 Graphs of Acyclic Cubical Complexes H A N S -J U ¨ R G E N B A N D E L T A N D V I C T O R C H E P O I It is well known that chordal graphs are exactly the graphs of acyclic simplicial complexes . In this note we consider the analogous class of graphs associated with acyclic cubical complexes . These graphs can be characterized among median graphs by forbidden convex subgraphs . They possess a number of properties (in analogy to chordal graphs) not shared by all median graphs . In particular , we disprove a conjecture of Mulder on star contraction of median graphs . A restricted class of cubical complexes for which this conjecture would hold true is related to perfect graphs . ÷ 1996 Academic Press Limited 1 . C U B I C A L C O M P L E X E S A cubical complex is a finite set _ of (graphic) cubes of any dimensions which is closed under taking subcubes and non-empty intersections . If we replace each cube of _ by a solid cube , then we obtain the geometric realization of _ , called a cubical polyhedron ; for further information consult van de Vel’s book [18] . Vertices of a cubical complex _ are 0-dimensional cubes of _ . In the ( underlying ) graph G of the complex two vertices of _ are adjacent if they constitute a 1-dimensional cube . Particular cubical complexes arise from median graphs . In what follows we consider only finite graphs . -
Reconstructing Unrooted Phylogenetic Trees from Symbolic Ternary Metrics
Reconstructing unrooted phylogenetic trees from symbolic ternary metrics Stefan Gr¨unewald CAS-MPG Partner Institute for Computational Biology Chinese Academy of Sciences Key Laboratory of Computational Biology 320 Yue Yang Road, Shanghai 200032, China Email: [email protected] Yangjing Long School of Mathematics and Statistics Central China Normal University, Luoyu Road 152, Wuhan, Hubei 430079, China Email: [email protected] Yaokun Wu Department of Mathematics and MOE-LSC Shanghai Jiao Tong University Dongchuan Road 800, Shanghai 200240, China Email: [email protected] Abstract In 1998, B¨ocker and Dress presented a 1-to-1 correspondence between symbolically dated rooted trees and symbolic ultrametrics. We consider the corresponding problem for unrooted trees. More precisely, given a tree T with leaf set X and a proper vertex coloring of its interior vertices, we can map every triple of three different leaves to the color of its median vertex. We characterize all ternary maps that can be obtained in this way in terms of 4- and 5-point conditions, and we show that the corresponding tree and its coloring can be reconstructed from a ternary map that satisfies those conditions. Further, we give an additional condition that characterizes whether the tree is binary, and we describe an algorithm that reconstructs general trees in a bottom-up fashion. Keywords: symbolic ternary metric ; median vertex ; unrooted phylogenetic tree 1 Introduction A phylogenetic tree is a rooted or unrooted tree where the leaves are labeled by some objects of interest, usually taxonomic units (taxa) like species. The edges have a positive edge length, thus the tree defines a metric on the taxa set. -
On the Generalized Θ-Number and Related Problems for Highly Symmetric Graphs
On the generalized #-number and related problems for highly symmetric graphs Lennart Sinjorgo ∗ Renata Sotirov y Abstract This paper is an in-depth analysis of the generalized #-number of a graph. The generalized #-number, #k(G), serves as a bound for both the k-multichromatic number of a graph and the maximum k-colorable subgraph problem. We present various properties of #k(G), such as that the series (#k(G))k is increasing and bounded above by the order of the graph G. We study #k(G) when G is the graph strong, disjunction and Cartesian product of two graphs. We provide closed form expressions for the generalized #-number on several classes of graphs including the Kneser graphs, cycle graphs, strongly regular graphs and orthogonality graphs. Our paper provides bounds on the product and sum of the k-multichromatic number of a graph and its complement graph, as well as lower bounds for the k-multichromatic number on several graph classes including the Hamming and Johnson graphs. Keywords k{multicoloring, k-colorable subgraph problem, generalized #-number, Johnson graphs, Hamming graphs, strongly regular graphs. AMS subject classifications. 90C22, 05C15, 90C35 1 Introduction The k{multicoloring of a graph is to assign k distinct colors to each vertex in the graph such that two adjacent vertices are assigned disjoint sets of colors. The k-multicoloring is also known as k-fold coloring, n-tuple coloring or simply multicoloring. We denote by χk(G) the minimum number of colors needed for a valid k{multicoloring of a graph G, and refer to it as the k-th chromatic number of G or the multichromatic number of G. -
Finite Primitive Distance-Transitive Graphs
CORE Metadata, citation and similar papers at core.ac.uk Provided by Elsevier - Publisher Connector European Journal of Combinatorics 28 (2007) 517–532 www.elsevier.com/locate/ejc Finite primitive distance-transitive graphs John van Bon Dipartimento di Matematica, Università della Calabria, 87036 Arcavacata di Rende, Italy Available online 8 September 2005 Abstract The status of the project of classifying finite primitive distance-transitive graphs is surveyed. Particular attention is given to the structure of the proof of the recently obtained classification of finite primitive affine distance-transitive graphs. © 2005 Elsevier Ltd. All rights reserved. 1. Introduction Let Γ be a finite connected undirected graph of diameter d, without loops or multiple edges. Let V Γ denote the vertex set of Γ and Aut(Γ ) denote the automorphism group of Γ .LetG be a subgroup of Aut(Γ ). We say that G acts distance-transitively on Γ if G acts transitively on each of the sets Γi ={(x, y) ∈ V Γ × V Γ | d(x, y) = i}, for i = 0, 1,...,d. In this case G will be called a distance-transitive group and a graph Γ admitting such a group will be called a distance-transitive graph. Observe that if Γ is a distance-transitive graph, then Aut(Γ ) is a distance-transitive group, but there might be many subgroups of it still acting distance-transitively on Γ . Distance-transitive graphs are vertex transitive and they are highly symmetric; in a certain sense they have the largest group of automorphisms possible. Some well known examples of distance- transitive graphs are the Hamming graphs, Johnson graphs and Grassmann graphs; see [15] for a detailed description of these graphs and for many others. -
A Note on the Automorphism Group of the Hamming Graph
Transactions on Combinatorics ISSN (print): 2251-8657, ISSN (on-line): 2251-8665 Vol. 10 No. 2 (2021), pp. 129-136. ⃝c 2021 University of Isfahan www.ui.ac.ir www.ui.ac.ir A NOTE ON THE AUTOMORPHISM GROUP OF THE HAMMING GRAPH SEYED MORTEZA MIRAFZAL∗ AND MEYSAM ZIAEE Abstract. Let m > 1 be an integer and Ω be an m-set. The Hamming graph H(n; m) has Ωn as its vertex-set, with two vertices are adjacent if and only if they differ in exactly one coordinate. In this paper, we provide a new proof on the automorphism group of the Hamming graph H(n; m). Although our result is not new (CE Praeger, C Schneider, Permutation groups and Cartesian decompositions, Cambridge University Press, 2018), we believe that our proof is shorter and more elementary than the known proofs for determining the automorphism group of Hamming graph. 1. Introduction Let m > 1 be an integer and Ω be an m-set. The Hamming graph H(n; m) has Ωn as its vertex-set, with two vertices are adjacent if and only if they differ in exactly one coordinate. The latter graph is very famous and much is known about it. For instance this graph is actually the Cartesian product of n complete graphs Km, that is, Km□ ··· □Km. Without lose of generality, we can assume that Ω = f1; 2; : : : ; mg. In general, the connection between Hamming graphs and coding theory is of major importance. If m = 2, then H(n; m) = Qn, where Qn is the hypercube of dimension n.