Total K-Uniform Graphs Are Indeed Total Domination Version of K-Uniform Graphs Introduced in [10]
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Automorphisms of the Double Cover of a Circulant Graph of Valency at Most 7
AUTOMORPHISMS OF THE DOUBLE COVER OF A CIRCULANT GRAPH OF VALENCY AT MOST 7 ADEMIR HUJDUROVIC,´ ¯DOR¯DE MITROVIC,´ AND DAVE WITTE MORRIS Abstract. A graph X is said to be unstable if the direct product X × K2 (also called the canonical double cover of X) has automorphisms that do not come from automorphisms of its factors X and K2. It is nontrivially unstable if it is unstable, connected, and non-bipartite, and no two distinct vertices of X have exactly the same neighbors. We find all of the nontrivially unstable circulant graphs of valency at most 7. (They come in several infinite families.) We also show that the instability of each of these graphs is explained by theorems of Steve Wilson. This is best possible, because there is a nontrivially unstable circulant graph of valency 8 that does not satisfy the hypotheses of any of Wilson's four instability theorems for circulant graphs. 1. Introduction Let X be a circulant graph. (All graphs in this paper are finite, simple, and undirected.) Definition 1.1 ([16]). The canonical bipartite double cover of X is the bipartite graph BX with V (BX) = V (X) 0; 1 , where × f g (v; 0) is adjacent to (w; 1) in BX v is adjacent to w in X: () Letting S2 be the symmetric group on the 2-element set 0; 1 , it is clear f g that the direct product Aut X S2 is a subgroup of Aut BX. We are interested in cases where this subgroup× is proper: Definition 1.2 ([12, p. -
Graph Operations and Upper Bounds on Graph Homomorphism Counts
Graph Operations and Upper Bounds on Graph Homomorphism Counts Luke Sernau March 9, 2017 Abstract We construct a family of countexamples to a conjecture of Galvin [5], which stated that for any n-vertex, d-regular graph G and any graph H (possibly with loops), n n d d hom(G, H) ≤ max hom(Kd,d,H) 2 , hom(Kd+1,H) +1 , n o where hom(G, H) is the number of homomorphisms from G to H. By exploiting properties of the graph tensor product and graph exponentiation, we also find new infinite families of H for which the bound stated above on hom(G, H) holds for all n-vertex, d-regular G. In particular we show that if HWR is the complete looped path on three vertices, also known as the Widom-Rowlinson graph, then n d hom(G, HWR) ≤ hom(Kd+1,HWR) +1 for all n-vertex, d-regular G. This verifies a conjecture of Galvin. arXiv:1510.01833v3 [math.CO] 8 Mar 2017 1 Introduction Graph homomorphisms are an important concept in many areas of graph theory. A graph homomorphism is simply an adjacency-preserving map be- tween a graph G and a graph H. That is, for given graphs G and H, a function φ : V (G) → V (H) is said to be a homomorphism from G to H if for every edge uv ∈ E(G), we have φ(u)φ(v) ∈ E(H) (Here, as throughout, all graphs are simple, meaning without multi-edges, but they are permitted 1 to have loops). -
Results on Domination Number and Bondage Number for Some Families of Graphs
International Journal of Pure and Applied Mathematics Volume 107 No. 3 2016, 565-577 ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version) url: http://www.ijpam.eu AP doi: 10.12732/ijpam.v107i3.5 ijpam.eu RESULTS ON DOMINATION NUMBER AND BONDAGE NUMBER FOR SOME FAMILIES OF GRAPHS G. Hemalatha1, P. Jeyanthi2 § 1Department of Mathematics Shri Andal Alagar College of Engineering Mamandur, Kancheepuram, Tamil Nadu, INDIA 2Research Centre, Department of Mathematics Govindammal Aditanar College for Women Tiruchendur 628 215, Tamil Nadu, INDIA Abstract: Let G = (V, E) be a simple graph on the vertex set V . In a graph G, A set S ⊆ V is a dominating set of G if every vertex in VS is adjacent to some vertex in S. The domination number of a graph Gγ(G)] is the minimum size of a dominating set of vertices in G. The bondage number of a graph G[Bdγ(G)] is the cardinality of a smallest set of edges whose removal results in a graph with domination number larger than that of G. In this paper we establish domination number and the bondage number for some families of graphs. AMS Subject Classification: 05C69 Key Words: dominating set, bondage number, domination number, cocktail party graph, coxeter graph, crown graph, cubic symmetric graph, doyle graph, folkman graph, levi graph, icosahedral graph 1. Introduction The domination in graphs is one of the major areas in graph theory which attracts many researchers because it has the potential to solve many real life problems involving design and analysis of communication network, social net- Received: October 17, 2015 c 2016 Academic Publications, Ltd. -
Constructing Arbitrarily Large Graphs with a Specified Number Of
Electronic Journal of Graph Theory and Applications 4 (1) (2019), 1–8 Constructing Arbitrarily Large Graphs with a Specified Number of Hamiltonian Cycles Michael Haythorpea aSchool of Computer Science, Engineering and Mathematics, Flinders University, 1284 South Road, Clovelly Park, SA 5042, Australia michael.haythorpe@flinders.edu.au Abstract A constructive method is provided that outputs a directed graph which is named a broken crown graph, containing 5n − 9 vertices and k Hamiltonian cycles for any choice of integers n ≥ k ≥ 4. The construction is not designed to be minimal in any sense, but rather to ensure that the graphs produced remain non-trivial instances of the Hamiltonian cycle problem even when k is chosen to be much smaller than n. Keywords: Hamiltonian cycles, Graph Construction, Broken Crown Mathematics Subject Classification : 05C45 1. Introduction The Hamiltonian cycle problem (HCP) is a famous NP-complete problem in which one must determine whether a given graph contains a simple cycle traversing all vertices of the graph, or not. Such a simple cycle is called a Hamiltonian cycle (HC), and a graph containing at least one arXiv:1902.10351v1 [math.CO] 27 Feb 2019 Hamiltonian cycle is said to be a Hamiltonian graph. Typically, randomly generated graphs (such as Erdos-R˝ enyi´ graphs), if connected, are Hamil- tonian and contain many Hamiltonian cycles. Although HCP is an NP-complete problem, for these graphs it is often fairly easy for a sophisticated heuristic (e.g. see Concorde [1], Keld Helsgaun’s LKH [4] or Snakes-and-ladders Heuristic [2]) to discover one of the multitude of Hamiltonian cy- cles through a clever search. -
Prime Labeling of Union Graphs
International Journal of Scientific Research in _____________________________ Research Paper . Mathematical and Statistical Sciences Vol.6, Issue.1, pp.86-93, February (2019) E-ISSN: 2348-4519 DOI: https://doi.org/10.26438/ijsrmss/v6i1.8693 Prime labeling of Union Graphs S K Patel1, A N Kansagara2* 1 Department of Mathematics, Government Engineering College, Bhuj-370 001. India. 2 Department of Mathematics, Shree D K V Arts and Science College, Jamnagar-361 008. India. *Corresponding author: [email protected] Tel: +91 94267 57504 Available online at: www.isroset.org Received: 01/Feb/2019, Accepted: 11/Feb/2019, Online: 28/Feb/2019 Abstract- A graph with vertices is called prime graph if its vertices can be labeled with first positive integers such that each pair of adjacent vertices have relatively prime labels. Such a labeling is called prime labeling. It is known that graphs like gear graphs, crown graphs, helm graphs, book graphs and are prime. In this paper, we study the prime labeling of union of these graphs. Keyword: Prime labeling, union of graphs, Corona product, Cartesian Product. AMS Subject Classification(2010): 05C7 I. INTRODUCTION (iii) whereas and are odd integers (iv) , where is any integer. All graphs considered in this paper are simple, finite, and 2. undirected. For a graph , and denotes the 3. , where is an odd integer. vertex set and edge set of respectively whereas 4. For arbitrary integers if and denotes the cardinality of the respective sets. For , then . terms not defined here, we refer the reader to [6]. Paths and cycles are the immediate examples of Now we discuss some basic definions which will be used in prime graphs but on the other hand the conjecture that trees main results. -
Smith Forms for Adjacency Matrices of Circulant Graphs
Smith forms for adjacency matrices of circulant graphs Gerald Williamsa aDepartment of Mathematical Sciences, University of Essex, Colchester, Essex CO4 3SQ, U.K. Abstract We calculate the Smith normal form of the adjacency matrix of each of the following graphs or their complements (or both): complete graph, cycle graph, square of the cycle, power graph of the cycle, distance matrix graph of cycle, Andr´asfaigraph, Doob graph, cocktail party graph, crown graph, prism graph, M¨obiusladder. The proofs operate by finding the abelianisation of a cyclically presented group whose relation matrix is column equivalent to the required adjacency matrix. Keywords: Smith normal form, circulant graph, adjacency matrix. 2000 MSC: 05C50, 15A21, 20F05. 1. Introduction The circulant matrix circn(a0; : : : ; an−1) is the n × n matrix whose first row is (a0; : : : ; an−1) and where row (i+1) (0 ≤ i ≤ n−2) is a cyclic shift of row i by one column. A circulant graph is a graph that is isomorphic to a graph whose adjacency matrix is circulant. We shall write A(Γ) for the adjacency matrix of a graph Γ. Given graphs Γ; Γ0 if det(A(Γ)) =6 det(A(Γ0)) (in particular if precisely one of A(Γ), A(Γ0) is singular) then Γ; Γ0 are non-isomorphic. Similarly, if rank(A(Γ)) =6 rank(A(Γ0)) then Γ; Γ0 are non-isomorphic. Singularity, rank, and determinants of various families of circulant graphs are considered (for example) in [3],[7],[16],[23]. For an n × n integer matrix M, the Smith normal form of M, written SNF(M), is the n × n diagonal integer matrix S = diagn(d0; : : : ; dn−1) where d0; : : : ; dn−1 2 N [ f0g and dijdi+1 (0 ≤ i ≤ n − 2) is such that there exist invertible integer matrices P; Q such that PMQ = S. -
Lombardi Drawings of Graphs 1 Introduction
Lombardi Drawings of Graphs Christian A. Duncan1, David Eppstein2, Michael T. Goodrich2, Stephen G. Kobourov3, and Martin Nollenburg¨ 2 1Department of Computer Science, Louisiana Tech. Univ., Ruston, Louisiana, USA 2Department of Computer Science, University of California, Irvine, California, USA 3Department of Computer Science, University of Arizona, Tucson, Arizona, USA Abstract. We introduce the notion of Lombardi graph drawings, named after the American abstract artist Mark Lombardi. In these drawings, edges are represented as circular arcs rather than as line segments or polylines, and the vertices have perfect angular resolution: the edges are equally spaced around each vertex. We describe algorithms for finding Lombardi drawings of regular graphs, graphs of bounded degeneracy, and certain families of planar graphs. 1 Introduction The American artist Mark Lombardi [24] was famous for his drawings of social net- works representing conspiracy theories. Lombardi used curved arcs to represent edges, leading to a strong aesthetic quality and high readability. Inspired by this work, we intro- duce the notion of a Lombardi drawing of a graph, in which edges are drawn as circular arcs with perfect angular resolution: consecutive edges are evenly spaced around each vertex. While not all vertices have perfect angular resolution in Lombardi’s work, the even spacing of edges around vertices is clearly one of his aesthetic criteria; see Fig. 1. Traditional graph drawing methods rarely guarantee perfect angular resolution, but poor edge distribution can nevertheless lead to unreadable drawings. Additionally, while some tools provide options to draw edges as curves, most rely on straight-line edges, and it is known that maintaining good angular resolution can result in exponential draw- ing area for straight-line drawings of planar graphs [17,25]. -
The Operations Invariant Properties on Graphs Yanzhong Hu, Gang
International Conference on Education Technology and Information System (ICETIS 2013) The Operations Invariant Properties on Graphs Yanzhong Hu, Gang Cheng School of Computer Science, Hubei University of Technology, Wuhan, 430068, China [email protected], [email protected] Keywords:Invariant property; Hamilton cycle; Cartesian product; Tensor product Abstract. To determine whether or not a given graph has a Hamilton cycle (or is a planar graph), defined the operations invariant properties on graphs, and discussed the various forms of the invariant properties under the circumstance of Cartesian product graph operation and Tensor product graph operation. The main conclusions include: The Hamiltonicity of graph is invariant concerning the Cartesian product, and the non-planarity of the graph is invariant concerning the tensor product. Therefore, when we applied these principles into practice, we testified that Hamilton cycle does exist in hypercube and the Desargues graph is a non-planarity graph. INTRODUCTION The Planarity, Eulerian feature, Hamiltonicity, bipartite property, spectrum, and so on, is often overlooked for a given graph. The traditional research method adopts the idea of direct proof. In recent years, indirect proof method is put forward, (Fang Xie & Yanzhong Hu, 2010) proves non- planarity of the Petersen graph, by use the non-planarity of K3,3, i.e., it induces the properties of the graph G with the properties of the graph H, here H is a sub-graph of G. However, when it comes to induce the other properties, the result is contrary to what we expect. For example, Fig. 1(a) is a Hamiltonian graph which is a sub-graph of Fig. -
Pattern Avoidance and Fiber Bundle Structures on Schubert Varieties
Thursday 2.10 Auditorium B Pattern Avoidance and Fiber Bundle Structures on Schubert Varieties Timothy Alland [email protected] SUNY Stony Brook (This talk is based on joint work with Edward Richmond.) MSC2000: 05E15, 14M15 Permutations are known to index type A Schubert varieties. A question one can ask regarding a specific Schubert variety is whether it is an iterated fiber bundle of Grass- mannian Schubert varieties, that is, if it has a complete parabolic bundle structure. We have found that a Schubert variety has a complete parabolic bundle structure if and only if its associated permutation avoids the patterns 3412, 52341, and 635241. We find this by first identifying when the standard projection from the Schubert variety in the complete flag variety to the Schubert variety in the Grassmannian is a fiber bundle using what we have called \split pattern avoidance". In this talk, I will demonstrate how we were able to move from a characterization of this projection in terms of the support and left descents of the permutation's parabolic decomposition to one that applies split pattern avoidance. I will also give a flavor of the proof of how the three patterns mentioned above determine whether a Schubert variety has a complete parabolic bundle structure. Monday 3.55 Auditorium B A simple proof of Shamir's conjecture Peter Allen [email protected] LSE (This talk is based on joint work with Julia B¨ottcher, Ewan Davies, Matthew Jenssen, Yoshiharu Kohayakawa, Barnaby Roberts.) MSC2000: 05C80 It is well known (and easy to show) that the threshold for a perfect matching in the bino- log n mial random graph G(n; p) is p = Θ n , coinciding with the threshold for every vertex to be in an edge (and much more is known). -
Arxiv:2108.05223V2 [Math.CO] 12 Aug 2021 the Line Graph of the Crown
The line graph of the crown graph is distance integral S. Morteza Mirafzal Department of Mathematics Lorestan University, Khorramabad, Iran E-mail: [email protected] E-mail: [email protected] Abstract The distance eigenvalues of a connected graph G are the eigenvalues of its distance matrix D(G). A graph is called distance integral if all of its distance eigenvalues are integers. Let n ≥ 3 be an integer. A crown graph Cr(n) is a graph obtained from the complete bipartite graph Kn,n by removing a perfect matching. Let L(Cr(n)) denote the line graph of the crown graph Cr(n). In this paper, by using the orbit partition method in algebraic graph theory, we determine the set of all distance eigenvalues of L(Cr(n)) and show that this graph is distance integral. 1 Introduction and Preliminaries arXiv:2108.05223v4 [math.CO] 8 Sep 2021 In this paper, a graph G = (V, E) is considered as an undirected simple graph where V = V (G) is the vertex-set and E = E(G) is the edge-set. For all the terminology and notation not defined here, we follow [3,4,5,6,7]. Let G = (V, E) be a graph and A = A(G) be an adjacency matrix of G. The characteristic polynomial of G is defined as P (G; x)= P (x)= |xI − A|. A zero of p(x) is called an eigenvalue of the graph G. A graph is called integral, if all the eigenvalues are integers. The study of integral graphs was initiated by 2010 Mathematics Subject Classification:05C50 Keywords: crown graph, distance integral, vertex-transitive, orbit partition Date: 1 2 S. -
Not Every Bipartite Double Cover Is Canonical
Volume 82 BULLETIN of the February 2018 INSTITUTE of COMBINATORICS and its APPLICATIONS Editors-in-Chief: Marco Buratti, Donald Kreher, Tran van Trung Boca Raton, FL, U.S.A. ISSN1182-1278 BULLETIN OF THE ICA Volume 82 (2018), Pages 51{55 Not every bipartite double cover is canonical Tomaˇz Pisanski University of Primorska, FAMNIT, Koper, Slovenia and IMFM, Ljubljana, Slovenia. [email protected]. Abstract: It is explained why the term bipartite double cover should not be used to designate canonical double cover alias Kronecker cover of graphs. Keywords: Kronecker cover, canonical double cover, bipartite double cover, covering graphs. Math. Subj. Class. (2010): 05C76, 05C10. It is not uncommon to use different terminology or notation for the same mathematical concept. There are several reasons for such a phenomenon. Authors independently discover the same object and have to name it. It is• quite natural that they choose different names. Sometimes the same concept appears in different mathematical schools or in• different disciplines. By using particular terminology in a given context makes understanding of such a concept much easier. Sometimes original terminology is not well-chosen, not intuitive and it is difficult• to relate the name of the object to its meaning. A name that is more appropriate for a given concept is preferred. One would expect terminology to be as simple as possible, and as easy to connect it to the concept as possible. However, it should not be too simple. Received: 8 October 2018 51 Accepted: 21 January 2018 In other words it should not introduce ambiguity. Unfortunately, the term bipartite double cover that has been used lately in several places to replace an older term canonical double cover, also known as the Kronecker cover, [1,4], is ambiguous. -
Nearly Tight Approximability Results for Minimum Biclique Cover and Partition
Nearly Tight Approximability Results for Minimum Biclique Cover and Partition Parinya Chalermsook1, Sandy Heydrich1;3, Eugenia Holm2, and Andreas Karrenbauer1∗ 1 Max Planck Institute for Informatics, Saarbr¨ucken, Germany 2 Dept. for Computer and Information Science, University of Konstanz, Germany 3 Saarbr¨ucken Graduate School of Computer Science {andreas.karrenbauer,parinya.chalermsook,sandy.heydrich}@mpi-inf.mpg.de [email protected] Abstract. In this paper, we consider the minimum biclique cover and minimum biclique partition problems on bipartite graphs. In the min- imum biclique cover problem, we are given an input bipartite graph G = (V; E), and our goal is to compute the minimum number of complete bipartite subgraphs that cover all edges of G. This problem, besides its correspondence to a well-studied notion of bipartite dimension in graph theory, has applications in many other research areas such as artificial intelligence, computer security, automata theory, and biology. Since it is NP-hard, past research has focused on approximation algorithms, fixed parameter tractability, and special graph classes that admit polynomial time exact algorithms. For the minimum biclique partition problem, we are interested in a biclique cover that covers each edge exactly once. We revisit the problems from approximation algorithms' perspectives and give nearly tight lower and upper bound results. We first show that both problems are NP-hard to approximate to within a factor of n1−" (where n is the number of vertices in the input graph). Using a stronger complexity assumption, the hardness becomes Ω~(n), where Ω~(·) hides lower order terms. Then we show that approximation factors of the form n=(log n)γ for some γ > 0 can be obtained.