DOCSLIB.ORG

On Induced Subgraphs with All Degrees Odd

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
On Induced Subgraphs with All Degrees Odd ON INDUCED SUBGRAPHS WITH ALL DEGREES ODD A.D. SCOTT Abstract. Gallai proved that the vertex set of any graph can be partitioned into two sets, each inducing a subgraph with all degrees even. We prove that every connected graph of even order has a vertex partition into sets inducing subgraphs with all degrees odd, and give bounds for the number of sets of this type required for vertex partitions and vertex covers. We also give results on the partitioning and covering problems for random graphs. Note: For the final version of this paper, see the journal publication. 1. Introduction Gallai (see [10], Exercise 5.19) proved that every graph G has a ver- tex partition into two sets, each of which induces a subgraph with all degrees even. Thus every graph G contains an induced subgraph with all degrees even on at least djGj=2e vertices. For induced subgraphs with all degrees odd, however, the situation is less clear. Clearly no subgraph with odd degrees can have an isolated vertex, so we must restrict our attention to graphs without isolated vertices. Let us write f(G) for the maximum order of an induced subgraph of G with all degrees odd and f(n) for the minimum of f(G) over the set of graphsp of order n without isolated vertices. Caro [5] proved that f(n) ≥ c n for n ≥ 2. It was proved in [12] that (1) f(n) ≥ cn= log n: Best possible bounds for f(T ) where T is a tree have been proved by Radcliffe and Scott [11], while Berman, Wang and Wargo [3] have proved optimal bounds for graphs with maximum degree 3. It has been conjectured (see [5]) that there is c > 0 such that f(n) ≥ cn for n ≥ 2. In this paper we prove results about partitioning and covering by induced subgraphs with all degrees odd. We begin by asking under what conditions a graph G has a vertex partition into sets that induce subgraphs with all degrees odd. A graph with all degrees odd must have even order, so clearly every component of a graph with such a 1 2 A.D. SCOTT vertex partition must have even order. Surprisingly, this trivial neces- sary condition is in fact sufficient, and we prove this in x2. It would be interesting to determine the number of sets required for a vertex partition of a graph of order n into sets inducing subgraphs with all degrees odd. Itp is known that o(n) sets suffice and we give an example showing that c n sets may be required. In x3, we look at vertex coverings by sets that induce subgraphs with all degrees odd. It turns out that we can in general achieve this with far fewer sets than for a vertex partition: we prove that every graph has a vertex covering of this type with c log2 n sets and give an example showing that we may need c log n sets. We would expect that for most graphs we could do rather better than this, and in x4 we prove that this is the case. In particular, we 1 prove that almost every graph G 2 G(n; 2 ) has a vertex partition (and hence a vertex covering) with three sets inducing subgraphs with all degrees odd. In x5, we turn to residue classes modulo k. In particular, following Caro [5] and Caro, Krasikov and Roditty [6], for a graph G without isolated vertices, we define fk(G) to be the maximum order of an in- duced subgraph of G with all degrees congruent to 1 modulo k. We give a lower bound on fk(G) in terms of the chromatic number of G, and prove that every connected claw-free graph G of order n satis- p fies fk(G) ≥ (1 + o(1)) n=12. This improves upon a result of Caro [5], who gave a lower bound of c(n log n)1=3. Best possible bounds for fk(T ), where T is a tree, are given by Berman, Radcliffe, Scott, Wang and Wargo [2]. Finally, in x6, we consider some open questions. 2. Partitions into induced subgraphs Under what conditions does a graph G have a vertex partition into sets inducing subgraphs with all degrees odd? Given a graph G, we say that a partition V1;:::;Vk of V (G) is a good partition of G if G[Vi] has all degrees odd for i = 1; : : : ; k. Any subgraph with all degrees odd must have an even number of vertices, so if G has a good parti- tion then it must have an even number of vertices in each component. Surprisingly, this trivial condition is also sufficient. Theorem 1. Let G be a graph. Then G has a good partition if and only if every component of G has even order. Proof. If a graph G has a good partition then it is clearly necessary that every component of G have even order. We prove by induction ON INDUCED SUBGRAPHS WITH ALL DEGREES ODD 3 on jGj that if every component of G has even order then G has a good partition. If jGj = 2 then the statement is trivial. We suppose that jGj > 2, that every component of G has even order, and that the inductive statement is true for graphs of smaller order. We may assume that G is connected (and thus has even order), or else we could partition every component of G separately. If, for every edge xy in G, we have d(x) = 1 or d(y) = 1, then G is a star: since jGj is even, G must itself have all degrees odd, so we can take the trivial partition. Otherwise, we may assume that we can find an edge xy with d(x) > 1 and d(y) > 1. Let G0 = G n fx; yg. If G0 is connected, then by the inductive hypothesis we can find some 0 good partition V1;:::;Vk of G , in which case V1;:::;Vk; fx; yg is a good partition of G. Also, if any component S of G0 has even order then we can find a good partition of S, and since GnS is connected we can find a good partition of G n S and combine these partitions to give a good partition of G. Thus we may assume that every component of G0 has odd order. 0 Let X1;:::;Xi be the components of G adjacent to x but not to y; 0 let Y1;:::;Yj be the components of G adjacent to y but not to x; and 0 let Z1;:::;Zk be the components of G adjacent to both x and y. Since G has even order, i + j + k must be even. If k > 0 then let i k [ [ X = fxg [ V (Xm) [ V (Zm); m=1 m=2 and j [ Y = fyg [ V (Ym): m=1 Exactly one of jXj and jY j is odd: adding V (Z1) to that set gives a partition of V (G) into two sets inducing non-empty connected graphs with even order. Both graphs have a good partition by the inductive hypothesis, and these can be combined to obtain a good partition for G. We may therefore assume that k = 0, so that i and j are both even or both odd. Now if i and j are both odd, then let i [ X = fxg [ V (Xm); m=1 4 A.D. SCOTT and j [ Y = fyg [ V (Ym): m=1 Both G[X] and G[Y ] are connected graphs with even order smaller than jGj, so both have good partitions, and hence G does as well. Otherwise, both i and j are even (and i + j ≥ 2). Let i [ X = fx; yg [ V (Xm); m=1 and i [ Y = fx; yg [ V (Ym): m=1 Let V1;:::;Vp be a good partition of G[X] and W1;:::;Wq be a good partition of G[Y ]. We may assume that x 2 V1 and y 2 W1, and so y 2 V1 and x 2 W1. Then V1 [ W1;V2;:::;Vp;W2;:::;Wq is a good partition of G. Can we say anything further about the subgraphs induced by the vertex sets in a good partition? We know from Theorem 1 that if G is a connected graph of even order, then G has a good partition. We say that G is a basic odd graph if G is minimal with respect to these partitions: in other words, G has all degrees odd, and the only good partition of G is the trivial partition fV (G)g. It is easy to characterize the basic odd graphs. Lemma 2. A graph G is a basic odd graph iff it is a tree with all degrees odd. Proof. Suppose first that G is a tree with all degrees odd and it has a good partition into sets V1;:::;Vk, with k ≥ 2. Let xy be any edge in G between V1 and V (G) n V1, and let Gx and Gy be the components of G with the edge xy removed, where x 2 V (Gx) and y 2 V (Gy). Since every vertex of Gx has odd degree (in G and hence in Gx), except for x, which has even degree in Gx, we must have jGxj odd, and similarly jGyj odd. However, there are clearly no edges other than xy between Vi \ V (Gx) and Vi \ V (Gy) , for i = 1; : : : ; k, so V1 \ V (Gx);:::;Vk \ V (Gx) must be a good partition of Gx, which is impossible, since jGxj is odd.
Load more

Recommended publications
  • Maximizing the Order of a Regular Graph of Given Valency and Second Eigenvalue∗
    SIAM J. DISCRETE MATH. c 2016 Society for Industrial and Applied Mathematics Vol. 30, No. 3, pp. 1509–1525 MAXIMIZING THE ORDER OF A REGULAR GRAPH OF GIVEN VALENCY AND SECOND EIGENVALUE∗ SEBASTIAN M. CIOABA˘ †,JACKH.KOOLEN‡, HIROSHI NOZAKI§, AND JASON R. VERMETTE¶ Abstract. From Alon√ and Boppana, and Serre, we know that for any given integer k ≥ 3 and real number λ<2 k − 1, there are only finitely many k-regular graphs whose second largest eigenvalue is at most λ. In this paper, we investigate the largest number of vertices of such graphs. Key words. second eigenvalue, regular graph, expander AMS subject classifications. 05C50, 05E99, 68R10, 90C05, 90C35 DOI. 10.1137/15M1030935 1. Introduction. For a k-regular graph G on n vertices, we denote by λ1(G)= k>λ2(G) ≥ ··· ≥ λn(G)=λmin(G) the eigenvalues of the adjacency matrix of G. For a general reference on the eigenvalues of graphs, see [8, 17]. The second eigenvalue of a regular graph is a parameter of interest in the study of graph connectivity and expanders (see [1, 8, 23], for example). In this paper, we investigate the maximum order v(k, λ) of a connected k-regular graph whose second largest eigenvalue is at most some given parameter λ. As a consequence of work of Alon and Boppana and of Serre√ [1, 11, 15, 23, 24, 27, 30, 34, 35, 40], we know that v(k, λ) is finite for λ<2 k − 1. The recent result of Marcus, Spielman, and Srivastava [28] showing the existence of infinite families of√ Ramanujan graphs of any degree at least 3 implies that v(k, λ) is infinite for λ ≥ 2 k − 1.
    [Show full text]
  • Distances, Diameter, Girth, and Odd Girth in Generalized Johnson Graphs
    Distances, Diameter, Girth, and Odd Girth in Generalized Johnson Graphs Ari J. Herman Dept. of Mathematics & Statistics Portland State University, Portland, OR, USA Mathematical Literature and Problems project in partial fulfillment of requirements for the Masters of Science in Mathematics Under the direction of Dr. John Caughman with second reader Dr. Derek Garton Abstract Let v > k > i be non-negative integers. The generalized Johnson graph, J(v; k; i), is the graph whose vertices are the k-subsets of a v-set, where vertices A and B are adjacent whenever jA \ Bj = i. In this project, we present the results of the paper \On the girth and diameter of generalized Johnson graphs," by Agong, Amarra, Caughman, Herman, and Terada [1], along with a number of related additional results. In particular, we derive general formulas for the girth, diameter, and odd girth of J(v; k; i). Furthermore, we provide a formula for the distance between any two vertices A and B in terms of the cardinality of their intersection. We close with a number of possible future directions. 2 1. Introduction In this project, we present the results of the paper \On the girth and diameter of generalized Johnson graphs," by Agong, Amarra, Caughman, Herman, and Terada [1], along with a number of related additional results. Let v > k > i be non-negative integers. The generalized Johnson graph, X = J(v; k; i), is the graph whose vertices are the k-subsets of a v-set, with adjacency defined by A ∼ B , jA \ Bj = i: (1) Generalized Johnson graphs were introduced by Chen and Lih in [4].
    [Show full text]
  • ON SOME PROPERTIES of MIDDLE CUBE GRAPHS and THEIR SPECTRA Dr.S
    Science, Technology and Development ISSN : 0950-0707 ON SOME PROPERTIES OF MIDDLE CUBE GRAPHS AND THEIR SPECTRA Dr.S. Uma Maheswari, Lecturer in Mathematics, J.M.J. College for Women (Autonomous) Tenali, Andhra Pradesh, India Abstract: In this research paper we begin with study of family of n For example, the boundary of a 4-cube contains 8 cubes, dimensional hyper cube graphs and establish some properties related to 24 squares, 32 lines and 16 vertices. their distance, spectra, and multiplicities and associated eigen vectors and extend to bipartite double graphs[11]. In a more involved way since no complete characterization was available with experiential results in several inter connection networks on this spectra our work will add an element to existing theory. Keywords: Middle cube graphs, distance-regular graph, antipodalgraph, bipartite double graph, extended bipartite double graph, Eigen values, Spectrum, Adjacency Matrix.3 1) Introduction: An n dimensional hyper cube Qn [24] also called n-cube is an n dimensional analogue of Square and a Cube. It is closed compact convex figure whose 1-skelton consists of groups of opposite parallel line segments aligned in each of spaces dimensions, perpendicular to each other and of same length. 1.1) A point is a hypercube of dimension zero. If one moves this point one unit length, it will sweep out a line segment, which is the measure polytope of dimension one. A projection of hypercube into two- dimensional image If one moves this line segment its length in a perpendicular direction from itself; it sweeps out a two-dimensional square.
    [Show full text]
  • Some Results on 2-Odd Labeling of Graphs
    International Journal of Recent Technology and Engineering (IJRTE) ISSN: 2277-3878, Volume-8 Issue-5, January 2020 Some Results on 2-odd Labeling of Graphs P. Abirami, A. Parthiban, N. Srinivasan Abstract: A graph is said to be a 2-odd graph if the vertices of II. MAIN RESULTS can be labelled with integers (necessarily distinct) such that for In this section, we establish 2-odd labeling of some graphs any two vertices which are adjacent, then the modulus difference of their labels is either an odd integer or exactly 2. In this paper, such as wheel graph, fan graph, generalized butterfly graph, we investigate 2-odd labeling of some classes of graphs. the line graph of sunlet graph, and shell graph. Lemma 1. Keywords: Distance Graphs, 2-odd Graphs. If a graph has a subgraph that does not admit a 2-odd I. INTRODUCTION labeling then G cannot have a 2-odd labeling. One can note that the Lemma 1 clearly holds since if has All graphs discussed in this article are connected, simple, a 2-odd labeling then this will yield a 2-odd labeling for any subgraph of . non-trivial, finite, and undirected. By , or simply , Definition 1: [5] we mean a graph whose vertex set is and edge set is . The wheel graph is a graph with vertices According to J. D. Laison et al. [2] a graph is 2-odd if there , formed by joining the central vertex to all the exists a one-to-one labeling (“the set of all vertices of . integers”) such that for any two vertices and which are Theorem 1: adjacent, the integer is either exactly 2 or an The wheel graph admits a 2-odd labeling for all .
    [Show full text]
  • The Distance-Regular Antipodal Covers of Classical Distance-Regular Graphs
    COLLOQUIA MATHEMATICA SOCIETATIS JANOS BOLYAI 52. COMBINATORICS, EGER (HUNGARY), 198'7 The Distance-regular Antipodal Covers of Classical Distance-regular Graphs J. T. M. VAN BON and A. E. BROUWER 1. Introduction. If r is a graph and "Y is a vertex of r, then let us write r,("Y) for the set of all vertices of r at distance i from"/, and r('Y) = r 1 ('Y)for the set of all neighbours of 'Y in r. We shall also write ry ...., S to denote that -y and S are adjacent, and 'YJ. for the set h} u r b) of "/ and its all neighbours. r i will denote the graph with the same vertices as r, where two vertices are adjacent when they have distance i in r. The graph r is called distance-regular with diameter d and intersection array {bo, ... , bd-li c1, ... , cd} if for any two vertices ry, oat distance i we have lfH1(1')n nr(o)j = b, and jri-ib) n r(o)j = c,(o ::; i ::; d). Clearly bd =co = 0 (and C1 = 1). Also, a distance-regular graph r is regular of degree k = bo, and if we put ai = k- bi - c; then jf;(i) n r(o)I = a, whenever d("f, S) =i. We shall also use the notations k, = lf1b)I (this is independent of the vertex ry),). = a1,µ = c2 . For basic properties of distance-regular graphs, see Biggs [4]. The graph r is called imprimitive when for some I~ {O, 1, ... , d}, I-:/= {O}, If -:/= {O, 1, ..
    [Show full text]
  • A Short Proof of the Odd-Girth Theorem
    A short proof of the odd-girth theorem E.R. van Dam M.A. Fiol Dept. Econometrics and O.R. Dept. de Matem`atica Aplicada IV Tilburg University Universitat Polit`ecnica de Catalunya Tilburg, The Netherlands Barcelona, Catalonia [email protected] [email protected] Submitted: Apr 27, 2012; Accepted: July 25, 2012; Published: Aug 9, 2012 Mathematics Subject Classifications: 05E30, 05C50 Abstract Recently, it has been shown that a connected graph Γ with d+1 distinct eigenvalues and odd-girth 2d + 1 is distance-regular. The proof of this result was based on the spectral excess theorem. In this note we present an alternative and more direct proof which does not rely on the spectral excess theorem, but on a known characterization of distance-regular graphs in terms of the predistance polynomial of degree d. 1 Introduction The spectral excess theorem [10] states that a connected regular graph Γ is distance- regular if and only if its spectral excess (a number which can be computed from the spectrum of Γ) equals its average excess (the mean of the numbers of vertices at maximum distance from every vertex), see [5, 9] for short proofs. Using this theorem, Van Dam and Haemers [6] proved the below odd-girth theorem for regular graphs. Odd-girth theorem. A connected graph with d + 1 distinct eigenvalues and finite odd- girth at least 2d + 1 is a distance-regular generalized odd graph (that is, a distance-regular graph with diameter D and odd-girth 2D + 1). In the same paper, the authors posed the problem of deciding whether the regularity condition is necessary or, equivalently, whether or not there are nonregular graphs with d+1 distinct eigenvalues and odd girth 2d+1.
    [Show full text]
  • Tilburg University an Odd Characterization of the Generalized
    Tilburg University An odd characterization of the generalized odd graphs van Dam, E.R.; Haemers, W.H. Published in: Journal of Combinatorial Theory, Series B, Graph theory DOI: 10.1016/j.jctb.2011.03.001 Publication date: 2011 Document Version Peer reviewed version Link to publication in Tilburg University Research Portal Citation for published version (APA): van Dam, E. R., & Haemers, W. H. (2011). An odd characterization of the generalized odd graphs. Journal of Combinatorial Theory, Series B, Graph theory, 101(6), 486-489. https://doi.org/10.1016/j.jctb.2011.03.001 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 Take down policy 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. Download date: 03. okt. 2021 An odd characterization of the generalized odd graphs¤ Edwin R. van Dam and Willem H. Haemers Tilburg University, Dept. Econometrics and O.R., P.O. Box 90153, 5000 LE Tilburg, The Netherlands, e-mail: [email protected], [email protected] Abstract We show that any connected regular graph with d + 1 distinct eigenvalues and odd-girth 2d + 1 is distance-regular, and in particular that it is a generalized odd graph.
    [Show full text]
  • Distance-Transitive Graphs
    Distance-Transitive Graphs Submitted for the module MATH4081 Robert F. Bailey (4MH) Supervisor: Prof. H.D. Macpherson May 10, 2002 2 Robert Bailey Department of Pure Mathematics University of Leeds Leeds, LS2 9JT May 10, 2002 The cover illustration is a diagram of the Biggs-Smith graph, a distance-transitive graph described in section 11.2. Foreword A graph is distance-transitive if, for any two arbitrarily-chosen pairs of vertices at the same distance, there is some automorphism of the graph taking the first pair onto the second. This project studies some of the properties of these graphs, beginning with some relatively simple combinatorial properties (chapter 2), and moving on to dis- cuss more advanced ones, such as the adjacency algebra (chapter 7), and Smith’s Theorem on primitive and imprimitive graphs (chapter 8). We describe four infinite families of distance-transitive graphs, these being the Johnson graphs, odd graphs (chapter 3), Hamming graphs (chapter 5) and Grass- mann graphs (chapter 6). Some group theory used in describing the last two of these families is developed in chapter 4. There is a chapter (chapter 9) on methods for constructing a new graph from an existing one; this concentrates mainly on line graphs and their properties. Finally (chapter 10), we demonstrate some of the ideas used in proving that for a given integer k > 2, there are only finitely many distance-transitive graphs of valency k, concentrating in particular on the cases k = 3 and k = 4. We also (chapter 11) present complete classifications of all distance-transitive graphs with these specific valencies.
    [Show full text]
  • Regular Graphs
    Taylor & Francis I-itt¿'tt¡'tnul "llultilintur ..llsahru- Vol. f4 No.1 1006 i[r-it O T¡yld &kilrr c@p On outindependent subgraphs of strongly regular graphs M. A. FIOL* a¡rd E. GARRIGA l)epartarnent de Matenr¿itica Aplicirda IV. t.luiversit¿rr Politdcnica de Catalunv¡- Jordi Gi'on. I--l- \{t\dul C-1. canrpus Nord. 080-lJ Barcelc¡'a. Sp'i¡r Conrr¡¡¡¡¡.n,"d br W. W¿rtkins I Rr,t t,llr,r/ -aj .llrny'r -rllllj) \n trutl¡dcpe¡dollt suhsr¿tnh ot'a gt'aph f. \\ith respccr to:ul ind!'pcndc¡t vcrtr\ subsct ('C l . rs thc suhl¡.aph f¡-induced bv th!'\ert¡ccs in l'',(', lA'!'s¡ud\ the casc'shcn f is stronslv rcgular. shen: the rcst¡lts ol'de C¿rcn ll99ti. The spc,ctnt ol'conrplcrnenttrn .ubgraph, in qraph. a slron{¡\ rr'!ulitr t-un¡lrt\nt J.rrrnt(l .,1 Ctunl¡in¿!orir:- l9(5)- 559 565.1- allou u: to dc'rivc'thc *lrolc spcctrulrt ol- f¡.. IUorcrrver. rr'lrcn ('rtt¿rins thr. [lollllr¡n Lor,isz bound l'or the indePcndr'nce llu¡rthcr. f( ii a rcsul:rr rr:rph {in lüct. d¡st:rnce-rc.rtular il' I- is I \toore graph). This arliclc'i: nltinlr deltrled to stutll: the non-lcgulal case. As lr nlain result. se chitractcrizc thc structurc ol- i¿^ s ht'n (' is ¡he n..ighhorhood of cirher ¡nc \!.rtc\ ()r ¡ne cdse. Ai,r rrr¡riA. Stlonsh rcuular lra¡rh: Indepcndt-nt set: Sr)ett¡.unr .
    [Show full text]
  • Distance-Transitive Representations of the Symmetric Groups
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector JOURNAL OF COMBINATORIAL THEORY, Series B 41, 255-274 (1986) Distance-Transitive Representations of the Symmetric Groups A. A. TVANOV Institute for System Studies, Academy OJ Sciences of the USSR, 9, Prospect 60 Let Oktyabrya 117312, Moscow, USSR Communicated by Norman L. Bigg.r Received April 1, 1985 Let f be a graph and G < Am(f). The group G is said to act distance-transitively on r if, for any vertices x, y, U, u such that a(x, y) = a(u, u), there is an element g E G mapping x into u and y into u. If G acts distance-transitively on r then the per- mutation group induced by the action of G on the vertex set of r is called the dis- tance-transitive representation of G. In the paper all distance-transitive represen- tations of the symmetric groups S, are classilied. Moreover, all pairs (G, r) such that G acts distance-transitively on r and G = S, for some n are described. The classification problem for these pairs was posed by N. Biggs (Ann. N.Y. Acad. Sci. 319 (1979) 71-81). The problem is closely related to the general question about distance-transitive graphs with given automorphism group. 0 1986 Academic PISS, IK 1. IN~~OUCTI~N r will refer to a connected, undirected, finite graph without loops and multiple edges, consisting of a vertex set V(r), edge set E(T) and having automorphism group Aut(T).
    [Show full text]
  • Quantum Automorphism Groups of Finite Graphs
    Quantum automorphism groups of finite graphs Dissertation zur Erlangung des Grades des Doktors der Naturwissenschaften der Fakult¨atf¨urMathematik und Informatik der Universit¨atdes Saarlandes Simon Schmidt Saarbr¨ucken, 2020 Datum der Verteidigung: 2. Juli 2020 Dekan: Prof. Dr. Thomas Schuster Pr¨ufungsausschuss: Vorsitzende: Prof. Dr. Gabriela Weitze-Schmith¨usen Berichterstatter: Prof. Dr. Moritz Weber Prof. Dr. Roland Speicher Prof. Dr. Julien Bichon Akademischer Mitarbeiter: Dr. Tobias Mai ii Abstract The present work contributes to the theory of quantum permutation groups. More specifically, we develop techniques for computing quantum automorphism groups of finite graphs and apply those to several examples. Amongst the results, we give a criterion on when a graph has quantum symmetry. By definition, a graph has quantum symmetry if its quantum automorphism group does not coincide with its classical automorphism group. We show that this is the case if the classical automorphism group contains a pair of disjoint automorphisms. Furthermore, we prove that several families of distance-transitive graphs do not have quantum symmetry. This includes the odd graphs, the Hamming graphs H(n; 3), the Johnson graphs J(n; 2), the Kneser graphs K(n; 2) and all cubic distance-transitive graphs of order ≥ 10. In particular, this implies that the Petersen graph does not have quantum symmetry, answering a question asked by Banica and Bichon in 2007. Moreover, we show that the Clebsch graph does have quantum symmetry and −1 prove that its quantum automorphism group is equal to SO5 answering a question asked by Banica, Bichon and Collins. More generally, for odd n, the quantum −1 automorphism group of the folded n-cube graph is SOn .
    [Show full text]
  • A Survey on the Missing Moore Graph ∗
    A survey on the missing Moore graph ∗ C. Dalf´o Departament de Matem`atica Universitat de Lleida Igualada (Barcelona), Catalonia [email protected] Abstract This is a survey on some known properties of the possible Moore graph (or graphs) Υ with degree 57 and diameter 2. Moreover, we give some new results about it, such as the following. When we consider the distance partition of Υ induced by a vertex subset C, the following graphs are distance-regular: The induced graph of the vertices at distance 1 from C when C is a set of 400 independent vertices; the induced graphs of the vertices at distance 2 from C when C is a vertex or an edge, and the line graph of Υ. Besides, Υ is an edge-distance-regular graph. Keywords: Moore graphs, Distance-regular graph, Outindependent graph, Spectral graph theory, Automorphisms. Mathematics Subject Classifications: 05C50; 05C25; 20B25; 20C15. 1 Introduction The Moore bound M(∆;D) is an upper bound on the largest possible number of vertices of a graph G with maximum degree ∆ and diameter D. Trivially, if ∆ = 1, then D = 1 and M(1; 1) = 2. If ∆ = 2, then M(2;D) = 2D + 1. For ∆ > 2, (∆ − 1)D − 1 M(∆;D) = 1 + ∆ : ∆ − 2 ∗This research is partially supported by the project 2017SGR1087 of the Agency for the Management of University and Research Grants (AGAUR) of the Government of Catalonia. This research has also received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sk lodowska-Curie grant agree- ment No 734922.
    [Show full text]