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2007, European Journal of Combinatorics
AI
This paper investigates partial cubes and their associated τ-graphs, extending the characterization of resonance graphs in catacondensed hexagonal graphs. It establishes that every graph is a τ-graph of some median graph, proves the connectivity conditions for τ-graphs, and presents conditions under which median graphs are K_n-free. Key results include the implications of these relationships for mathematical chemistry and the characterization of graphs in relation to certain graph classes. The work concludes with open problems, fostering further research.
Discrete Mathematics, 1998
Let G be a median graph on n vertices and m edges and let k be the number of equivalence classes of the Djoković's relation Θ defined on the edge-set of G. Then 2n − m − k ≤ 2. Moreover, 2n − m − k = 2 if and only if G is cube-free.
Discussiones Mathematicae Graph Theory, 2010
The periphery graph of a median graph is the intersection graph of its peripheral subgraphs. We show that every graph without a universal vertex can be realized as the periphery graph of a median graph. We characterize those median graphs whose periphery graph is the join
Discussiones Mathematicae Graph Theory, 2005
Median graphs are characterized among direct products of graphs on at least three vertices. Beside some trivial cases, it is shown that one component of G × P 3 is median if and only if G is a tree in that the distance between any two vertices of degree at least 3 is even. In addition, some partial results considering median graphs of the form G × K 2 are proved, and it is shown that the only nonbipartite quasimedian direct product is K 3 × K 3 .
Discrete Mathematics, 2009
The crossing graph G # of a partial cube G has the equivalence classes of the Djoković-Winkler relation Θ as vertices, two Θ-classes being adjacent if they appear on some common isometric cycle. The following question from [12, Problem 7.3] is treated: Let G be a median graph and n ≥ 4. Does an induced cycle C n in G # necessarily force an induced cogwheel M n in G? It is shown that the answer is positive for n = 4, 5 and negative for n ≥ 6. On the other hand it is proved that if G # contains an induced cycle C n , n ≥ 4, then G contains some induced cogwheel M m , 4 ≤ m ≤ n. A refinement of the expansion procedure for partial cubes is obtained along the way.
For a partial cube (that is, an isometric subgraph of a hypercube) G, quotient graphs G # , G τ , and G ∼ have the equivalence classes of the Djoković-Winkler relation as the vertex set, while edges are defined in three different natural ways. Several results on these quotients are proved and the concepts are compared. For instance, for every graph G there exists a median graph M such that G = M τ . Triangle-free and complete quotient graphs are treated and it is proved that for a median graph G its τ -graph is triangle-free if and only if G contains no convex K 1,3 . Connectedness and the question of when quotients yield the same graphs are also treated.
arXiv: Combinatorics, 2018
Let $G$ be a simple graph of order $n$ with degree sequence $(d)=(d_1,d_2,\ldots,d_n)$ and conjugate degree sequence $(d^*)=(d_1^*,d_2^*,\ldots,d_n^*)$. In \cite{AkbariGhorbaniKoolenObudi2010,DasMojallalGutman2017} it was proven that $\mathcal{E}(G)\leq \sum_{i=1}^{n} \sqrt{d_i}$ and $\sum_{i=1}^{n} \sqrt{d_i^*} \leq LEL(G) \leq IE(G) \leq \sum_{i=1}^{n} \sqrt{d_i}$, where $\mathcal{E}(G)$, $LEL(G)$ and $IE(G)$ are the energy, the Laplacian-energy-like invariant and the incidence energy of $G$, respectively, and in \cite{DasMojallalGutman2017} it was concluded that the class of all connected simple graphs of order $n$ can be dividend into four subclasses according to the position of $\mathcal{E}(G)$ in the order relations above. Then, they proposed a problem about characterizing all graphs in each subclass. In this paper, we attack this problem. First, we count the number of graphs of order $n$ in each of four subclasses for every $1\leq n \leq 8$ using a Sage code. Second, we prese...
Networks, 2010
The distance DG(v) of a vertex v in an undirected graph G is the sum of the distances between v and all other vertices of G. The set of vertices in G with maximum (minimum) distance is the antimedian (median) set of a graph G. It is proved that for arbitrary graphs G and J and a positive integer r ≥ 2, there exists a connected graph H such that G is the antimedian and J the median subgraphs of H, respectively, and that dH (G, J) = r. When both G and J are connected, G and J can in addition be made convex subgraphs of H.
Discrete Applied Mathematics, 2000
A median of a k-tuple = (x 1; : : : ; x k ) of vertices of a ÿnite connected graph G is a vertex x for which k i=1 d(x; xi) is minimum, where d is the geodesic metric on G. The function M with domain the set of all k-tuples with k ¿ 0 and deÿned by M ( ) = {x | x is a median of } is called the median function on G. In this paper a new characterization of the median function is given for G a median graph. This is used to give a characterization of the median function on median semilattices. ? 2000 Elsevier Science B.V. All rights reserved. MSC: primary 05C12; 05C75; secondary 06A12; 90A08
For a set $S$ of vertices and the vertex $v$ in a connected graph $G$, $\displaystyle\max_{x \in S}d(x,v)$ is called the $S$-eccentricity of $v$ in $G$. The set of vertices with minimum $S$-eccentricity is called the $S$-center of $G$. Any set $A$ of vertices of $G$ such that $A$ is an $S$-center for some set $S$ of vertices of $G$ is called a center set. We identify the center sets of certain classes of graphs namely, Block graphs, $K_{m,n}$, $K_n-e$, wheel graphs, odd cycles and symmetric even graphs and enumerate them for many of these graph classes. We also introduce the concept of center number which is defined as the number of distinct center sets of a graph and determine the center number of some graph classes.
2011
This dissertation contains two parts: lattice theory and graph theory. In the lattice theory part, we have two main subjects. First, the class of all distributive lattices is one of the most familiar classes of lattices. We introduce "7r-versions" of five familiar equivalent conditions for distributivity by applying the various conditions to 3-element antichains only. We prove that they are inequivalent concepts, and characterize them via exclusion systems. A lattice L satisfies D 0n if a A (b Vc) < (a A b) Vc for all 3-element antichains {a, b, c}. We consider a congruence relation ~ whose blocks are the maximal autonomous chains and define the orderskeleton of a lattice L to be L := L/~. We prove that the following are equivalent for a lattice L: (i) L satisfies D Qw , (ii) L satisfies any of the five 7r-versions of distributivity, (Hi) the order-skeleton L is distributive. Second, the symmetric difference notion for Boolean algebra is well-known. Matousek introduced the orthocomplemented difference lattices (ODLs), which are ortholattices associated with a symmetric difference. He proved that the class of ODLs forms a variety. We focus on the class of all ODLs that are set-representable and prove that this class is not locally finite by constructing an infinite set-representable ODL that is generated by three elements. In the graph theory part, we prove generating theorems and splitter theorems for 5-regular graphs. A generating theorem for a certain class of graphs tells us how to iii iv generate all graphs in this class from a few graphs by using some graph operations. A splitter theorem tells us how to build up any graph G from any graph HUG "contains" H. In this dissertation, we find generating theorems for 5-regular graphs and 5-regular loopless graphs for various edge-connectivities. We also find splitter theorems for 5-regular graphs for various edge-connectivities.
2007
A profile on a graph G is a finite sequence of vertices of G. The remoteness of a vertex u is the sum of distances to the vertices of the profile. The set of vertices that maximize (minimize) the remoteness is the antimedian (median) set of the profile. It is proved that for every connected graph G there exists a graph H such that G is a convex subgraph of H and V (G) is the antimedian set of the profile consisting of V (H). Using linear programming it is also proved that for an arbitrary graph G and S ⊆ V (G) it can be decided in polynomial time whether S is the antimedian set of some profile. Both results are extended to median sets as well. Graphs in which every antimedian set is connected are also considered.
Discrete Mathematics, 1993
A notion of parallelism is defined in finite median graphs and a number of properties about geodesics and the existence of cubes are obtained. Introducing sites as a double structure of partial order and graph on a set, it is shown that all median graphs can be constructed from sites and, in fact, that the categories of sites and pointed median graphs are equivalent, generalizing Birkhoff's duality.
SIAM Journal on Discrete Mathematics, 1999
Let M (m, n) be the complexity of checking whether a graph G with m edges and n vertices is a median graph. We show that the complexity of checking whether G is triangle-free is at most M (m, m). Conversely, we prove that the complexity of checking whether a given graph is a median graph is at most O(m log n) + T (m log n, n), where T (m, n) is the complexity of finding all triangles of the graph. We also demonstrate that, intuitively speaking, there are as many median graphs as there are triangle-free graphs. Finally, these results enable us to prove that the complexity of recognizing planar median graphs is linear.
Proceedings of the 1st annual computer science conference on Program information abstracts - CWC '73, 1973
European Journal of Combinatorics, 2003
Three characterizations of quasi-median graphs are proved, for instance, they are partial Hamming graphs without convex house and convex Q3− such that certain relations on their edge sets coincide. Expansion procedures, weakly 2-convexity, and several relations on ...
Discrete Applied Mathematics, 2022
In this note, we give answers to three questions from the paper [A. Das, Triameter of graphs, Discuss. Math. Graph Theory 41 (2021), 601-616.]. Namely, we obtain a tight lower bound for the triameter of trees in terms of order and number of leaves. We show that in a connected block graph any triametral triple of vertices contains a diametral pair and that any diametral pair of vertices can be extended to a triametral triple. We also present several open problems concerning the interplay between triametral triples, diametral pairs and peripheral vertices in median and distance-hereditary graphs.
Journal of Computer and System Sciences, 2022
The median of a set of vertices P of a graph G is the set of all vertices x of G minimizing the sum of distances from x to all vertices of P. In this paper, we present a linear time algorithm to compute medians in median graphs, improving over the existing quadratic time algorithm. We also present a linear time algorithm to compute medians in the 1-cube complexes associated with median graphs. Median graphs constitute the principal class of graphs investigated in metric graph theory and have a rich geometric and combinatorial structure, due to their bijections with CAT(0) cube complexes and domains of event structures. Our algorithm is based on the majority rule characterization of medians in median graphs and on a fast computation of parallelism classes of edges (Θ-classes or hyperplanes) via Lexicographic Breadth First Search (LexBFS). To prove the correctness of our algorithm, we show that any LexBFS ordering of the vertices of G satisfies the following fellow traveler property of independent interest: the parents of any two adjacent vertices of G are also adjacent. Using the fast computation of the Θ-classes, we also compute the Wiener index (total distance) of G in linear time and the distance matrix in optimal quadratic time.
Doklady Mathematics, 2010
Mathematical Programming
The median of a graph G with weighted vertices is the set of all vertices x minimizing the sum of weighted distances from x to the vertices of G. For any integer p ≥ 2, we characterize the graphs in which, with respect to any non-negative weights, median sets always induce connected subgraphs in the pth power G p of G. This extends some characterizations of graphs with connected medians (case p = 1) provided by Bandelt and Chepoi (2002). The characteristic conditions can be tested in polynomial time for any p. We also show that several important classes of graphs in metric graph theory, including bridged graphs (and thus chordal graphs), graphs with convex balls, bucolic graphs, and bipartite absolute retracts, have G 2-connected medians. Extending the result of Bandelt and Chepoi that basis graphs of matroids are graphs with connected medians, we characterize the isometric subgraphs of Johnson graphs and of halved-cubes with connected medians.
Discrete Applied Mathematics, 2009
A profile on a graph G is any nonempty multiset whose elements are vertices from G. The corresponding remoteness function associates to each vertex x ∈ V (G) the sum of distances from x to the vertices in the profile. Starting from some nice and useful properties of the remoteness function in hypercubes, the remoteness function is studied in arbitrary median graphs with respect to their isometric embeddings in hypercubes. In particular, a relation between the vertices in a median graph G whose remoteness function is maximum (antimedian set of G) with the antimedian set of the host hypercube is found. While for odd profiles the antimedian set is an independent set that lies in the strict boundary of a median graph, there exist median graphs in which special even profiles yield a constant remoteness function. We characterize such median graphs in two ways: as the graphs whose periphery transversal number is 2, and as the graphs with the geodetic number equal to 2. Finally, we present an algorithm that, given a graph G on n vertices and m edges, decides in O(m log n) time whether G is a median graph with geodetic number 2.
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