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1.
The clique graph K(G) of a given graph G is the intersection graph of the collection of maximal cliques of G. Given a family ℱ of graphs, the clique‐inverse graphs of ℱ are the graphs whose clique graphs belong to ℱ. In this work, we describe characterizations for clique‐inverse graphs of K3‐free and K4‐free graphs. The characterizations are formulated in terms of forbidden induced subgraphs. © 2000 John Wiley & Sons, Inc. J Graph Theory 35: 257–272, 2000  相似文献   

2.
In this paper, we shall prove that a projective‐planar (resp., toroidal) triangulation G has K6 as a minor if and only if G has no quadrangulation isomorphic to K4 (resp., K5 ) as a subgraph. As an application of the theorems, we can prove that Hadwiger's conjecture is true for projective‐planar and toroidal triangulations. © 2009 Wiley Periodicals, Inc. J Graph Theory 60: 302‐312, 2009  相似文献   

3.
Let cl(G) denote Ryjá?ek's closure of a claw‐free graph G. In this article, we prove the following result. Let G be a 4‐connected claw‐free graph. Assume that G[NG(T)] is cyclically 3‐connected if T is a maximal K3 in G which is also maximal in cl(G). Then G is hamiltonian. This result is a common generalization of Kaiser et al.'s theorem [J Graph Theory 48(4) (2005), 267–276] and Pfender's theorem [J Graph Theory 49(4) (2005), 262–272]. © 2011 Wiley Periodicals, Inc. J Graph Theory  相似文献   

4.
The choosability of a graph G is the minimum k such that having k colors available at each vertex guarantees a proper coloring. Given a toroidal graph G, it is known that , and if and only if G contains K7. Cai et al. (J Graph Theory 65(1) (2010), 1–15) proved that a toroidal graph G without 7‐cycles is 6‐choosable, and if and only if G contains K6. They also proved that a toroidal graph G without 6‐cycles is 5‐choosable, and conjectured that if and only if G contains K5. We disprove this conjecture by constructing an infinite family of non‐4‐colorable toroidal graphs with neither K5 nor cycles of length at least 6; moreover, this family of graphs is embeddable on every surface except the plane and the projective plane. Instead, we prove the following slightly weaker statement suggested by Zhu: toroidal graphs containing neither (a K5 missing one edge) nor 6‐cycles are 4‐choosable. This is sharp in the sense that forbidding only one of the two structures does not ensure that the graph is 4‐choosable.  相似文献   

5.
In this article, we show that every simple r‐regular graph G admits a balanced P4‐decomposition if r ≡ 0(mod 3) and G has no cut‐edge when r is odd. We also show that a connected 4‐regular graph G admits a P4‐decomposition if and only if |E(G)| ≡ 0(mod 3) by characterizing graphs of maximum degree 4 that admit a triangle‐free Eulerian tour. © 1999 John Wiley & Sons, Inc. J Graph Theory 31: 135–143, 1999  相似文献   

6.
Generalizing the well‐known concept of an i‐perfect cycle system, Pasotti [Pasotti, in press, Australas J Combin] defined a Γ‐decomposition (Γ‐factorization) of a complete graph Kv to be i‐perfect if for every edge [x, y] of Kv there is exactly one block of the decomposition (factor of the factorization) in which x and y have distance i. In particular, a Γ‐decomposition (Γ‐factorization) of Kv that is i‐perfect for any i not exceeding the diameter of a connected graph Γ will be said a Steiner (Kirkman) Γ‐system of order v. In this article we first observe that as a consequence of the deep theory on decompositions of edge‐colored graphs developed by Lamken and Wilson [Lamken and Wilson, 2000, J Combin Theory Ser A 89, 149–200], there are infinitely many values of v for which there exists an i‐perfect Γ‐decomposition of Kv provided that Γ is an i‐equidistance graph, namely a graph such that the number of pairs of vertices at distance i is equal to the number of its edges. Then we give some concrete direct constructions for elementary abelian Steiner Γ‐systems with Γ the wheel on 8 vertices or a circulant graph, and for elementary abelian 2‐perfect cube‐factorizations. We also present some recursive constructions and some results on 2‐transitive Kirkman Γ‐systems. © 2008 Wiley Periodicals, Inc. J Combin Designs 17: 197–209, 2009  相似文献   

7.
Let ? be a symmetric binary function, positive valued on positive arguments. A graph G = (V,E) is a ?‐tolerance graph if each vertex υ ∈ V can be assigned a closed interval Iυ and a positive tolerance tυ so that xyE ? | IxIy|≥ ? (tx,ty). An Archimedean function has the property of tending to infinity whenever one of its arguments tends to infinity. Generalizing a known result of [15] for trees, we prove that every graph in a large class (which includes all chordless suns and cacti and the complete bipartite graphs K2,k) is a ?‐tolerance graph for all Archimedean functions ?. This property does not hold for most graphs. Next, we present the result that every graph G can be represented as a ?G‐tolerance graph for some Archimedean polynomial ?G. Finally, we prove that there is a ?universal”? Archimedean function ? * such that every graph G is a ?*‐tolerance graph. © 2002 Wiley Periodicals, Inc. J Graph Theory 41: 179–194, 2002  相似文献   

8.
Let G=(V(G),E(G)) be a graph. A (n,G, λ)‐GD is a partition of the edges of λKn into subgraphs (G‐blocks), each of which is isomorphic to G. The (n,G,λ)‐GD is named as graph design for G or G‐decomposition. The large set of (n,G,λ)‐GD is denoted by (n,G,λ)‐LGD. In this work, we obtain the existence spectrum of (n,P3,λ)‐LGD. © 2002 Wiley Periodicals, Inc. J Combin Designs 10: 151–159, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/jcd.10008  相似文献   

9.
The following question was raised by Bruce Richter. Let G be a planar, 3‐connected graph that is not a complete graph. Denoting by d(v) the degree of vertex v, is G L‐list colorable for every list assignment L with |L(v)| = min{d(v), 6} for all vV(G)? More generally, we ask for which pairs (r, k) the following question has an affirmative answer. Let r and k be the integers and let G be a K5‐minor‐free r‐connected graph that is not a Gallai tree (i.e. at least one block of G is neither a complete graph nor an odd cycle). Is G L‐list colorable for every list assignment L with |L(v)| = min{d(v), k} for all vV(G)? We investigate this question by considering the components of G[Sk], where Sk: = {vV(G)|d(v)8k} is the set of vertices with small degree in G. We are especially interested in the minimum distance d(Sk) in G between the components of G[Sk]. © 2011 Wiley Periodicals, Inc. J Graph Theory 71:18–30, 2012  相似文献   

10.
Let G be a graph of order 4k and let δ(G) denote the minimum degree of G. Let F be a given connected graph. Suppose that |V(G)| is a multiple of |V(F)|. A spanning subgraph of G is called an F‐factor if its components are all isomorphic to F. In this paper, we prove that if δ(G)≥5/2k, then G contains a K4?‐factor (K4? is the graph obtained from K4 by deleting just one edge). The condition on the minimum degree is best possible in a sense. In addition, the proof can be made algorithmic. © 2002 John Wiley & Sons, Inc. J Graph Theory 39: 111–128, 2002  相似文献   

11.
Let G be a planar triangle‐free graph and let C be a cycle in G of length at most 8. We characterize all situations where a 3‐coloring of C does not extend to a proper 3‐coloring of the whole graph.  相似文献   

12.
We define the A4structure of a graph G to be the 4‐uniform hypergraph on the vertex set of G whose edges are the vertex subsets inducing 2K2, C4, or P4. We show that perfection of a graph is determined by its A4‐structure. We relate the A4‐structure to the canonical decomposition of a graph as defined by Tyshkevich [Discrete Math 220 (2000) 201–238]; for example, a graph is indecomposable if and only if its A4‐structure is connected. We also characterize the graphs having the same A4‐structure as a split graph.  相似文献   

13.
For which groups G of even order 2n does a 1‐factorization of the complete graph K2n exist with the property of admitting G as a sharply vertex‐transitive automorphism group? The complete answer is still unknown. Using the definition of a starter in G introduced in 4 , we give a positive answer for new classes of groups; for example, the nilpotent groups with either an abelian Sylow 2‐subgroup or a non‐abelian Sylow 2‐subgroup which possesses a cyclic subgroup of index 2. Further considerations are given in case the automorphism group G fixes a 1‐factor. © 2005 Wiley Periodicals, Inc. J Combin Designs  相似文献   

14.
《Journal of Graph Theory》2018,89(2):194-213
We first prove that for every vertex x of a 4‐connected graph G, there exists a subgraph H in G isomorphic to a subdivision of the complete graph K4 on four vertices such that is connected and contains x. This implies an affirmative answer to a question of Kühnel whether every 4‐connected graph G contains a subdivision H of K4 as a subgraph such that is connected. The motor for our induction is a result of Fontet and Martinov stating that every 4‐connected graph can be reduced to a smaller one by contracting a single edge, unless the graph is the square of a cycle or the line graph of a cubic graph. It turns out that this is the only ingredient of the proof where 4‐connectedness is used. We then generalize our result to connected graphs of minimum degree at least 4 by developing the respective motor, a structure theorem for the class of simple connected graphs of minimum degree at least 4. A simple connected graph G of minimum degree 4 cannot be reduced to a smaller such graph by deleting a single edge or contracting a single edge and simplifying if and only if it is the square of a cycle or the edge disjoint union of copies of certain bricks as follows: Each brick is isomorphic to K3, K5, K2, 2, 2, , , or one the four graphs , , , obtained from K5 and K2, 2, 2 by deleting the edges of a triangle, or replacing a vertex x by two new vertices and adding four edges to the endpoints of two disjoint edges of its former neighborhood, respectively. Bricks isomorphic to K5 or K2, 2, 2 share exactly one vertex with the other bricks of the decomposition, vertices of degree 4 in any other brick are not contained in any further brick of the decomposition, and the vertices of a brick isomorphic to K3 must have degree 4 in G and have pairwise no common neighbors outside that brick.  相似文献   

15.
Let G = (V(G),E(G)) be a graph. A (ν, G, λ)‐GD is a partition of all the edges of λKν into subgraphs (G‐blocks), each of which is isomorphic to G. The (ν, G, λ)‐GD is named as graph design for G or G‐decomposition. The large set of (ν, G, λ)‐GD is denoted by (ν, G, λ)‐LGD. In this paper, we obtain a general result by using the finite fields, that is, if qk ≥ 2 is an odd prime power, then there exists a (q,Pk, k ? 1)‐LGD. © 2005 Wiley Periodicals, Inc. J Combin Designs.  相似文献   

16.
Given a 3‐colorable graph G together with two proper vertex 3‐colorings α and β of G, consider the following question: is it possible to transform α into β by recoloring vertices of G one at a time, making sure that all intermediate colorings are proper 3‐colorings? We prove that this question is answerable in polynomial time. We do so by characterizing the instances G, α, β where the transformation is possible; the proof of this characterization is via an algorithm that either finds a sequence of recolorings between α and β, or exhibits a structure which proves that no such sequence exists. In the case that a sequence of recolorings does exist, the algorithm uses O(|V(G)|2) recoloring steps and in many cases returns a shortest sequence of recolorings. We also exhibit a class of instances G, α, β that require Ω(|V(G)|2) recoloring steps. © 2010 Wiley Periodicals, Inc. J Graph Theory 67: 69‐82, 2011  相似文献   

17.
The prism over a graph G is the Cartesian product GK2 of G with the complete graph K2. If the prism over G is hamiltonian, we say that G is prism‐hamiltonian. We prove that triangulations of the plane, projective plane, torus, and Klein bottle are prism‐hamiltonian. We additionally show that every 4‐connected triangulation of a surface with sufficiently large representativity is prism‐hamiltonian, and that every 3‐connected planar bipartite graph is prism‐hamiltonian. © 2007 Wiley Periodicals, Inc. J Graph Theory 57: 181–197, 2008  相似文献   

18.
Let n≥2 be an integer. The complete graph Kn with a 1‐factor F removed has a decomposition into Hamilton cycles if and only if n is even. We show that KnF has a decomposition into Hamilton cycles which are symmetric with respect to the 1‐factor F if and only if n≡2, 4 mod 8. We also show that the complete bipartite graph Kn, n has a symmetric Hamilton cycle decomposition if and only if n is even, and that if F is a 1‐factor of Kn, n, then Kn, nF has a symmetric Hamilton cycle decomposition if and only if n is odd. © 2010 Wiley Periodicals, Inc. J Combin Designs 19:1‐15, 2010  相似文献   

19.
In this paper necessary and sufficient conditions are found for an edge‐colored graph H to be the homomorphic image of a 2‐factorization of a complete multipartite graph G in which each 2‐factor of G has the same number of components as its corresponding color class in H. This result is used to completely solve the problem of finding hamilton decompositions of Ka,b ? E(U) for any 2‐factor U of Ka,b. © 2001 John Wiley & Sons, Inc. J Combin Designs 9: 460–467, 2001  相似文献   

20.
We derive decomposition theorems for P6, K1 + P4‐free graphs, P5, K1 + P4‐free graphs and P5, K1 + C4‐free graphs, and deduce linear χ‐binding functions for these classes of graphs (here, Pn (Cn) denotes the path (cycle) on n vertices and K1 + G denotes the graph obtained from G by adding a new vertex and joining it with every vertex of G). Using the same techniques, we also obtain an optimal χ‐binding function for P5, C4‐free graphs which is an improvement over that given in [J. L. Fouquet, V. Giakoumakis, F. Maire, and H. Thuillier, 11 , Discrete Math, 146, 33–44.]. © 2006 Wiley Periodicals, Inc. J Graph Theory 54: 293–306, 2007  相似文献   

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