无交双圈图的邻接矩阵的奇异性

一个无交双圈图G的邻接矩阵是奇异的当且仅当G含有4m(m∈N)阶圈，或G含有完美匹配和G—V(c1)，G-V(c2)均含有完美匹配且G中含有4κ1 3与4e1 1(κ1，e1∈N)阶圈，或G、G-V(c1)、G—V(c2)、G—V(c1)-V(c2)均无完美匹配．无交双圈图的邻接矩阵的最大行列式值为16。     

子图匹配数与图无符号拉普拉斯谱(英文)

设H是图G的一个子图.图G中同构于H的点不交的子图构成的集合称为G的一个H-匹配.图G的H-匹配的最大基数称为是G的H-匹配数,记为ν(H,G).本文主要研究ν(H,G)与G的无符号拉普拉斯谱的关系,同时也讨论了ν(H,G)与G的拉普拉斯谱的关系.     

循环图C_(2n)(1,(2n+1)/3)的匹配可扩性

称图G是k-偶匹配可扩的,是指G的每一个基数不大于k(1≤k≤(|V(G)|-2)/2)的偶匹配M都可以扩充为G的一个完美匹配.根据循环图的性质研究了图C_(2n)(1,(2n+1)/3)的匹配可扩性,证明了对于任意的n(n≥4),C_(2n)(1,(2n+1)/3)是3-偶匹配可扩的.     

奇图的匹配可扩性

设G是一个图,n,k和d是三个非负整数,满足n+2k+d≤|V(G)|-2,|V(G)|和n+d有相同的奇偶性.如果删去G中任意n个点后所得的图有k-匹配,并且任一k-匹配都可以扩充为一个亏d-匹配,那么称G是一个(n,k,d)-图.Liu和Yu[1]首先引入了(n,k,d)-图的概念,并且给出了(n,k,d)-图的一个刻划和若干性质. (0,k,1)-图也称为几乎k-可扩图.在本文中,作者改进了(n,k,d)-图的刻划,并给出了几乎k-可扩图和几乎k-可扩二部图的刻划,进而研究了几乎k-可扩图与n-因子临界图之间的关系.     

具有｜V（G）｜＋2个最大匹配的因子临界图G^＊

在连通图G中,如果对任意的V∈V(G),G-v有完美匹配,则称G是因子临界图.该文刻画了具有|V(G)| +2个最大匹配的因子临界图.进而,刻画了一些特殊的双因子临界图.     

匹配可扩图的结构

设G是一个有限的简单连通图。D（G）表示V（G）的一个子集，它的每一个点至少有一个最大匹配不覆盖它。A（G）表示V（G）-D（G）的一个子集，它的每一个点至少和D（G）的一个点相邻。最后设C（G）=V（G）-A（G）-D（G）。在这篇章中，下面的被获得。⑴设u∈V（G）。若n≥1和G是n-可扩的，则（a)C(G-u)=φ和A（G-u)∪{u}是一个独立集，（b)G的每个完美匹配包含D（G-u)的每个分支的一个几乎守美匹配，并且它匹配A（G-u)∪{u}的所有点与D（G-4）的不同分支的点。⑵若G是2-可扩的，则对于u∈V（G），A（G-u）∪{u}是G的一个最大障碍且G的最大障碍的个数是2或是│V(G)│.⑶设X=Cay(Q,S),则对于u∈Q,(a)A(X-u)=φ=C（G-u)和X-u是一个因子临界图，或（b)C(X-u)=φ和X的两部是A（X-u)∪{u}和D（X-u)且│A(X-u)∪{u}│=│D(X-u）│。⑷设X=Cay(Q,S),则对于u∈Q，A（X-u)∪{u}是X的一个最大障碍且X的最大障碍的个数是2或是│Q│。     

变换图的正则性和谱半径

在前人对八种变换图研究的基础上,探讨了变换后满足正则性的原图的性质,得到了如下结果:G~( )及G~(---)是正则图当且仅当G是正则图;G~( -)和G~(-- )为正则图的充要条件是G为C_n、K_(2,n-2)或K_4;G~( - )和G~(- -)是正则图当且仅当G为C_5、K_7、K_2、K_(3,3)或G_0;G~(- )和G~( --)是正则的当且仅当G是(n-1)/2-正则图．同时还讨论了变换图的谱半径上界,并对这些上界进行了估计．     

特征标次数图是一种连通图的单群

对有限单群G,假设其不可约特征标次数图Δ(G)连通,且图顶点集ρ(G)=π_1∪π_2∪{p},其中|π_1|,|π_2|≥1,π_1∩π_2=θ,且π_1与π_2中顶点不相邻.证明了Δ(G)满足上面的假设的有限单群G只有4种:M_(11),J_1,PSL_3(4)或²B_2(q2B_2(q²),其中q2),其中q²一1是Mersenne素数.     

关于几类图的L(2,1)标号问题

图G的L( 2 ,1 )标号是一个从顶点集V(G)到非负整数集的函数f(x) ,使得若d(x ,y) =1 ,则｜f(x) -f(y) ｜≥ 2 ;若d(x ,y) =2 ,则｜f(x) -f(y) ｜≥ 1 .图G的L( 2 ,1 ) 标号数λ(G)是使得G有max{f(v) ∶v∈V(G) }=k的L( 2 ,1 )标号中的最小数k .Griggs和Yeh猜想对最大度为Δ的一般图G ,有λ(G) ≤Δ2 .本文给出了Kneser图 ,Mycieklski图 ,Descartes图 ,Halin图的λ值的上界 ,并证明了上述猜想对以上几类图成立     

最大匹配数的一个问题

一、本文仅讨论简单图。图G的1-因子数记为F(G)。 f(G)记使如下事实正确的最大的K:“假如G是一个n-连通图且G有1-因子,则G至少有k个1-因子”。包含G的所有的点,且每个点的度为0或1的G的子图叫G的一个匹配M,有最大的边数的匹配称为最大匹配。假如匹配M的一个点v的度为0,称v为在M里的分离点。以M(G)表示G的最大匹配的集合。假如图G的一个点v所关连的每一条边都属于G的一个最大匹配,称点v被M(G)完全覆盖。     

Zagreb indices of graphs

The first Zagreb index M₁(G) is equal to the sum of squares of the degrees of the vertices, and the second Zagreb index M₂(G) is equal to the sum of the products of the degrees of pairs of adjacent vertices of the underlying molecular graph G. In this paper, we obtain lower and upper bounds on the first Zagreb index M₁(G) of G in terms of the number of vertices (n), number of edges (m), maximum vertex degree (Δ), and minimum vertex degree (δ). Using this result, we find lower and upper bounds on M₂(G). Also, we present lower and upper bounds on M₂(G) +M₂(G) in terms of n, m, Δ, and δ, where G denotes the complement of G. Moreover, we determine the bounds on first Zagreb coindex M₁(G) and second Zagreb coindex M₂(G). Finally, we give a relation between the first Zagreb index and the second Zagreb index of graph G.     

Algorithms for finding a Kth best valued assignment

We consider a new problem, the Kth best valued assignment problem. Given a bipartite graph G and a cost vector w on its edge set, this is the problem of finding a perfect matching M_k in G such that there exist perfect matchings M₁,…,M_K−1 satisfying w(M₁) < < w(M_K−1) < w(M_K), and w(M_K) < w(M) for all perfect matchings M with w(M) ≠ w(M₁),…,w(M_K). Here w(M) denotes the sum of costs of edges in M. In this paper, we propose two algorithms for solving this problem and verify the efficiency of our algorithms by our preliminary computational experiments.     

ContributionsChromatic Ramsey numbers

Suppose G is a graph. The chromatic Ramsey number r_c(G) of G is the least integer m such that there exists a graph F of chromatic number m for which the following is true: for any 2-colouring of the edges of F there is a monochromatic subgraph isomorphic to G. Let M_n = min[r_c(G): χ(G) = n]. It was conjectured by Burr et al. (1976) that M_n = (n − 1)² + 1. This conjecture has been confirmed previously for n 4. In this paper, we shall prove that the conjecture is true for n = 5. We shall also improve the upper bounds for M₆ and M₇.     

Neighborhood unions and cyclability of graphs

A graph G is said to be cyclable if for each orientation of G, there exists a set S of vertices such that reversing all the arcs of with one end in S results in a hamiltonian digraph. Let G be a 3-connected graph of order n36. In this paper, we show that if for any three independent vertices x₁, x₂ and x₃, |N(x₁)N(x₂)|+|N(x₂)N(x₃)|+|N(x₃)N(x₁)|2n+1, then G is cyclable.     

Maximal IM-unextendable graphs

A graph G is maximal IM-unextendable if G is not induced matching extendable and, for every two nonadjacent vertices x and y, G+xy is induced matching extendable. We show in this paper that a graph G is maximal IM-unextendable if and only if G is isomorphic to M_r(K_s(K_n1K_n2K_nt)), where M_r is an induced matching of size r, r1, t=s+2, and each n_i is odd.     

Bandwidth of the composition of two graphs

The bandwidth B(G) of a graph G is the minimum of the quantity max{|f(x)−f(y)| : xyE(G)} taken over all proper numberings f of G. The composition of two graphs G and H, written as G[H], is the graph with vertex set V(G)×V(H) and with (u₁,v₁) is adjacent to (u₂,v₂) if either u₁ is adjacent to u₂ in G or u₁=u₂ and v₁ is adjacent to v₂ in H. In this paper, we investigate the bandwidth of the composition of two graphs. Let G be a connected graph. We denote the diameter of G by D(G). For two distinct vertices x,yV(G), we define w_G(x,y) as the maximum number of internally vertex-disjoint (x,y)-paths whose lengths are the distance between x and y. We define w(G) as the minimum of w_G(x,y) over all pairs of vertices x,y of G with the distance between x and y is equal to D(G). Let G be a non-complete connected graph and let H be any graph. Among other results, we prove that if |V(G)|=B(G)D(G)−w(G)+2, then B(G[H])=(B(G)+1)|V(H)|−1. Moreover, we show that this result determines the bandwidth of the composition of some classes of graphs composed with any graph.     

On strong 1-factors and Hamilton weights of cubic graphs

A 1-factor M of a cubic graph G is strong if |M∩T|=1 for each 3-edge-cut T of G. It is proved in this paper that a cubic graph G has precisely three strong 1-factors if and only if the graph can be obtained from K₄ via a series of ↔Y operations. Consequently, the graph G admits a Hamilton weight and is uniquely edge-3-colorable.     

Pseudo-orthogonality for graph 1-Laplacian eigenvectors and applications to higher Cheeger constants and data clustering

The data clustering problem consists in dividing a data set into prescribed groups of homogeneous data. This is an NP-hard problem that can be relaxed in the spectral graph theory, where the optimal cuts of a graph are related to the eigenvalues of graph 1-Laplacian. In this paper, we first give new notations to describe the paths, among critical eigenvectors of the graph 1-Laplacian, realizing sets with prescribed genus. We introduce the pseudo-orthogonality to characterize m₃(G), a special eigenvalue for the graph 1-Laplacian. Furthermore, we use it to give an upper bound for the third graph Cheeger constant h₃(G), that is, h₃(G) 6 m₃(G). This is a first step for proving that the k-th Cheeger constant is the minimum of the 1-Laplacian Raylegh quotient among vectors that are pseudo-orthogonal to the vectors realizing the previous k - 1 Cheeger constants. Eventually, we apply these results to give a method and a numerical algorithm to compute m₃(G), based on a generalized inverse power method.     

The chromatic difference sequence of the Cartesian product of graphs

The chromatic difference sequence cds(G) of a graph G with chromatic number n is defined by cds(G) = (a(1), a(2),…, a(n)) if the sum of a(1), a(2),…, a(t) is the maximum number of vertices in an induced t-colorable subgraph of G for t = 1, 2,…, n. The Cartesian product of two graphs G and H, denoted by G□H, has the vertex set V(G□H = V(G) x V(H) and its edge set is given by (x₁, y₁)(x₂, y₂) ε E(G□H) if either x₁ = x₂ and y₁ y₂ ε E(H) or y₁ = y₂ and x₁x₂ ε E(G).

We obtained four main results: the cds of the product of bipartite graphs, the cds of the product of graphs with cds being nondrop flat and first-drop flat, the non-increasing theorem for powers of graphs and cds of powers of circulant graphs.