关于带宽极值问题的一些结果

本文研究的问题是确定f(p,B）的值，也就是给定顶点数p和带宽B，求满足最大度不超过B的连通图的最小边数，本文给出了一些f(p,B）的值及相应极图。     

分子图的Zagreb拓扑指标的逆问题

研究了化学分子图的Zagreb指标的逆问题，解决了对于给定的怎样的数存在分子图，其Zagreb指标值等于该数的问题，对n个顶点m条边的简单连通图，给出了其具有最小Zagreb指标值的充分必要条件，并给出了其具有最大Zagreb指标值的必要条件，为利用计算机搜索具有给定Zagreb指标值的所有分子图界定了顶点数和边数的范围，从而提高了计算机搜索的效率，这在组合化学中具有重要的意义。     

图的孤立断裂度

连通图G的孤立断裂度isc(G)=max{i(G-S)-|S|:S∈C(G)},其中i(G-S)是G-S中的孤立点数,C(G)是G的点割集.本文研究了孤立断裂度和图的其它一些参数的关系.讨论了孤立断裂度取特殊值的一些图,证明了圈、连通二部图、连通二部图的联图以及树和圈的补图的孤立断裂度都达到最小.给出了具有给定阶数和最大度的村的最大、最小孤立断裂度.     

关于图的最小Q-特征值

研究了基于n阶二部图和s阶完全图构造的一个图类,得到了该图类的无符号拉普拉斯最小特征值(即最小Q-特征值)的一个可达上界为s.基于此,对于任意给定的正整数s和正偶数n,构造了最小Q-特征值为s的一类n+s阶图.另外,对于任意给定的最小度δ和阶数n,在满足2≤δ≤n-1/2条件下,构造了最小Q-特征值为δ-1的一类n阶图.     

图有分数因子的度条件

本文研究了图有分数因子的度条件,得到了下面的结果：令k(?)1是一个整数,G是一个连通的n阶图,n(?)4k-3且最小度δ(G)(?)k,若对于每一对不相邻的顶点u,v∈V(G)都有max{d_G(u),d_G(v)}(?)n/2,则G有分数k-因子．并指出该结果在一定意义上是最好可能的。     

随机图的广义3-连通度

图的广义连通度的概念是由Chartrand等人引入的.令S表示图G的一个非空顶点集,κ(S)表示图G中连结S的内部不交树的最大数目.那么,对任意一个满足2≤r≤n的整数r,定义G的广义r-连通度为所有κ(S)中的最小值,其中S取遍G的顶点集合的r-元子集.显然,κ_2(G)=κ(G),即为图G的顶点连通度.所以广义连通度是经典连通度的一个自然推广.本文研究了随机图的广义3-连通度,证明了对任一给定的整数k,k≥1,p=(log n+(k+1)log long n-log lon logn)/n是关于性质κ_3(G(n,p))≥k的紧阈值函数.我们得到的结果可以看作是Bollobas和Thomason给出的关于经典连通度结果的推广.     

单圈图的最低及最高电阻地位

一个连通图的一个顶点的电阻地位是这个顶点到该图的其它所有顶点的电阻距离之和. 一个连通图的最低(最高)电阻地位是这个图的所有顶点的电阻地位的最小值(最大值). 我们确定了在给定阶数的单圈图中最低(最高)电阻地位的极值和相应的极图,还讨论了单圈图的最低(最高)电阻地位与围长的关系.     

PSL（2,13）的最小级连通3度弧传递陪集图表示

通过寻找给定群G的图表示,对PSL（2,13）的连通3度弧传递陪集图表示进行研究,得到了如下结果：PSL（2,13）的最小级连通3度弧传递陪集图表示的级是182.并且给出了该陪集图表示的例子.     

临界κ连通图中的点度数

如果在—个κ连通图G中删掉任意一个顶点后得到的图都不再是κ连通,则称G为临界κ连通.Chartrand,Kaugars和Lick证明了每一个临界κ连通图(κ≥2)都含有一个度数小于(3κ-1)/2的顶点.Hamidoune进一步证明了每一个临界k连通图都至少含有两个这样的顶点,并且这一下界是最优的.在本文中,我们证明如果一个临界κ连通图恰好含有两个度数小于(3κ-1)/2的顶点,则这两个顶点的度数一定是κ.     

最大边连通和super-边连通超图的充分条件

设H是连通超图。若超图H的边连通度等于其最小度,则称H是最大边连通的。若超图H的每个最小边割总是由关联于某个最小度顶点的边集所构成,则称H是super-边连通的。首先给出一致线性超图是最大边连通超图的度序列条件。其次,给出一致线性超图是super-边连通超图的度条件。这些结果分别推广了Dankelmann和Volkmann（1997）以及Hellwig和Volkmann（2005）在图上的相关结论。     

三圈图的Harary指数

图G的Harary指数是指图G中所有顶点对间的距离倒数之和. 三圈图是指边数等于顶点数加2的连通图. 研究了三圈图的Harary数, 给出了所有三圈图中具有极大Harary指数的图的结构以及含有三个圈的三圈图中具有次大Harary指数的图的结构.     

On the spectral radius of bicyclic graphs with n vertices and diameter d

Bicyclic graphs are connected graphs in which the number of edges equals the number of vertices plus one. In this paper we determine the graph with the largest spectral radius among all bicyclic graphs with n vertices and diameter d. As an application, we give first three graphs among all bicyclic graphs on n vertices, ordered according to their spectral radii in decreasing order.     

Homomorphisms of 2-edge-colored graphs

In this paper, we study homomorphisms of 2-edge-colored graphs, that is graphs with edges colored with two colors. We consider various graph classes (outerplanar graphs, partial 2-trees, partial 3-trees, planar graphs) and the problem is to find, for each class, the smallest number of vertices of a 2-edge-colored graph H such that each graph of the considered class admits a homomorphism to H.     

Homomorphisms of 2-edge-colored graphs

In this paper, we study homomorphisms of 2-edge-colored graphs, that is graphs with edges colored with two colors. We consider various graph classes (outerplanar graphs, partial 2-trees, partial 3-trees, planar graphs) and the problem is to find, for each class, the smallest number of vertices of a 2-edge-colored graph H such that each graph of the considered class admits a homomorphism to H.     

On vertex symmetric digraphs

The problem of how “near” we can come to a n-coloring of a given graph is investigated. I.e., what is the minimum possible number of edges joining equicolored vertices if we color the vertices of a given graph with n colors. In its generality the problem of finding such an optimal color assignment to the vertices (given the graph and the number of colors) is NP-complete. For each graph G, however, colors can be assigned to the vertices in such a way that the number of offending edges is less than the total number of edges divided by the number of colors. Furthermore, an Ω(epn) deterministic algorithm for finding such an n-color assignment is exhibited where e is the number of edges and p is the number of vertices of the graph (e?p?n). A priori solutions for the minimal number of offending edges are given for complete graphs; similarly for equicolored K_m in K_p and equicolored graphs in K_p.     

Enumeration of Full Graphs: Onset of the Asymptotic Region

A full graph on n vertices, as defined by Fulkerson, is a representation of the intersection and containment relations among a system of n sets. It has an undirected edge between vertices representing intersecting sets and a directed edge from a to b if the corresponding set A contains B;. Kleitman, Lasaga, and Cowen gave a unified argument to show that asymptotically, almost all full graphs can be obtained by taking an arbitrary undirected graph on the n vertices, distinguishing a clique in this graph, which need not be maximal, and then adding directed edges going out from each vertex in the clique to all vertices to which there is not already an existing undirected edge. Call graphs of this type members of the dominant class. This article obtains the first upper and lower bounds on how large n has to be, so that the asymptotic behavior is indeed observed. It is shown that when n > 170, the dominant class predominates, while when n < 17, the full graphs in the dominant class compose less than half of the total number of realizable full graphs on n vertices.     

Intersections of graphs

The study of graph homomorphisms has a long and distinguished history, with applications in many areas of graph theory. There has been recent interest in counting homomorphisms, and in particular on the question of finding upper bounds for the number of homomorphisms from a graph G into a fixed image graph H. We introduce our techniques by proving that the lex graph has the largest number of homomorphisms into K₂ with one looped vertex (or equivalently, the largest number of independent sets) among graphs with fixed number of vertices and edges. Our main result is the solution to the extremal problem for the number of homomorphisms into P, the completely looped path of length 2 (known as the Widom–Rowlinson model in statistical physics). We show that there are extremal graphs that are threshold, give explicitly a list of five threshold graphs from which any threshold extremal graph must come, and show that each of these “potentially extremal” threshold graphs is in fact extremal for some number of edges. Copyright © 2011 Wiley Periodicals, Inc. J Graph Theory 67: 261–284, 2011     

Graphs with equal eternal vertex cover and eternal domination numbers

Mobile guards on the vertices of a graph are used to defend it against an infinite sequence of attacks on either its vertices or its edges. If attacks occur at vertices, this is known at the eternal domination problem. If attacks occur at edges, this is known as the eternal vertex cover problem. We focus on the model in which all guards can move to neighboring vertices in response to an attack. Motivated by the question of which graphs have equal eternal vertex cover and eternal domination numbers, a number of results are presented; one of the main results of the paper is that the eternal vertex cover number is greater than the eternal domination number (in the all-guards move model) in all graphs of minimum degree at least two.     

Nontrivial discontinuities of the Golovach functions for trees

The ɛ-search problem on graphs is considered. Properties of the Golovach function, which associates each nonnegative number ɛ with the ɛ-search number, are studied. It is known that the Golovach function is piecewise constant, nonincreasing, and right continuous. Golovach and Petrov proved that the Golovach function for a complete graph on more than five vertices may have nonunit jumps. The jumps of the Golovach function for the case of trees are considered. Examples of trees which disprove the conjecture that the Golovach function has only unit jumps for any planar graph are given. For these examples, the Golovach function is constructed. It is shown that the Golovach function for trees with at most 27 edges has only unit jumps. The same assertion is proved for trees containing at most 28 edges all of whose vertices have degree at most 3. The examples mentioned above have minimum number of edges.     

Traceability in Small Claw-Free Graphs

We prove that a claw-free, 2-connected graph with fewer than 18 vertices is traceable, and we determine all non-traceable, claw-free, 2-connected graphs with exactly 18 vertices and a minimal number of edges. This complements a result of Matthews on Hamiltonian graphs.