图同构的一个充分必要条件

图同构的判定性问题是图论理论中的一个难问题,至今没有得到彻底解决。Ulam曾经提出过一个判定图同构的猜想,也称为图的重构猜想。提出了一个新的判定图同构的充分必要条件,即在子图同构的前提下,根据新增顶点及相应关联边的关系,判断母图同构的充分必要条件。基于具有同构关系的对应点无限衍生技术,采用数学归纳法证明了这个充分必要条件的成立。     

2r-正则图连通圈网络的Hamilton分解

互连网络是超级计算机的重要组成部分。互连网络通常模型化为一个图,图的顶点代表处理机,图的边代表通信链路。2010年师海忠提出互连网络的正则图连通圈网络模型,设计出了多种互连网络,也提出了一系列猜想。文中证明了2r -正则图连通圈网络可分解为边不交的一个Hamilton圈和一个完美对集的并,从而证明了当原图为2r-正则连通图时,这一系列猜想成立。     

求解图同构的判定算法

图同构的判定性问题是图论理论中的一个难题,至今没有得到彻底解决。受Ulam猜想的启发,提出了一个新的判定图同构的充分必要条件：在子图同构的前提下,根据新增顶点及相应关联边的关系,利用子图同构函数,判断父图同构的充分必要条件。基于具有同构关系的对应点无限衍生技术,采用反证法证明了这个充分必要条件的成立。设计并实现了图同构的一个判定算法,通过实例验证了算法的正确性和有效性。     

相交双圈图的代数连通度的排序

简单连通图若边数等于顶点数加1,且图中所含的两个圈至少有两个公共顶点,则称该图为相交双圈图。主要给出了相交双圈图中第五到第十大代数连通度的图类。     

如何判断有向图的连通性？

(一)问题 2000年第24期擂台赛的问题是有向图的连通性判断。 对一个有向图,忽略所有有向边的方向性而得到对应的一个无向图,如果该无向图是连通的,即其中任意两点有通路相连,则称原有向图是弱连通的:如果在原有向图中任意两点间至少有一点至另一点的通路,则称该图是单向连通的:而如果原图任两点之间一定既有甲至乙,也有乙至甲的通路,即有双向通路,则称该图是强连通的。 例如,图1是一个非弱连通非单向连通与非强连通图:图     

网络图论研究出现多处前沿性突破

新疆大学数学物理研究所承担的国家自然科学基金项目：图论及其在化学和网络优化中的应用，已在国际上的ＩＳＩＰ、ＭＲ、ＳＣＩ等文献上发表了２０篇论文。它的一系列成果有：首次引入Ｋ-图共振图概念，并建立了判定Ｋ－圈共振图的充分必要条件；建立了广义分子键序模型；首次基于广义六角系统的性质定义了广义六隅体及其结构，定义了超环及超六隅体旋转变换图等新概念，并证明了该变换图的有向树结构；首次建立了辨识Ｋｅｋｕｌｅａｎ六角系统的典型Ｐ—Ｖ路消去法，建立了生成该系统所有１＝因子的有效算法；首次给出了临界Ｋ－边连通图…     

基于分层递阶商空间链的图连通性研究

图连通性的判定对于路径规划中任意两点间路径相通性判断以及连通块的划分都具有重要意义。从节点的边连通关系着手分析图的结构层次,通过构建图的分层递阶商空间链,分析不同层次商空间链中各节点分布情况,得出新的图连通性判定方法。与以往各判定方法相比,该方法具有易实现、效率高的优点,不仅能有效地判定图是否连通,还能确定图的连通分支数以及哪些节点位于同一连通分支中。     

一些新的Hamilton图的必要条件

寻求Hamilton图的适当的特征刻画是图论的一个重大未解决问题，根据图的结构特征，设计了图的顶点的分层方法，研究了Hamilton图中层与层间对外顶点数和对外边数应该满足的关系，分析了Hamilton图中每层顶点数与每层对外项点数的关系，探讨了图与其Hamilton演化图的Hamilton性关系，最后得到一些新的Hamilton图的必要条件。所获得的新的Hamilton图的必要条件实用性强，使用方便，能判断一些原必要条件不能判断的非Hamilton图。     

用转换成多级图的方法判定图的H性质

给出了哈密顿图判定问题的一个算法。思想是先将简单无向图转换成多级图，然后证明简单无向图中哈密顿回路存在性与多级图中简单路径(定义见正文)存在性的等价性，最后通过多级图中简单路径存在性的判定实现简单无向图H性质判定。     

无向图中连通支配集问题的精确算法

图◢G=（V,E）的一个支配集DV是一个顶点子集,使得图中每一个顶点要么在D中,要么至少与D中的一个顶点相连。连通支配集问题是找到一个顶点数最小的支配集S,并且S的导出子图G［S］是连通图。◣该问题是一个经典的NP难问题,可应用于连通设施选址、自适应网络等领域。针对无向图中连通支配集问题,仔细分析该问题的图结构性质,挖掘出若干有效的约简规则和分支规则,设计了一个分支搜索算法,并采用了测量治之方法分析算法的运行时间,最终得到了一个运行时间复杂◢度为O*△（1.93n△）的精确◣算法。     

Planar Graph Coloring Avoiding Monochromatic Subgraphs: Trees and Paths Make It Difficult

We consider the problem of coloring a planar graph with the minimum number of colors so that each color class avoids one or more forbidden graphs as subgraphs. We perform a detailed study of the computational complexity of this problem. We present a complete picture for the case with a single forbidden connected (induced or noninduced) subgraph. The 2-coloring problem is NP-hard if the forbidden subgraph is a tree with at least two edges, and it is polynomially solvable in all other cases. The 3-coloring problem is NP-hard if the forbidden subgraph is a path with at least one edge, and it is polynomially solvable in all other cases. We also derive results for several forbidden sets of cycles. In particular, we prove that it is NP-complete to decide if a planar graph can be 2-colored so that no cycle of length at most 5 is monochromatic.     

采用线性不定方程组的Hamiltonian存在判定条件

本文基于{0，1}线性不定方程组和顶边关联矩阵．提出了一个基于无向Hamiltonian图的充要判定定理。并证明了满足该定理的不定方程组解向量对应给定圄的Hamiltonian回路中边的集合，本文还推导出两个可以基于矩阵秩的Hamiltonian回路存在的必要判据。     

A new heuristic for detecting non-Hamiltonicity in cubic graphs

We analyse a polyhedron which contains the convex hull of all Hamiltonian cycles of a given undirected connected cubic graph. Our constructed polyhedron is defined by polynomially-many linear constraints in polynomially-many continuous (relaxed) variables. Clearly, the emptiness of the constructed polyhedron implies that the graph is non-Hamiltonian. However, whenever a constructed polyhedron is non-empty, the result is inconclusive. Hence, the following natural question arises: if we assume that a non-empty polyhedron implies Hamiltonicity, how frequently is this diagnosis incorrect? We prove that, in the case of bridge graphs, the constructed polyhedron is always empty. We also demonstrate that some non-bridge non-Hamiltonian cubic graphs induce empty polyhedra as well. We compare our approach to the famous Dantzig–Fulkerson–Johnson relaxation of a TSP, and give empirical evidence which suggests that the latter is infeasible if and only if our constructed polyhedron is also empty. By considering special edge cut sets which are present in most cubic graphs, we describe a heuristic approach, built on our constructed polyhedron, for which incorrect diagnoses of non-Hamiltonian graphs as Hamiltonian appear to be very rare. In particular, for cubic graphs containing up to 18 vertices, only four out of 45,982 undirected connected cubic graphs were so misdiagnosed. By constrast, we demonstrate that an equivalent heuristic, when built on the Dantzig–Fulkerson–Johnson relaxation of a TSP, is mostly unsuccessful in identifying additional non-Hamiltonian graphs. These empirical results suggest that polynomial algorithms based on our constructed polyhedron may be able to correctly identify Hamiltonicity of a cubic graph in all but rare cases.     

k元组合的Hamiltonan回路快速搜索算法

通过定义k元组合的方式给出了一个逐步搜索图（有向或元向）的全部Hamiltonan回路的新算法和判定图的哈密顿特性的充要条件。使用该算法可准确地求出Hamiltonan图的全部Hamiltonan回路,不必生成基本回路。     

Packing triangles in bounded degree graphs

We consider the two problems of finding the maximum number of node disjoint triangles and edge disjoint triangles in an undirected graph. We show that the first (respectively second) problem is polynomially solvable if the maximum degree of the input graph is at most 3 (respectively 4), whereas it is APX-hard for general graphs and NP-hard for planar graphs if the maximum degree is 4 (respectively 5) or more.     

Weighted nearest neighbor algorithms for the graph exploration problem on cycles

In the graph exploration problem, a searcher explores the whole set of nodes of an unknown graph. We assume that all the unknown graphs are undirected and connected. The searcher is not aware of the existence of an edge until he/she visits one of its endpoints. The searcher's task is to visit all the nodes and go back to the starting node by traveling a tour as short as possible. One of the simplest strategies is the nearest neighbor algorithm (NN), which always chooses the unvisited node nearest to the searcher's current position. The weighted NN (WNN) is an extension of NN, which chooses the next node to visit by using the weighted distance. It is known that WNN with weight 3 is 16-competitive for planar graphs. In this paper we prove that NN achieves the competitive ratio of 1.5 for cycles. In addition, we show that the analysis for the competitive ratio of NN is tight by providing an instance for which the bound of 1.5 is attained, and NN is the best for cycles among WNN with all possible weights. Furthermore, we prove that no online algorithm to explore cycles is better than 1.25-competitive.     

Algorithms to Compute Minimum Cycle Basis in Directed Graphs

We consider the problem of computing a minimum cycle basis in a directed graph G with m arcs and n vertices. The arcs of G have non-negative weights assigned to them. In this problem a {-1,0,1} incidence vector is associated with each cycle and the vector space over generated by these vectors is the cycle space of G. A set of cycles is called a cycle basis of G if it forms a basis for its cycle space. A cycle basis where the sum of weights of the cycles is minimum is called a minimum cycle basis of G. This paper presents an algorithm, which is the first polynomial-time algorithm for computing a minimum cycle basis in G. We then improve it to an algorithm. The problem of computing a minimum cycle basis in an undirected graph has been well studied. In this problem a {0,1} incidence vector is associated with each cycle and the vector space over generated by these vectors is the cycle space of the graph. There are directed graphs in which the minimum cycle basis has lower weight than any cycle basis of the underlying undirected graph. Hence algorithms for computing a minimum cycle basis in an undirected graph cannot be used as black boxes to solve the problem in directed graphs.     

耦合强度对量子绝热算法求解最大割的影响

本文分析基于量子绝热近似的不同顶点的最大割问题求解.该算法将无向图的顶点等效为量子比特,各个顶点间的边等效为两个量子比特之间的耦合,边的权重值等效为量子比特间的耦合强度.采用Python语言编写算法程序,模拟了6–13个顶点的完全无向图的最大割问题求解情况.实验结果表明,当完全无向图顶点个数取为8,12,13,同时耦合强度为1.0时,所求解最大割问题哈密顿量的期望值不收敛.进一步调整模拟计算中量子比特间耦合强度数值,观察期望值变化.实验发现,对于顶点数为12的完全无向图,耦合强度取0.95时,其期望值获得收敛.对于顶点数为8和13的完全无向图情形,当耦合强度取0.75时,所计算得到的期望值随演化时间变化收敛.由此推测超过13个顶点的完全无向图在用量子绝热算法求解最大割问题时,可将量子比特耦合强度归一化到0.75左右,使期望值有效收敛.     

Delaunay and Voronoi tessellations and minimal simple cycles in triangular region and regular-3 undirected planar graphs

Linear time algorithms are presented which generate the Voronoi tessellation from a Delaunay tessellation and vice versa when the tessellations of convex polygons and triangles are stored as edge-adjacent undirected graphs. These two geometric algorithms lead naturally to two further linear and quadratic graph algorithms for determining the minimal simple cycles or regions in both triangular and regular-3 undirected planar graphs.