Independent Directed Triangles in a Directed Graph

If D is a directed graph of order n≥3 with , then D contains independent directed triangles. Moreover, the condition on the minimum degree is sharp in general. Revised: April 19, 1999     

Many Empty Triangles have a Common Edge

Given a finite point set $X$ X in the plane, the degree of a pair $\{x,y\} \subset X$ { x , y } ? X is the number of empty triangles $t=\mathrm {conv} \{x,y,z\},$ t = conv { x , y , z } , where empty means $t\cap X=\{x,y,z\}.$ t ∩ X = { x , y , z } . Define $\deg X$ deg X as the maximal degree of a pair in $X.$ X . Our main result is that if $X$ X is a random sample of $n$ n independent and uniform points from a fixed convex body, then $\deg X \ge cn/\ln n$ deg X ≥ cn / ln n in expectation.     

The Number of Independent Sets in a Graph with Small Maximum Degree

Let ind(G) be the number of independent sets in a graph G. We show that if G has maximum degree at most 5 then

不含相邻三角形的平面图的无圈边着色

An acyclic edge coloring of a graph G is a proper edge coloring such that there are no bichromatic cycles.The acyclic edge chromatic number of a graph G is the minimum number k such that there exists an acyclic edge coloring using k colors and is denoted by χ’ a(G).In this paper we prove that χ ’ a(G) ≤(G) + 5 for planar graphs G without adjacent triangles.     

The Edge Rotation Graph

Let P be a set of n points on the plane in general position, n ≥  3. The edge rotation graph ${\mathcal{E} \mathcal{R} \mathcal{G}(P,k)}$ of P is the graph whose vertices are the plane geometric graphs on P with exactly k edges, two of which are adjacent if one can be obtained from the other by an edge rotation. In this paper we study some structural properties of ${\mathcal{E} \mathcal{R} \mathcal{G}(P,k)}$ , such as its connectivity and diameter. We show that if the vertices of ${\mathcal{E} \mathcal{R} \mathcal{G}(P,k)}$ are not triangulations of P, then it is connected and has diameter O(n ²). We also show that the chromatic number of ${\mathcal{E} \mathcal{R} \mathcal{G}(P,k)}$ is O(n), and show how to compute an implicit coloring of its vertices. We also study edge rotations in edge-labelled geometric graphs.     

图映射的渐近稳定集

设f为图G的连续自映射．在本文中，我们讨论了图映射的渐近稳定集，并给出了f的不动点为渐近稳定的一个充分必要条件．     

Sets of all periodic points of a toral automorphism

In this article, subsets of T² that can arise as sets of all periodic points of a continuous 2-dimensional toral automorphism are characterized. Here the torus T² is viewed as [0,1)×[0,1) as a group under coordinatewise addition modulo 1.     

On Computing the Subset Graph of a Collection of Sets

Let a given collection of sets have size N measured by the sum of the cardinalities. Yellin and Jutla presented an algorithm which constructed the partial order induced by the subset relation (a “subset graph”) in a claimed O(N²/log N) operations over a dictionary ADT, and exhibited a collection whose subset graph had Θ(N²/log² N) edges. This paper first establishes a matching upper bound on the number of edges in a subset graph. It also presents a finer analysis of the algorithm, which confirms the claimed upper bound and shows it to be tight. A simple implementation requiring O(1) bit-parallel operations per ADT operation is presented, along with a variant of the algorithm with an implementation requiring O(N²/log N) RAM operations.     

Convex Sets Under Some Graph Operations

 Given a connected graph G, we say that a set C⊆V(G) is convex in G if, for every pair of vertices x,y∈C, the vertex set of every x-y geodesic in G is contained in C. The cardinality of a maximal proper convex set in G is the convexity number of G. In this paper, we characterize the convex sets of graphs resulting from some binary operations, and compute the convexity numbers of the resulting graphs. Received: October, 2001 Final version received: September 4, 2002 Acknowledgments. The authors would like to thank the referee for the helpful suggestions and useful comments.     

Edge Weighting Functions on Dominating Sets

In this article, we use edge weighting functions on dominating sets to show that if we impose a regularity condition on a graph, then upper bounds on both the upper domination number and the upper total domination number can be greatly improved. More precisely, we prove that for if G is a k‐regular graph on n vertices, then the upper domination number of G is at most , and the upper total domination number of G is at most . Furthermore, we show that these bounds are sharp and characterize the infinite families of graphs that achieve equality in both these bounds.     

最小1-因子覆盖集的若干结果

假设G是一个1-可扩图．G的1-因子覆盖是G的某些1-因子的集合M使得∪M∈M M=F（G）．1-因子数目最小的1．因子覆盖称为excessive factorization．一个excessive factorization中的1．因子数目称为图G的excessive index，记为x：（G）．本文我们基于G的耳朵分解和E（C）的依赖关系给出了X＇e（G）的上界．对任意正整数k≥3，我们构造出一个图G使得A（G）=3而X＇e（G）=k．进而，我们考虑了乘积图的excessive index．     

Forbidden Triples Containing a Complete Graph and a Complete Bipartite Graph of Small Order

For a graph G and a set \({\mathcal{F}}\) of connected graphs, G is said be \({\mathcal{F}}\) -free if G does not contain any member of \({\mathcal{F}}\) as an induced subgraph. We let \({\mathcal{G} _{3}(\mathcal{F})}\) denote the set of all 3-connected \({\mathcal{F}}\) -free graphs. This paper is concerned with sets \({\mathcal{F}}\) of connected graphs such that \({\mathcal{F}}\) contains no star, \({|\mathcal{F}|=3}\) and \({\mathcal{G}_{3}(\mathcal{F})}\) is finite. Among other results, we show that for a connected graph T( ≠ K ₁) which is not a star, \({\mathcal{G}_{3}(\{K_{4},K_{2,2},T\})}\) is finite if and only if T is a path of order at most 6.     

Partitioning the Vertices of a Graph into Two Total Dominating Sets

A total dominating set in a graph G is a set S of vertices of G such that every vertex in G is adjacent to a vertex of S. We study graphs whose vertex set can be partitioned into two total dominating sets. In particular, we develop several sufficient conditions for a graph to have a vertex partition into two total dominating sets. We also show that with the exception of the cycle on five vertices, every selfcomplementary graph with minimum degree at least two has such a partition.     

图的反符号边k-控制数

图的符号边控制数有着许多重要的应用背景.已知它的计算是NP-完全问题,因而确定其精确值有重要意义.本文给出了一般图的反符号边k-控制数的若干上界.     

On Small Blocking Sets

Received: August 5, 1997     

Coloring the Vertices of a Graph With Measurable Sets in a Probability Space

We introduce a class of “chromatic” graph parameters that include the chromatic number, the circular chromatic number, the fractional chromatic number, and an uncountable horde of others. We prove some basic results about this class and pose some problems.     

论简单图所含k阶i爪独立集个数的可重构性

设犐是图犌的一个含有犽个点的独立集（简称犽独立集）．如果犐不是犌的其它任何独立集的真子集,则称犐为犌的一个极大独立集．犌中所含的极大犽独立集的个数记为犿（犵犽,犌）．设犵犽是图犌的任一个犽独立集,如果存在｛狏１,狏２,…,狏犻｝犞（犌）－犵犽,犻≥１,使得（１）对任意犼∈ ｛１,２,…,犻｝,犵犽＋｛狏犼｝的都是犌的（犽＋１） 独立集;（２）对任意狌∈犞（犌）－犵犽－｛狏１,狏２,…,狏犻｝,犵犽＋｛狌｝的都不是犌的独立集;则称犵犽为犌的一个犻爪犽独立集,犌所含的犻爪犽独立集的个数记为犿犻（犵犽,犌）．该文证明了对简单图犌,犿犻（犵犽,犌）和犿（犵犽,犌）都是可重构的．另外,用同样的方法可以证明犌中的极大犽团的个数及犻爪犽团的个数也是可重构的．