Bounded families for the on-line t-relaxed coloring

We introduce a new generalization of the on-line coloring game. We define the concept of bounded family for on-line t-relaxed colorings. This extends the concept of on-line competitive coloring algorithms to t-relaxed colorings. We characterize the trees T for which the family of T-free graphs is bounded and show that the corresponding bounding function is linear.     

Edge coloring of bipartite graphs with constraints

A classical result from graph theory states that the edges of an l-regular bipartite graph can be colored using exactly l colors so that edges that share an endpoint are assigned different colors. In this paper, we study two constrained versions of the bipartite edge coloring problem.

Full-size image

随机图的点可区别V-全染色算法

图[G]的点可区别V-全染色就是相邻的边、顶点与其关联边必须染不同的颜色,同时要求所有顶点的色集合也不相同,所用的最少颜色数称为图[G]的点可区别V-全色数。根据点可区别V-全染色的约束规则,设计了一种启发式的点可区别V-全染色算法,该算法借助染色矩阵及色补集合逐步迭代交换,每次迭代交换后判断目标函数值,当目标函数值满足要求时染色成功。给出了算法的详细描述、算法分析和算法测试结果,对给定点数的图进行了点可区别V-全染色猜想的验证。实验结果表明,该算法有很好的执行效率并可以得到给定图的点可区别V-全色数,并且算法的时间复杂度不超过[O(n3)]。     

On acyclic edge coloring of toroidal graphs

Let c be a proper edge coloring of a graph G. If there exists no bicolored cycle in G with respect to c, then c is called an acyclic edge coloring of G. Let G be a planar graph with maximum degree Δ and girth g. In Dong and Xu (2010) [8], Dong and Xu proved that G admits an acyclic edge coloring with Δ(G) colors if Δ?8 and g?7, or Δ?6 and g?8, or Δ?5 and g?9, or Δ?4 and g?10, or Δ?3 and g?14. In this note, we fix a small gap in the proof of Dong and Xu (2010) [8], and generalize the above results to toroidal graphs.     

A note on 2-facial coloring of plane graphs

An l-facial coloring of a plane graph is a vertex coloring such that any two different vertices joined by a facial walk of length at most l receive distinct colors. It is known that every plane graph admits a 2-facial coloring using 8 colors [D. Král, T. Madaras, R. Škrekovski, Cyclic, diagonal and facial coloring, European J. Combin. 3-4 (26) (2005) 473-490]. We improve this bound for plane graphs with large girth and prove that if G is a plane graph with girth g?14 (resp. 10, 8) then G admits a 2-facial coloring using 5 colors (resp. 6, 7). Moreover, we give exact bounds for outerplanar graphs and K₄-minor free graphs.     

Fast algorithms for the dominating set problem on permutation graphs

AnO(n log logn) (resp.O(n² log² n)) algorithm is presented to solve the minimum cardinality (resp. weight) dominating set problem on permutation graphs, assuming the input is a permutation. The best-known previous algorithm was given by FÄrber and Keil, where they use dynamic programming to get anO(n² (resp.O(n³)) algorithm. Our improvement is based on the following three factors: (1) an observation on the order among the intermediate terms in the dynamic programming, (2) a new construction formula for the intermediate terms, and (3) efficient data structures for manipulating these terms.This research was supported in part by the National Science Foundation under Grant CCR-8905415 to Northwestern University.     

Total coloring of planar graphs with maximum degree 7

In the paper, it is proved that a planar graph of maximum degree Δ?7 is (Δ+1)-totally-colorable if no 3-cycle has a common vertex with a 4-cycle or no 3-cycle is adjacent to a cycle of length less than 6.     

Oriented colorings of 2-outerplanar graphs

A graph G is 2-outerplanar if it has a planar embedding such that the subgraph obtained by removing the vertices of the outer face is outerplanar. The oriented chromatic number of an oriented graph H is defined as the minimum order of an oriented graph H^′ such that H has a homomorphism to H^′. In this paper, we prove that 2-outerplanar graphs are 4-degenerate. We also show that oriented 2-outerplanar graphs have a homomorphism to the Paley tournament QR₆₇, which implies that their (strong) oriented chromatic number is at most 67.     

Some results on acyclic edge coloring of plane graphs

An acyclic edge coloring of a graph is a proper edge coloring without bichromatic cycles. The acyclic chromatic index of a graph G, denoted by α^′(G), is the minimum number k such that G admits an acyclic edge coloring using k colors. Let G be a plane graph with maximum degree Δ and girth g. In this paper, we prove that α^′(G)=Δ(G) if one of the following conditions holds: (1) Δ?8 and g?7; (2) Δ?6 and g?8; (3) Δ?5 and g?9; (4) Δ?4 and g?10; (5) Δ?3 and g?14. We also improve slightly a result of A. Fiedorowicz et al. (2008) [7] by showing that every triangle-free plane graph admits an acyclic edge coloring using at most Δ(G)+5 colors.     

图的点可区别边染色算法研究

针对一般图设计了一种新型的点可区别边染色算法。该算法把概率思想和图染色相结合, 根据点可区别边染色的约束规则确立目标函数, 利用交换规则逐步寻优, 当目标函数的值满足要求时染色成功。给出详细算法步骤并进行了测试和分析, 实验结果表明该算法可以求出满足猜想的点可区别边色数。     

On the total choosability of planar graphs and of sparse graphs

In the current paper, we prove the 11-total choosability of planar graphs with maximum degree Δ?8, the (Δ+1)-total choosability of 5-cycle-free planar graphs with maximum degree Δ?8, the 5-total choosability of graphs with maximum degree Δ=4 and maximum average degree mad<3, and the 4-total choosability of subcubic graphs with maximum average degree .     

Fractional programming formulation for the vertex coloring problem

We devise a new formulation for the vertex coloring problem. Different from other formulations, decision variables are associated with pairs of vertices. Consequently, colors will be distinguishable. Although the objective function is fractional, it can be replaced by a piece-wise linear convex function. Numerical experiments show that our formulation has significantly good performance for dense graphs.     

The computational complexity of the backbone coloring problem for bounded-degree graphs with connected backbones

Given a graph G, a spanning subgraph H of G   and an integer λ≥2$\lambda \ge 2$, a λ-backbone coloring of G with backbone H is a proper vertex coloring of G   using colors 1,2,…$1,2,\dots$, in which the color difference between vertices adjacent in H is greater than or equal to λ. The backbone coloring problem is that of finding such a coloring whose maximum color does not exceed a given limit k  . In this paper, we study the backbone coloring problem for bounded-degree graphs with connected backbones and we give a complete computational complexity classification of this problem. We present a polynomial algorithm for optimal backbone coloring for subcubic graphs with arbitrary backbones. We also prove that the backbone coloring problem for graphs with arbitrary backbones and with fixed maximum degree (at least 4) is NP-complete. Furthermore, we show that for the special case of graphs with fixed maximum degree at least 5 and λ≥4$\lambda \ge 4$ the problem remains NP-complete even for spanning tree backbones.     

Constructing Plane Spanners of Bounded Degree and Low Weight

Given a set S of n points in the plane, we give an O(n log n)-time algorithm that constructs a plane t-spanner for S, with t ≈ 10, such that the degree of each point of S is bounded from above by 27, and the total edge length is proportional to the weight of a minimum spanning tree of S. Previously, no algorithms were known for constructing plane t-spanners of bounded degree.     

The distant-2 chromatic number of random proximity and random geometric graphs

We are interested in finding bounds for the distant-2 chromatic number of geometric graphs drawn from different models. We consider two undirected models of random graphs: random geometric graphs and random proximity graphs for which sharp connectivity thresholds have been shown. We are interested in a.a.s. connected graphs close just above the connectivity threshold. For such subfamilies of random graphs we show that the distant-2-chromatic number is Θ(logn) with high probability. The result on random geometric graphs is extended to the random sector graphs defined in [J. Díaz, J. Petit, M. Serna. A random graph model for optical networks of sensors, IEEE Transactions on Mobile Computing 2 (2003) 143-154].     

On the bi-enhancement of chordal-bipartite probe graphs

Given a class C of graphs, a graph G=(V,E) is said to be a C-probe graph if there exists a stable (i.e., independent) set of vertices X⊆V and a set F of pairs of vertices of X such that the graph G^′=(V,E∪F) is in the class C. Recently, there has been increasing interest and research on a variety of C-probe graph classes, such as interval probe graphs, chordal probe graphs and chain probe graphs.In this paper we focus on chordal-bipartite probe graphs. We prove a structural result that if B is a bipartite graph with no chordless cycle of length strictly greater than 6, then B is chordal-bipartite probe if and only if a certain “enhanced” graph B^∗ is a chordal-bipartite graph. This theorem is analogous to a result on interval probe graphs in Zhang (1994) [18] and to one on chordal probe graphs in Golumbic and Lipshteyn (2004) [11].     

NIC-平面图中的轻边存在性及其定向染色

如果一个图[G]画在平面上有交叉[c],则该交叉可以与产生它的两条边所关联的4个顶点所构成的点集合[{v1,v2,v3,v4}]建立一个对应关系[θ:c→{v1,v2,v3,v4}]。如果对于[G]中任何两个不同的交叉（如果存在的话）[c1]与[c2]都有[|θ(c1)?θ(c2)|≤1],则称图[G]为NIC-平面图。证明了每个围长至少为5且最小度为4的NIC-平面图含有一条边,其2个顶点的度数都是4,从而每个围长至少为5的NIC-平面图的定向染色数至多为67。     

Cliques, holes and the vertex coloring polytope

Certain subgraphs of a given graph G restrict the minimum number χ(G) of colors that can be assigned to the vertices of G such that the endpoints of all edges receive distinct colors. Some of such subgraphs are related to the celebrated Strong Perfect Graph Theorem, as it implies that every graph G contains a clique of size χ(G), or an odd hole or an odd anti-hole as an induced subgraph. In this paper, we investigate the impact of induced maximal cliques, odd holes and odd anti-holes on the polytope associated with a new 0-1 integer programming formulation of the graph coloring problem. We show that they induce classes of facet defining inequalities.     

On the oriented chromatic number of Halin graphs

An oriented k-coloring of an oriented graph G is a mapping such that (i) if xy∈E(G) then c(x)≠c(y) and (ii) if xy,zt∈E(G) then c(x)=c(t)⇒c(y)≠c(z). The oriented chromatic number of an oriented graph G is defined as the smallest k such that G admits an oriented k-coloring. We prove in this paper that every Halin graph has oriented chromatic number at most 9, improving a previous bound proposed by Vignal.