A Note on the Game Chromatic Index of Graphs

In this note we prove that the game chromatic index χ _g ^′(G) of a graph G of arboricity k is at most Δ + 3k − 1. This improves a bound obtained by Cai and Zhu [J. Graph Theory 36 (2001), 144–155] for k-degenerate graphs. Tomasz Bartnicki: Research of the first author is supported by a PhD grant from Polish Ministry of Science and Higher Education N201 2128 33. Received: November 1, 2006. Final version received: December 22, 2007.     

1-factor covers of regular graphs

We consider minimal 1-factor covers of regular multigraphs, focusing on those that are 1-factorizations. In particular, we classify cubic graphs such that every minimal 1-factor cover is also a 1-factorization, and also classify simple regular bipartite graphs with this property. For r>3, we show that there are finitely many simple r-regular graphs such that every minimal 1-factor cover is also a 1-factorization.     

On the fractional chromatic index of a graph and its complement

Jensen and Toft conjectured that for a graph with an even number of vertices, either the minimum number of colours in a proper edge colouring is equal to the maximum vertex degree, or this is true in its complement. We prove a fractional version of this conjecture.     

On a conjecture of the Randić index and the minimum degree of graphs

The Randi? index $R\left(G\right)$ of a graph $G$ is defined by $R\left(G\right)={\sum }_{uv}\frac{1}{\sqrt{d\left(u\right)d\left(v\right)}}$, where $d\left(u\right)$ is the degree of a vertex $u$ and the summation extends over all edges $uv$ of $G$. Delorme et al. (2002)  [6] put forward a conjecture concerning the minimum Randi? index among all$n$-vertex connected graphs with the minimum degree at least $k$. In this work, we show that the conjecture is true given the graph contains $k$ vertices of degree $n?1$. Further, it is true among $k$-trees.     

New results on chromatic index critical graphs

In this paper, we prove several new results on chromatic index critical graphs. We also prove that if G is a Δ(≥4)-critical graph, then

     

Hadwiger Number and the Cartesian Product of Graphs

The Hadwiger number η(G) of a graph G is the largest integer n for which the complete graph K _n on n vertices is a minor of G. Hadwiger conjectured that for every graph G, η(G) ≥ χ(G), where χ(G) is the chromatic number of G. In this paper, we study the Hadwiger number of the Cartesian product of graphs. As the main result of this paper, we prove that for any two graphs G ₁ and G ₂ with η(G ₁) = h and η(G ₂) = l. We show that the above lower bound is asymptotically best possible when h ≥ l. This asymptotically settles a question of Z. Miller (1978). As consequences of our main result, we show the following:

A theorem in edge colouring

We prove the following theorem: if G is an edge-chromatic critical multigraph with more than 3 vertices, and if x,y are two adjacent vertices of G, the edge-chromatic number of G does not change if we add an extra edge joining x and y.     

Achieving maximum chromatic index in multigraphs

Let G be a multigraph with maximum degree Δ and maximum edge multiplicity μ. Vizing’s Theorem says that the chromatic index of G is at most Δ+μ. If G is bipartite its chromatic index is well known to be exactly Δ. Otherwise G contains an odd cycle and, by a theorem of Goldberg, its chromatic index is at most , where g_o denotes odd-girth. Here we prove that a connected G achieves Goldberg’s upper bound if and only if G=μC_go and (g_o−1)∣2(μ−1). The question of whether or not G achieves Vizing’s upper bound is NP-hard for μ=1, but for μ≥2 we have reason to believe that this may be answerable in polynomial time. We prove that, with the exception of μK₃, every connected G with μ≥2 which achieves Vizing’s upper bound must contain a specific dense subgraph on five vertices. Additionally, if Δ≤μ², we prove that G must contain K₅, so G must be nonplanar. These results regarding Vizing’s upper bound extend work by Kierstead, whose proof technique influences us greatly here.     

Some sufficient conditions for a planar graph of maximum degree six to be Class 1

Let G be a planar graph of maximum degree 6. In this paper we prove that if G does not contain either a 6-cycle, or a 4-cycle with a chord, or a 5- and 6-cycle with a chord, then χ^′(G)=6, where χ^′(G) denotes the chromatic index of G.     

A proof of a conjecture on the Randić index of graphs with given girth

The Randić index R(G) of a graph G is defined by , where is the degree of a vertex u in G and the summation extends over all edges uv of G. Aouchiche, Hansen and Zheng proposed the following conjecture: For any connected graph on n≥3 vertices with Randić index R and girth g,

with equalities if and only if . This paper is devoted to giving a confirmative proof to this conjecture.     

Complete solution to a conjecture on the Randić index of triangle-free graphs

The Randić index R(G) of a graph G is defined by , where d(u) is the degree of a vertex u in G and the summation extends over all edges uv of G. A conjecture about the Randić index says that for any triangle-free graph G of order n with minimum degree δ≥k≥1, one has , where the equality holds if and only if G=K_k,n−k. In this short note we give a confirmative proof for the conjecture.     

A reduction of the Jacobian Conjecture to the symmetric case

The main result of this paper asserts that it suffices to prove the Jacobian Conjecture for all polynomial maps of the form , where is homogeneous (of degree 3) and is nilpotent and symmetric. Also a 6-dimensional counterexample is given to a dependence problem posed by de Bondt and van den Essen (2003).

     

图的色因子分解

设 ｎ１ ≥ ｎ ２ ≥ … ≥ ｎｋ ≥ ２ 是整数． 若图 Ｇ 能边分解成 Ｇ１  Ｇ２  …  Ｇｋ , 这里 χ（ Ｇｉ） ＝ ｎ ｉ, ｉ＝１,２,…,ｋ ,则称 Ｇ 有（ｎ１ , ｎ２ , …, ｎｋ ）色因子 分解． 本文改进 了 Ｈａｋｉｍ ｉ和 Ｓｃｈｍ ｅｉｃｈ ｅｌ 关于图的色因 子分解的结果,作为推 论,推广了 Ｍ ａｔｕｌａ 和 Ｈａｒａｒｙ 等人的结果     

A generalization of interval edge-colorings of graphs

An edge-coloring of a graph G with colors 1,2,…,t is called an interval (t,1)-coloring if at least one edge of G is colored by i, i=1,2,…,t, and the colors of edges incident to each vertex of G are distinct and form an interval of integers with no more than one gap. In this paper we investigate some properties of interval (t,1)-colorings. We also determine exact values of the least and the greatest possible number of colors in such colorings for some families of graphs.     

Vertex-, edge-, and total-colorings of Sierpiński-like graphs

Vertex-colorings, edge-colorings and total-colorings of the Sierpiński gasket graphs S_n, the Sierpiński graphs S(n,k), graphs S⁺(n,k), and graphs S⁺⁺(n,k) are considered. In particular, χ^″(S_n), χ^′(S(n,k)), χ(S⁺(n,k)), χ(S⁺⁺(n,k)), χ^′(S⁺(n,k)), and χ^′(S⁺⁺(n,k)) are determined.