Neighbor Sum Distinguishing Index

We consider proper edge colorings of a graph G using colors of the set {1, . . . , k}. Such a coloring is called neighbor sum distinguishing if for any pair of adjacent vertices x and y the sum of colors taken on the edges incident to x is different from the sum of colors taken on the edges incident to y. The smallest value of k in such a coloring of G is denoted by ndi_Σ(G). In the paper we conjecture that for any connected graph G ≠ C ₅ of order n ≥ 3 we have ndi_Σ(G) ≤ Δ(G) + 2. We prove this conjecture for several classes of graphs. We also show that ndi_Σ(G) ≤ 7Δ(G)/2 for any graph G with Δ(G) ≥ 2 and ndi_Σ(G) ≤ 8 if G is cubic.     

The cochromatic index of a graph

We discuss partitions of the edge set of a graph into subsets which are uniform in their internal relationships; i.e., the edges are independent, they are incident with a common vertex (a star), or three edges meet in a triangle. We define the cochromatic index z′(G) of G to be the minimum number of subsets into which the edge set of G can be partitioned so that the edges in any subset are either mutually adjacent or independent.Several bounds for z′(G) are discussed. For example, it is shown that δ(G) - 1 ? z′(G)? Δ(G) + 1, with the lower bound being attained only for a complete graph. Here δ(G) and Δ(G) denote the minimum and maximum degrees of G, respectively. The cochromatic index is also found for complete n-partite graphs.Given a graph G define a sequence of graphs G₀, G₁,…, G_k, with G₀=G and

, with k being the first value of i for which G_i is regular. Let φ_i(G) = |V(G) – V(G_i| + Δ (G_i) and setφ(G) = min_0?i?kφ_i(G). We show that φ(G) ? 1 ?z′(G)?φ(G) + 1. We then s that a graph G is of class A, B or C, if z′(G) = φ(G) ? 1, φ(G), orφ(G) + 1, respectively. Examples of graphs of each class are presented; in particular, it is shown that any bipartite graph belongs to class B.Finally, we show that if a, b and c are positive integers with a?b?c + 1 and a?c, then unless a = c = b - 1 = 1, there exists a graph G having δ(G) = a, Δ(G) =c, and z′(G) = b.     

Neighbor sum distinguishing colorings of graphs with maximum average degree less than $$\tfrac{{37}} {{12}}$$

Let G be a graph and let its maximum degree and maximum average degree be denoted by Δ(G) and mad(G), respectively. A neighbor sum distinguishing k-edge colorings of graph G is a proper k-edge coloring of graph G such that, for any edge uv ∈ E(G), the sum of colors assigned on incident edges of u is different from the sum of colors assigned on incident edges of v. The smallest value of k in such a coloring of G is denoted by χ′_∑(G). Flandrin et al. proposed the following conjecture that χ′_∑ (G) ≤ Δ(G) + 2 for any connected graph with at least 3 vertices and G ≠ C₅. In this paper, we prove that the conjecture holds for a normal graph with mad(G) < \(\tfrac{{37}}{{12}}\) and Δ(G) ≥ 7.     

Improvement on Brooks' chromatic bound for a class of graphs

Brooks' Theorem says that if for a graph G,Δ(G)=n, then G is n-colourable, unless (1) n=2 and G has an odd cycle as a component, or (2) n>2 and K_n+1 is a component of G. In this paper we prove that if a graph G has none of some three graphs (K_1,3;K₅?e and H) as an induced subgraph and if Δ(G)?6 and d(G)<Δ(G), then χ(G)<Δ(G). Also we give examples to show that the hypothesis Δ(G)?6 can not be non-trivially relaxed and the graph K₅?e can not be removed from the hypothesis. Moreover, for a graph G with none of K_1,3;K₅?e and H as an induced subgraph, we verify Borodin and Kostochka's conjecture that if for a graph G,Δ(G)?9 and d(G)<Δ(G), then χ(G)<Δ(G).     

The Size of Maximally Irregular Graphs and Maximally Irregular Triangle-Free Graphs

Let G be a graph. The irregularity index of G, denoted by t(G), is the number of distinct values in the degree sequence of G. For any graph G, t(G) ≤ Δ(G), where Δ(G) is the maximum degree. If t(G) = Δ(G), then G is called maximally irregular. In this paper, we give a tight upper bound on the size of maximally irregular graphs, and prove the conjecture proposed in [6] on the size of maximally irregular triangle-free graphs. Extremal graphs are also characterized.     

Hall parameters of complete and complete bipartite graphs

Given a graph G, for each υ ∈V(G) let L(υ) be a list assignment to G. The well‐known choice number c(G) is the least integer j such that if |L(υ)| ≥j for all υ ∈V(G), then G has a proper vertex colouring ? with ?(υ) ∈ L (υ) (?υ ∈V(G)). The Hall number h(G) is like the choice number, except that an extra non‐triviality condition, called Hall's condition, has to be satisfied by the list assignment. The edge‐analogue of the Hall number is called the Hall index, h′(G), and the total analogue is called the total Hall number, h″(G), of G. If the stock of colours from which L(υ) is selected is restricted to a set of size k, then the analogous numbers are called k‐restricted, or restricted, Hall parameters, and are denoted by h_k(G), h′_k(G) and h″_k(G). Our main object in this article is to determine, or closely bound, h′(K), h″(K_n), h′(K_m,n) and h′_k(K_m,n). We also answer some hitherto unresolved questions about Hall parameters. We show in particular that there are examples of graphs G with h′(G)?h′(G ? e)>1. We show that there are examples of graphs G and induced subgraphs H with h_k(G)<h_k(H) [this phenomenon cannot occur with unrestricted Hall numbers]. We also give an example of a graph G and an integer k such that h_k(G)<χ(G)<h(G). © 2002 Wiley Periodicals, Inc. J Graph Theory 41: 208–237, 2002     

Adjacent strong edge coloring of graphs

For a graph G(V, E), if a proper k-edge coloring ƒ is satisfied with C(u) ≠ C(v) for uv ∈ E(G), where $\text{C(u) = {ƒ(uv) | uv ∈ E}}$, then ƒ is called k-adjacent strong edge coloring of G, is abbreviated k-ASEC, and χ′_as(G) = min{k | k-ASEC of G} is called the adjacent strong edge chromatic number of G. In this paper, we discuss some properties of χ′_as(G), and obtain the χ′_as(G) of some special graphs and present a conjecture: if G are graphs whose order of each component is at least six, then χ′_as(G) ≤ Δ(G) + 2, where Δ(G) is the maximum degree of G.     

Edge-choosability of planar graphs without adjacent triangles or without 7-cycles

A graph G is edge-L-colorable, if for a given edge assignment L={L(e):e∈E(G)}, there exists a proper edge-coloring ? of G such that ?(e)∈L(e) for all e∈E(G). If G is edge-L-colorable for every edge assignment L with |L(e)|≥k for e∈E(G), then G is said to be edge-k-choosable. In this paper, we prove that if G is a planar graph with maximum degree Δ(G)≠5 and without adjacent 3-cycles, or with maximum degree Δ(G)≠5,6 and without 7-cycles, then G is edge-(Δ(G)+1)-choosable.     

Spanning Connectivity of the Power of a Graph and Hamilton-Connected Index of a Graph

Let G = (V, E) be a connected graph. The hamiltonian index h(G) (Hamilton-connected index hc(G)) of G is the least nonnegative integer k for which the iterated line graph L ^k (G) is hamiltonian (Hamilton-connected). In this paper we show the following. (a) If |V(G)| ≥ k + 1 ≥ 4, then in G ^k , for any pair of distinct vertices {u, v}, there exists k internally disjoint (u, v)-paths that contains all vertices of G; (b) for a tree T,  h(T) ≤ hc(T) ≤ h(T) + 1, and for a unicyclic graph G,  h(G) ≤ hc(G) ≤ max{h(G) + 1, k′ + 1}, where k′ is the length of a longest path with all vertices on the cycle such that the two ends of it are of degree at least 3 and all internal vertices are of degree 2; (c) we also characterize the trees and unicyclic graphs G for which hc(G) = h(G) + 1.     

The edge-face coloring of graphs embedded in a surface of characteristic zero

Let G be a graph embedded in a surface of characteristic zero with maximum degree Δ. The edge-face chromatic number χ_ef(G) of G is the least number of colors such that any two adjacent edges, adjacent faces, incident edge and face have different colors. In this paper, we prove that χ_ef(G)≤Δ+1 if Δ≥13, χ_ef(G)≤Δ+2 if Δ≥12, χ_ef(G)≤Δ+3 if Δ≥4, and χ_ef(G)≤7 if Δ≤3.     

Acyclic edge coloring of graphs with maximum degree 4

An acyclic edge coloring of a graph is a proper edge coloring such that there are no bichromatic cycles. The acyclic chromatic index of a graph is the minimum number k such that there is an acyclic edge coloring using k colors and is denoted by a′(G). It was conjectured by Alon, Sudakov, and Zaks that for any simple and finite graph G, a′(G)?Δ + 2, where Δ=Δ(G) denotes the maximum degree of G. We prove the conjecture for connected graphs with Δ(G)?4, with the additional restriction that m?2n?1, where n is the number of vertices and m is the number of edges in G. Note that for any graph G, m?2n, when Δ(G)?4. It follows that for any graph G if Δ(G)?4, then a′(G)?7. © 2009 Wiley Periodicals, Inc. J Graph Theory 61: 192–209, 2009     

Local condition for planar graphs of maximum degree 7 to be 8-totally colorable

The total chromatic number of a graph G, denoted by χ^″(G), is the minimum number of colors needed to color the vertices and edges of G such that no two adjacent or incident elements get the same color. It is known that if a planar graph G has maximum degree Δ≥9, then χ^″(G)=Δ+1. In this paper, we prove that if G is a planar graph with maximum degree 7, and for every vertex v, there is an integer k_v∈{3,4,5,6} so that v is not incident with any k_v-cycle, then χ^″(G)=8.     

Injective coloring of planar graphs

A coloring of a graph G is injective if its restriction to the neighborhood of any vertex is injective. The injective chromatic numberχ_i(G) of a graph G is the least k such that there is an injective k-coloring. In this paper we prove that if G is a planar graph with girth g and maximum degree Δ, then (1) χ_i(G)=Δ if either g≥20 and Δ≥3, or g≥7 and Δ≥71; (2) χ_i(G)≤Δ+1 if g≥11; (3) χ_i(G)≤Δ+2 if g≥8.     

On the chromatic index of multigraphs without large triangles

Let M be a multigraph. Vizing (Kibernetika (Kiev)1 (1965), 29–39) proved that χ′(M)≤Δ(M)+μ(M). Here it is proved that if χ′(M)≥Δ(M)+s, where $\text{1}\text{2}\text{(μ(M) + 1) < s}$ then M contains a 2s-sided triangle. In particular, (C′) if μ(M)≤2 and M does not contain a 4-sided triangle then χ′(M)≤Δ(M) + 1. Javedekar (J. Graph Theory4 (1980), 265–268) had conjectured that (C) if G is a simple graph that does not induce K_1,3 or K₅?e then χ(G)≤ω(G) + 1. The author and Schmerl (Discrete Math.45 (1983), 277–285) proved that (C′) implies (C); thus Javedekar's conjecture is true.     

Total chromatic number of graphs with small genus

Given a graph G, a total k-coloring of G is a simultaneous coloring of the vertices and edges of G with k colors. Denote χ_ve (G) the total chromatic number of G, and c(Σ) the Euler characteristic of a surfase Σ. In this paper, we prove that for any simple graph G which can be embedded in a surface Σ with Euler characteristic c(Σ), χ_ve (G) = Δ (G) + 1 if c(Σ) > 0 and Δ (G) ≥ 13, or, if c(Σ) = 0 and Δ (G) ≥ 14. This result generalizes results in [3], [4], [5] by Borodin.     

Dynamic list coloring of bipartite graphs

A dynamic coloring of a graph is a proper coloring of its vertices such that every vertex of degree more than one has at least two neighbors with distinct colors. The least number of colors in a dynamic coloring of G, denoted by χ₂(G), is called the dynamic chromatic number of G. The least integer k, such that if every vertex of G is assigned a list of k colors, then G has a proper (resp. dynamic) coloring in which every vertex receives a color from its own list, is called the choice number of G, denoted by ch(G) (resp. the dynamic choice number, denoted by ch₂(G)). It was recently conjectured (Akbari et al. (2009) [1]) that for any graph G, ch₂(G)=max(ch(G),χ₂(G)). In this short note we disprove this conjecture. We first give an example of a small planar bipartite graph G with ch(G)=χ₂(G)=3 and ch₂(G)=4. Then, for any integer k≥5, we construct a bipartite graph G_k such that ch(G_k)=χ₂(G_k)=3 and ch₂(G)≥k.     

Neighbor sum distinguishing total colorings via the Combinatorial Nullstellensatz

Let G=(V,E)be a graph andφbe a total coloring of G by using the color set{1,2,...,k}.Let f(v)denote the sum of the color of the vertex v and the colors of all incident edges of v.We say thatφis neighbor sum distinguishing if for each edge uv∈E(G),f(u)=f(v).The smallest number k is called the neighbor sum distinguishing total chromatic number,denoted byχ′′nsd(G).Pil′sniak and Wo′zniak conjectured that for any graph G with at least two vertices,χ′′nsd(G)(G)+3.In this paper,by using the famous Combinatorial Nullstellensatz,we show thatχ′′nsd(G)2(G)+col(G)-1,where col(G)is the coloring number of G.Moreover,we prove this assertion in its list version.     

Incidence coloring of the squares of some graphs

The incidence chromatic number of G, denoted by χ_i(G), is the least number of colors such that G has an incidence coloring. In this paper, we determine the incidence chromatic number of the powers of paths, trees, which are min{n,2k+1}, and Δ(T²)+1, respectively. For the square of a Halin graph, we give an upper bound of its incidence chromatic number.     

Total coloring of embedded graphs of maximum degree at least ten

A total k-coloring of a graph G is a coloring of V(G) ∪ E(G) using k colors such that no two adjacent or incident elements receive the same color.The total chromatic number χ〃(G) is the smallest integer k such that G has a total k-coloring.In this paper,it is proved that the total chromatic number of any graph G embedded in a surface Σ of Euler characteristic χ(Σ)≥0 is Δ(G) + 1 if Δ(G)≥10,where Δ(G) denotes the maximum degree of G.     

Neighbor sum distinguishing chromatic index of sparse graphs via the combinatorial Nullstellensatz

Let ?: E(G) → {1, 2, · · ·, k} be an edge coloring of a graph G. A proper edge-k-coloring of G is called neighbor sum distinguishing if \(\sum\limits_{e \mathrel\backepsilon u} {\phi \left( e \right)} \ne \sum\limits_{e \mathrel\backepsilon v} {\phi \left( e \right)} \) for each edge uv ∈ E(G). The smallest value k for which G has such a coloring is denoted by χ′_Σ(G), which makes sense for graphs containing no isolated edge (we call such graphs normal). It was conjectured by Flandrin et al. that χ′_Σ(G) ≤ Δ(G) + 2 for all normal graphs, except for C₅. Let mad(G) = \(\max \left\{ {\frac{{2\left| {E\left( h \right)} \right|}}{{\left| {V\left( H \right)} \right|}}|H \subseteq G} \right\}\) be the maximum average degree of G. In this paper, we prove that if G is a normal graph with Δ(G) ≥ 5 and mad(G) < 3 ? \(\frac{2}{{\Delta \left( G \right)}}\), then χ′_Σ(G) ≤ Δ(G) + 1. This improves the previous results and the bound Δ(G) + 1 is sharp.