New edge neighborhood graphs

Let G be an undirected simple connected graph, and e = uv be an edge of G. Let N _G(e) be the subgraph of G induced by the set of all vertices of G which are not incident to e but are adjacent to u or v. Let N _e be the class of all graphs H such that, for some graph G,N _G (e) H for every edge e of G. Zelinka [3] studied edge neighborhood graphs and obtained some special graphs in N _e. Balasubramanian and Alsardary [1] obtained some other graphs in N _e. In this paper we given some new graphs in N _e.     

Iterated clique graphs with increasing diameters

A simple argument by Hedman shows that the diameter of a clique graph G differs by at most one from that of K(G), its clique graph. Hedman described examples of a graph G such that diam(K(G)) = diam(G) + 1 and asked in general about the existence of graphs such that diam(Kⁱ(G)) = diam(G) + i. Examples satisfying this equality for i = 2 have been described by Peyrat, Rall, and Slater and independently by Balakrishnan and Paulraja. The authors of the former work also solved the case i = 3 and i = 4 and conjectured that such graphs exist for every positive integer i. The cases i ≥ 5 remained open. In the present article, we prove their conjecture. For each positive integer i, we describe a family of graphs G such that diam(Kⁱ(G)) = diam(G) + i. © 1998 John Wiley & Sons, Inc. J. Graph Theory 28: 147–154, 1998     

Iterated line graphs are maximally ordered

A graph G is k‐ordered if for every ordered sequence of k vertices, there is a cycle in G that encounters the vertices of the sequence in the given order. We prove that if G is a connected graph distinct from a path, then there is a number t_G such that for every t ≥ t_G the t‐iterated line graph of G, L^t (G), is (δ(L^t (G)) + 1)‐ordered. Since there is no graph H which is (δ(H)+2)‐ordered, the result is best possible. © 2006 Wiley Periodicals, Inc. J Graph Theory 52: 171–180, 2006     

Dirac-type generalizations concerning large cycles in graphs

Bondy conjectured a common generalization of various results in hamiltonian graph theory concerning Hamilton and dominating cycles by introducing a notion of PD_λ-cycles (cycles that dominate all paths of lengths at least λ). We show that the minimum degree version of Bondy’s conjecture is true (along with the reverse version) if PD_λ-cycles are replaced by CD_λ-cycles (cycles that dominate all cycles of lengths at least λ). Fraisse proved a minimum degree generalization including a theorem of Nash-Williams for Hamilton cycles as a special case. We present the reverse version of this result including a theorem of Voss and Zuluaga as a special case. Two earlier less known results (due to the author) are crucial for the proofs of these results. All results are sharp in all respects. A number of possible similar generalizations are conjectured as well.     

Constructing and classifying neighborhood anti-Sperner graphs

For a simple graph G let N_G(u) be the (open) neighborhood of vertex u∈V(G). Then G is neighborhood anti-Sperner (NAS) if for every u there is a v∈V(G)?{u} with N_G(u)⊆N_G(v). And a graph H is neighborhood distinct (ND) if every neighborhood is distinct, i.e., if N_H(u)≠N_H(v) when u≠v, for all u and v∈V(H).In Porter and Yucas [T.D. Porter, J.L. Yucas. Graphs whose vertex-neighborhoods are anti-sperner, Bulletin of the Institute of Combinatorics and its Applications 44 (2005) 69-77] a characterization of regular NAS graphs was given: ‘each regular NAS graph can be obtained from a host graph by replacing vertices by null graphs of appropriate sizes, and then joining these null graphs in a prescribed manner’. We extend this characterization to all NAS graphs, and give algorithms to construct all NAS graphs from host ND graphs. Then we find and classify all connected r-regular NAS graphs for r=0,1,…,6.     

Hamiltonian graphs with neighborhood intersections

In this paper, k + 1 real numbers c₁, c₂, ?, c_k+1 are found such that the following condition is sufficient for a k-connected graph of order n to be hamiltonian: for each independent vertex set of k + 1 vertices in G. where S_i = {v ? V:|N(v) ∩ S| = i} for 0 ≦ i ≦ k + 1. Such a set of k + 1 numbers is called an Hk-sequence. A sufficient condition for the existence of Hk-sequences is obtained that generalizes many known results involving sum of degrees, neighborhood unions, and/or neighborhood intersections.     

Hamiltonian graphs involving neighborhood unions

Dirac proved that a graph G is hamiltonian if the minimum degree , where n is the order of G. Let G be a graph and . The neighborhood of A is for some . For any positive integer k, we show that every (2k ? 1)‐connected graph of order n ≥ 16k³ is hamiltonian if |N(A)| ≥ n/2 for every independent vertex set A of k vertices. The result contains a few known results as special cases. The case of k = 1 is the classic result of Dirac when n is large and the case of k = 2 is a result of Broersma, Van den Heuvel, and Veldman when n is large. For general k, this result improves a result of Chen and Liu. The lower bound 2k ? 1 on connectivity is best possible in general while the lower bound 16k³ for n is conjectured to be unnecessary. © 2006 Wiley Periodicals, Inc. J Graph Theory 53: 83–100, 2006     

On random subgraphs of Kneser graphs and their generalizations

A series of results are obtained on the stability of the independence number of random subgraphs of distance graphs, which are natural generalizations of the classical Kneser graphs.     

1-Triangle graphs and perfect neighborhood sets

A graph is called a 1-triangle if, for its every maximal independent set I, every edge of this graph with both endvertices not belonging to I is contained exactly in one triangle with a vertex of I. We obtain a characterization of 1-triangle graphs which implies a polynomial time recognition algorithm. Computational complexity is establishedwithin the class of 1-triangle graphs for a range of graph-theoretical parameters related to independence and domination. In particular, NP-completeness is established for the minimum perfect neighborhood set problem in the class of all graphs.     

20-relative neighborhood graphs are hamiltonian

For any two points p and q in the Euclidean plane, define LUN_pq = { v | v ∈ R², d_pv < d_pq and d_qv < d_pq}, where d_uv is the Euclidean distance between two points u and v . Given a set of points V in the plane, let LUN_pq(V) = V ∩ LUN_pq. Toussaint defined the relative neighborhood graph of V, denoted by RNG(V) or simply RNG, to be the undirected graph with vertices V such that for each pair p,q ∈ V, (p,q) is an edge of RNG(V) if and only if LUN_pq (V) = ?. The relative neighborhood graph has several applications in pattern recognition that have been studied by Toussaint. We shall generalize the idea of RNG to define the k-relative neighborhood graph of V, denoted by kRNG(V) or simply kRNG, to be the undirected graph with vertices V such that for each pair p,q ∈ V, (p,q) is an edge of kRNG(V) if and only if | LUN_pq(V) | < k, for some fixed positive number k. It can be shown that the number of edges of a kRNG is less than O(kn). Also, a kRNG can be constructed in O(kn₂) time. Let E_c = {e_pq| p ∈ V and q ∈ V}. Then G_c = (V,E_c) is a complete graph. For any subset F of E_c, define the maximum distance of F as max_epq∈Fd_pq. A Euclidean bottleneck Hamiltonian cycle is a Hamiltonian cycle in graph G_c whose maximum distance is the minimum among all Hamiltonian cycles in graph G_c. We shall prove that there exists a Euclidean bottleneck Hamiltonian cycle which is a subgraph of 20RNG(V). Hence, 20RNGs are Hamiltonian.     

Hamilton连通性和邻域并条件

设G＝（V，E）为简单图，δ为图G的最小度，1987年Faudree等人给出NC＝min{|N(x)∪N(y)‖x,y∈V(G),xy∈N(G)}，有关文献曾研究3连通的H连通图，本文进一步得到：若G是n阶2连通图，且NC≥n-δ，则G除几个图外均是H连通图，从而，完成了邻并条件的H连通图问题。     

Odd neighborhood transversals on grid graphs

An odd neighborhood transversal of a graph is a set of its vertices that intersects the set of neighbors of each of its vertices in an odd number of elements. In the case of grid graphs this odd number will be either one or three. We characterize those grid graphs that have odd neighborhood transversals.     

Variable neighborhood search for extremal graphs 3

In the set of bicolored trees with given numbers of black and of white vertices we describe those for which the largest eigenvalue is extremal (maximal or minimal). The results are first obtained by the automated system AutoGraphiX, developed in GERAD (Montreal), and verified afterwards by theoretical means.     

Pan-connectedness of graphs with large neighborhood unions

Let G be a simple graph with n vertices. For any v ? V(G){v \in V(G)} , let N(v)={u ? V(G): uv ? E(G)}{N(v)=\{u \in V(G): uv \in E(G)\}} , NC(G) = min{|N(u) èN(v)|: u, v ? V(G){NC(G)= \min \{|N(u) \cup N(v)|: u, v \in V(G)} and uv \not ? E(G)}{uv \not \in E(G)\}} , and NC₂(G) = min{|N(u) èN(v)|: u, v ? V(G){NC_2(G)= \min\{|N(u) \cup N(v)|: u, v \in V(G)} and u and v has distance 2 in E(G)}. Let l ≥ 1 be an integer. A graph G on n ≥ l vertices is [l, n]-pan-connected if for any u, v ? V(G){u, v \in V(G)} , and any integer m with l ≤ m ≤ n, G has a (u, v)-path of length m. In 1998, Wei and Zhu (Graphs Combinatorics 14:263–274, 1998) proved that for a three-connected graph on n ≥ 7 vertices, if NC(G) ≥ n − δ(G) + 1, then G is [6, n]-pan-connected. They conjectured that such graphs should be [5, n]-pan-connected. In this paper, we prove that for a three-connected graph on n ≥ 7 vertices, if NC ₂(G) ≥ n − δ(G) + 1, then G is [5, n]-pan-connected. Consequently, the conjecture of Wei and Zhu is proved as NC ₂(G) ≥ NC(G). Furthermore, we show that the lower bound is best possible and characterize all 2-connected graphs with NC ₂(G) ≥ n − δ(G) + 1 which are not [4, n]-pan-connected.     

Pan-connectedness of graphs with large neighborhood unions

Let G be a simple graph with n vertices. For any , let , and , and and u and v has distance 2 in E(G)}. Let l ≥ 1 be an integer. A graph G on n ≥ l vertices is [l, n]-pan-connected if for any , and any integer m with l ≤ m ≤ n, G has a (u, v)-path of length m. In 1998, Wei and Zhu (Graphs Combinatorics 14:263–274, 1998) proved that for a three-connected graph on n ≥ 7 vertices, if NC(G) ≥ n − δ(G) + 1, then G is [6, n]-pan-connected. They conjectured that such graphs should be [5, n]-pan-connected. In this paper, we prove that for a three-connected graph on n ≥ 7 vertices, if NC ₂(G) ≥ n − δ(G) + 1, then G is [5, n]-pan-connected. Consequently, the conjecture of Wei and Zhu is proved as NC ₂(G) ≥ NC(G). Furthermore, we show that the lower bound is best possible and characterize all 2-connected graphs with NC ₂(G) ≥ n − δ(G) + 1 which are not [4, n]-pan-connected.      

Variable neighborhood search for extremal graphs 3

In the set of bicolored trees with given numbers of black and of white vertices we describe those for which the largest eigenvalue is extremal (maximal or minimal). The results are first obtained by the automated system AutoGraphiX, developed in GERAD (Montreal), and verified afterwards by theoretical means.     

Degree and neighborhood conditions for hamiltonicity of claw-free graphs

For a graph $H$, let ${\sigma }_t\left(H\right)=min\left\{{\Sigma }_{i=1}^t{d}_H\left(v_i\right)\right|\{v_1,v_2,\dots ,v_t\}\text{is an independent set in }H\}$ and let $U_t\left(H\right)=min\left\{\right|{?}_{i=1}^t{N}_H\left(v_i\right)\left|\right|\{v_1,v_2,?,v_t\}\text{is an independent set in }H\}$. We show that for a given number $?$ and given integers $p\ge t>0$, $k\in \{2,3\}$ and $N=N(p,?)$, if $H$ is a $k$-connected claw-free graph of order $n>N$ with $\delta \left(H\right)\ge 3$ and its Ryjác?ek’s closure $cl\left(H\right)=L\left(G\right)$, and if $d_t\left(H\right)\ge t(n+?)∕p$ where $d_t\left(H\right)\in \{{\sigma }_t\left(H\right),U_t\left(H\right)\}$, then either $H$ is Hamiltonian or $G$, the preimage of $L\left(G\right)$, can be contracted to a $k$-edge-connected $K_3$-free graph of order at most $max\{4p?5,2p+1\}$ and without spanning closed trails. As applications, we prove the following for such graphs $H$ of order $n$ with $n$ sufficiently large:(i) If $k=2$, $\delta \left(H\right)\ge 3$, and for a given $t$ ($1\le t\le 4$) $d_t\left(H\right)\ge \frac{tn}{4}$, then either $H$ is Hamiltonian or $cl\left(H\right)=L\left(G\right)$ where $G$ is a graph obtained from $K_{2,3}$ by replacing each of the degree 2 vertices by a $K_{1,s}$ ($s\ge 1$). When $t=4$ and $d_t\left(H\right)={\sigma }_4\left(H\right)$, this proves a conjecture in Frydrych (2001).(ii) If $k=3$, $\delta \left(H\right)\ge 24$, and for a given $t$ ($1\le t\le 10$) $d_t\left(H\right)>\frac{t(n+5)}{10}$, then $H$ is Hamiltonian. These bounds on $d_t\left(H\right)$ in (i) and (ii) are sharp. It unifies and improves several prior results on conditions involved ${\sigma }_t$ and $U_t$ for the hamiltonicity of claw-free graphs. Since the number of graphs of orders at most $max\{4p?5,2p+1\}$ are fixed for given $p$, improvements to (i) or (ii) by increasing the value of $p$ are possible with the help of a computer.