Vertex Set Research Articles

Strictly chordal graphs: Structural properties and integer Laplacian eigenvalues

In this paper, we establish the relation between classic invariants and integer Laplacian eigenvalues of strictly chordal graphs, pointing out how their computation can be efficiently implemented. We review results concerning general graphs showing that the number of universal vertices and the degrees of twins provide integer Laplacian eigenvalues and their respective multiplicities. For strictly chordal graphs, we show new results about integer Laplacian eigenvalues which are directly related to particular vertex sets of the graph.

Approximating Minimum k-Tree Cover of a Connected Graph Inspired by the Multi-Ferry Routing in Delay Tolerant Networks

In this paper, we consider the problem of covering the vertex set of a given graph by [Formula: see text] trees so as to minimize the maximum weight of the trees, as a subproblem of the multi-ferry scheduling problem proposed by Zhao and Ammar. After pointing out that the approximation ratio of a greedy scheme based on the Kruskal’s algorithm is provably bad, we show that the approximation ratio cannot be better than 3/2 for [Formula: see text] even when the edge selection criterion is modified so as to minimize the increase in the maximum weight in the collection of trees. We then propose two polynomial-time algorithms with a guaranteed approximation ratio. The first algorithm achieves 3-approximation for the class of graphs in which the edge weights satisfy the triangle inequality. The second algorithm achieves 4-approximation for any connected graph with arbitrary edge weights.

On Group-Vertex-Magic Labeling of Simple Graphs

Let A be an Abelian group with identity 0. The A-vertex-magic labeling of a graph G is a mapping from the set of vertices in G to A-{0} such that the sum of the labels of every open neighborhood vertex of v is equal, for every vertex v in G. In this article, we discuss group-vertex-magic labeling of some simple graphs by using the Abelian group Zk, with natural numbers k1. We investigated some classes of simple graphs are path graphs, complete graphs, cyclic graphs, and star graphs. The method we used in this article is literature study and then developing the properties of vertex-magic labeling of some simple graphs, that are path graphs, complete graphs, cyclic graphs, and star graphs. We obtain that complete graphs, cyclic graphs, and star graphs have Zk-vertex-magic labeling, while path graphs have vertex-magic labeling only for n=2,3.

An Inclusive Local Irregularity Vertex Coloring of Dutch Windmill Graph

Let G(V,E) is a simple and connected graph with V(G) as vertex set and E(G) as edge set. An inclusive local irregularity vertex coloring is a development of the topic of local irregularity vertex coloring. An inclusive local irregularity vertex coloring is defined by coloring the graph so that its weight value is obtained by adding up the labels of the neighboring vertex and its label. The inclusive local irregularity chromatic number is defined as the minimum number of colors obtained from coloring the vertex of the inclusive local irregularity in graph G. In this paper, we find the inclusive local irregularity vertex coloring and determine the chromatic number on the Dutch windmill graph using axiomatic deductive and pattern recognition methods. The results of this study are expected to be used as a basis for studies in the development of knowledge related to the inclusive local irregularity vertex coloring

Graph-codes

The symmetric difference of two graphs G1,G2 on the same set of vertices [n]={1,2,…,n} is the graph on [n] whose set of edges are all edges that belong to exactly one of the two graphs G1,G2. Let H be a fixed graph with an even (positive) number of edges, and let DH(n) denote the maximum possible cardinality of a family of graphs on [n] containing no two members whose symmetric difference is a copy of H. Is it true that DH(n)=o(2n2) for any such H? We discuss this problem, compute the value of DH(n) up to a constant factor for stars and matchings, and discuss several variants of the problem including ones that have been considered in earlier work.

A multioperator genetic algorithm for the traveling salesman problem with job-times

This paper addresses the traveling salesman problem with job-times (TSPJ). TSPJ considers two sets of equal size, a set of tasks and a set of vertices, where a traveler must visit each vertex exactly once, return to the starting vertex, and perform a unique task at each vertex. Each task is assigned to only one vertex, and each task is completed with a job-time that depends on each vertex. Thus, the objective is to minimize the time of the last task performed. However, due to its NP-hardness, existing algorithms do not optimally solve the TSPJ larger instances. Therefore, we propose an approach based on a multioperator genetic algorithm (MGA) that utilizes various initial population procedures, crossover and mutation operators. MGA applies five initial population procedures to generate a diverse population, and employs three crossover operators and six mutation operators, with four mutations focused on diversification and two designed to aid intensification. Furthermore, to improve the quality of the best individual found, a local search is used in every generation, and generational replacement with elitism is considered when generating a new population. MGA is evaluated on four sets of instances ranging in size from 17 to 1200 vertices: tsplib, small, medium, and large. Our approach outperforms the state-of-the-art algorithms on the four instance sets. This performance is attributed to the synergistic effect of the multioperators of crossover and mutation, along with effective parameter tuning.

Decomposing a triangle-free planar graph into a forest and a subcubic forest

We strengthen a result of Dross et al. (2017) that the vertex set of every triangle-free planar graph can be decomposed into a set that induces a forest and a set that induces a forest with maximum degree at most 5, showing that 5 can be replaced by 3.

Spanning trees in graphs without large bipartite holes

AbstractWe show that for any $\varepsilon \gt 0$ and $\Delta \in \mathbb{N}$ , there exists $\alpha \gt 0$ such that for sufficiently large $n$ , every $n$ -vertex graph $G$ satisfying that $\delta (G)\geq \varepsilon n$ and $e(X, Y)\gt 0$ for every pair of disjoint vertex sets $X, Y\subseteq V(G)$ of size $\alpha n$ contains all spanning trees with maximum degree at most $\Delta$ . This strengthens a result of Böttcher, Han, Kohayakawa, Montgomery, Parczyk, and Person.

On the vertex stability numbers of graphs

For an arbitrary invariant ρ(G) of a graph G the ρ-vertex stability number vsρ(G) (ρ-edge stability number esρ(G)) is the minimum number of vertices (edges) whose removal results in a graph H⊆G with ρ(H)≠ρ(G). If such a vertex set (edge set) does not exist, then we set vsρ(G)=∞ (esρ(G)=∞).In the first part of this paper we give some general results for the ρ-vertex stability number. We prove among others a result which implies the well-known Theorem of Gallai on independence and vertex covering of graphs. In the second part we focus on the invariant chromatic number χ(G) and determine vsχ(G) for several classes of graphs. Moreover, we consider the relationship between esχ(G) and vsχ(G) and prove that their difference and their ratio may get arbitrarily large.

Inducibility in the hypercube

AbstractLet be the hypercube of dimension and let and be subsets of the vertex set , called configurations in . We say that is an exact copy of if there is an automorphism of which sends onto . Let be an integer, let be a configuration in and let be a configuration in . We let be the maximum, over all configurations in , of the fraction of sub‐‐cubes of in which is an exact copy of , and we define the ‐cube density of to be the limit as goes to infinity of . We determine for several configurations in and as well as for an infinite family of configurations. There are strong connections with the inducibility of graphs.

Fast and error-adaptive influence maximization based on Count-Distinct sketches

Influence maximization (IM) is the problem of finding a seed vertex set that maximizes the expected number of vertices influenced under a given diffusion model. Due to the NP-Hardness of finding an optimal seed set, high-quality yet expensive approximation algorithms have been frequently used. In addition, lightweight, sketch-based approaches, which do not step-by-step simulate the influence process, have been proposed in the literature to cope with the scale of today's networks. In this work, we describe a fast, error-adaptive approach that leverages Count-Distinct sketches and hash-based fused sampling to avoid step-by-step simulations and estimate the number of influenced vertices throughout a diffusion. Furthermore, for faster estimations, the sketches of a vertex for consecutive simulations are stored adjacently in memory. This allows the proposed algorithm to estimate the number of influenced vertices for multiple simulations at once. For faster processing, the proposed method automatically rebuilds the sketches after observing estimation errors above a given threshold. Our experimental results show that the proposed algorithm yields seed sets with comparable quality while being up to 3,337× faster than a state-of-the-art, high-quality influence maximization algorithm. In addition, it is up to 63× faster than a sketch-based approach while producing seed sets with 2%–10% better influence scores.

Induced subgraphs and tree decompositions VII. Basic obstructions in H-free graphs

We say a class C of graphs is clean if for every positive integer t there exists a positive integer w(t) such that every graph in C with treewidth more than w(t) contains an induced subgraph isomorphic to one of the following: the complete graph Kt, the complete bipartite graph Kt,t, a subdivision of the (t×t)-wall or the line graph of a subdivision of the (t×t)-wall. In this paper, we adapt a method due to Lozin and Razgon (building on earlier ideas of Weißauer) to prove that the class of all H-free graphs (that is, graphs with no induced subgraph isomorphic to a fixed graph H) is clean if and only if H is a forest whose components are subdivided stars.Their method is readily applied to yield the above characterization. However, our main result is much stronger: for every forest H as above, we show that forbidding certain connected graphs containing H as an induced subgraph (rather than H itself) is enough to obtain a clean class of graphs. Along the proof of the latter strengthening, we build on a result of Davies and produce, for every positive integer η, a complete description of unavoidable connected induced subgraphs of a connected graph G containing η vertices from a suitably large given set of vertices in G. This is of independent interest, and will be used in subsequent papers in this series.

SOME GLOBAL EXISTENCE RESULTS ON LOCALLY FINITE GRAPHS

Abstract Let $G=(V, E)$ be a locally finite graph with the vertex set V and the edge set E, where both V and E are infinite sets. By dividing the graph G into a sequence of finite subgraphs, the existence of a sequence of local solutions to several equations involving the p-Laplacian and the poly-Laplacian systems is confirmed on each subgraph, and the global existence for each equation on graph G is derived by the convergence of these local solutions. Such results extend the recent work of Grigor’yan, Lin and Yang [J. Differential Equations, 261 (2016), 4924–4943; Rev. Mat. Complut., 35 (2022), 791–813]. The method in this paper also provides an idea for investigating similar problems on infinite graphs.

Toric Rings of Perfectly Matchable Subgraph Polytopes

The perfectly matchable subgraph polytope of a graph is a (0,1)-polytope associated with the vertex sets of matchings in the graph. In this paper, we study algebraic properties (compressedness, Gorensteinness) of the toric rings of perfectly matchable subgraph polytopes. In particular, we give a complete characterization of a graph whose perfectly matchable subgraph polytope is compressed.

The Degree and Codegree Threshold for Linear Triangle Covering in 3-Graphs

Given two $k$-uniform hypergraphs $F$ and $G$, we say that $G$ has an $F$-covering if every vertex in $G$ is contained in a copy of $F$. For $1\le i \le k-1$, let $c_i(n,F)$ be the least integer such that every $n$-vertex $k$-uniform hypergraph $G$ with $\delta_i(G)> c_i(n,F)$ has an $F$-covering. The covering problem has been systematically studied by Falgas-Ravry and Zhao [Codegree thresholds for covering 3-uniform hypergraphs, [SIAM J. Discrete Math., 2016]. Last year, Falgas-Ravry, Markström, and Zhao [Triangle-degrees in graphs and tetrahedron coverings in 3-graphs, Combinatorics, Probability and Computing, 2021] asymptotically determined $c_1(n, F)$ when $F$ is the generalized triangle. In this note, we give the exact value of $c_2(n, F)$ and asymptotically determine $c_1(n, F)$ when $F$ is the linear triangle $C_6^3$, where $C_6^3$ is the 3-uniform hypergraph with vertex set $\{v_1,v_2,v_3,v_4,v_5,v_6\}$ and edge set $\{v_1v_2v_3,v_3v_4v_5,v_5v_6v_1\}$.

On the Maximum $F_5$-Free Subhypergraphs of a Random Hypergraph

Denote by $F_5$ the $3$-uniform hypergraph on vertex set $\{1,2,3,4,5\}$ with hyperedges $\{123,124,345\}$. Balogh, Butterfield, Hu, and Lenz proved that if $p > K \log n /n$ for some large constant $K$, then every maximum $F_5$-free subhypergraph of $G^3(n,p)$ is tripartite with high probability, and showed that if $p_0 = 0.1\sqrt{\log n} /n$, then with high probability there exists a maximum $F_5$-free subhypergraph of $G^3(n,p_0)$ that is not tripartite. In this paper, we sharpen the upper bound to be best possible up to a constant factor. We prove that if $p > C \sqrt{\log n} /n $ for some large constant $C$, then every maximum $F_5$-free subhypergraph of $G^3(n, p)$ is tripartite with high probability.

Some results on a supergraph of the sum annihilating ideal graph of a commutative ring

The rings considered in this paper are commutative with identity which are not integral domains. Let [Formula: see text] be a ring. An ideal [Formula: see text] of [Formula: see text] is said to be an annihilating ideal if there exists [Formula: see text] such that [Formula: see text]. Let [Formula: see text] denote the set of all annihilating ideals of [Formula: see text] and we denote [Formula: see text] by [Formula: see text]. With [Formula: see text], in this paper, we associate an undirected graph denoted by [Formula: see text] whose vertex set is [Formula: see text] and two distinct vertices [Formula: see text] are adjacent in this graph if and only if either [Formula: see text] or [Formula: see text]. The aim of this paper is to study the interplay between some graph properties of [Formula: see text] and the algebraic properties of [Formula: see text] and to compare some graph properties of [Formula: see text] with the corresponding graph properties of the annihilating ideal graph of [Formula: see text] and the sum annihilating ideal graph of [Formula: see text].

Finding Minimum Connected Subgraphs With Ontology Exploration on Large RDF Data

In this paper, we study the following problem: given a knowledge graph (KG) and a set of input vertices (representing concepts or entities) and edge labels, we aim to find the smallest connected subgraphs containing all of the inputs. This problem plays a key role in KG-based search engines and natural language question answering systems, and it is a natural extension of the Steiner tree problem, which is known to be NP-hard. We present RECON, a system for finding approximate answers. RECON aims at achieving high accuracy with instantaneous response (i.e., sub-second/millisecond delay) over KGs with hundreds of millions edges without resorting to expensive computational resources. Furthermore, when no answer exists due to disconnection between concepts and entities, RECON refines the input to a semantically similar one based on the ontology, and attempts to find answers with respect to the refined input. We conduct a comprehensive experimental evaluation of RECON. In particular we compare it with five existing approaches for finding approximate Steiner trees. Our experiments on four large real and synthetic KGs show that RECON significantly outperforms its competitors and incurs a much smaller memory footprint.

Outer-Convex Hop Domination in Graphs Under Some Binary Operations

Let G be a graph with vertex and edge sets V (G) and E(G), respectively. A set C ⊆ V (G) is called an outer-convex hop dominating if for every two vertices x, y ∈ V (G) \ C, the vertex set of every x−y geodesic is contained in V (G) \ C and for every a ∈ V (G) \ C, there exists b ∈ C such that dG(a, b) = 2. The minimum cardinality of an outer-convex hop dominating set of G, denoted by ̃γconh(G), is called the outer-convex hop domination number of G. In this paper, we generate some formulas for the parameters of some special graphs and graphs under some binary operations by characterizing first the outer-convex hop dominating sets of each of thesegraphs. Moreover, we establish realization result that identifies and determines the connection of this parameter with the standard hop domination parameter. It shows that given any graph, this new parameter is always greater than or equal to the standard hop domination parameter.

Prime Graph Generation through Single Edge Addition: Characterizing a Class of Graphs

A graph G consists of a finite set V (G) of vertices with a collection E(G) of unordered pairs of distinct vertices called edge set of G. Let G be a graph. A set M of vertices is a module of G if, for vertices x and y in M and each vertex z outside M, {z, x} ∈ E(G) ⇐⇒ {z, y} ∈ E(G). Thus, a module of G is a set M of vertices indistinguishable by the vertices outside M. The empty set, the singleton sets and the full set of vertices represent the trivial modules. A graph is indecomposable if all its modules are trivial, otherwise it is decomposable. Indecomposable graphs with at least four vertices are prime graphs. The introduction and the study of the construction of prime graphs obtained from a given decomposable graph by adding one edge constitue the central points of this paper.