scholarly journals Uniquely $D$-Colourable Digraphs with Large Girth II: Simplification via Generalization

10.37236/9689 ◽  
2021 ◽  
Vol 28 (1) ◽  
Author(s):  
P. Mark Kayll ◽  
Esmaeil Parsa
Keyword(s):  
Graph Theory ◽  
Sparse Graphs ◽  
Large Girth ◽  
Positive Integers

We prove that for every digraph $D$ and every choice of positive integers $k$, $\ell$ there exists a digraph $D^*$ with girth at least $\ell$ together with a surjective acyclic homomorphism $\psi\colon D^*\to D$ such that: (i) for every digraph $C$ of order at most $k$, there exists an acyclic homomorphism $D^*\to C$ if and only if there exists an acyclic homomorphism $D\to C$; and (ii) for every $D$-pointed digraph $C$ of order at most $k$ and every acyclic homomorphism $\varphi\colon D^*\to C$ there exists a unique acyclic homomorphism $f\colon D\to C$ such that $\varphi=f\circ\psi$. This implies the main results in [A. Harutyunyan et al., Uniquely $D$-colourable digraphs with large girth, Canad. J. Math., 64(6) (2012), 1310—1328; MR2994666] analogously with how the work [J. Nešetřil and X. Zhu, On sparse graphs with given colorings and homomorphisms, J. Combin. Theory Ser. B, 90(1) (2004), 161—172; MR2041324] generalizes and extends [X. Zhu, Uniquely $H$-colorable graphs with large girth, J. Graph Theory, 23(1) (1996), 33—41; MR1402136].

Frozen (Δ + 1)-colourings of bounded degree graphs

2020 ◽  
pp. 1-14
Author(s):  
Marthe Bonamy ◽  
Nicolas Bousquet ◽  
Guillem Perarnau
Keyword(s):  
Graph Theory ◽  
Maximum Degree ◽  
Mixing Time ◽  
Glauber Dynamics ◽  
Regular Graphs ◽  
Connected Component ◽  
Bounded Degree ◽  
Large Girth ◽  
Bounded Degree Graphs

Abstract Let G be a graph on n vertices and with maximum degree Δ, and let k be an integer. The k-recolouring graph of G is the graph whose vertices are k-colourings of G and where two k-colourings are adjacent if they differ at exactly one vertex. It is well known that the k-recolouring graph is connected for $k\geq \Delta+2$ . Feghali, Johnson and Paulusma (J. Graph Theory83 (2016) 340–358) showed that the (Δ + 1)-recolouring graph is composed by a unique connected component of size at least 2 and (possibly many) isolated vertices. In this paper, we study the proportion of isolated vertices (also called frozen colourings) in the (Δ + 1)-recolouring graph. Our first contribution is to show that if G is connected, the proportion of frozen colourings of G is exponentially smaller in n than the total number of colourings. This motivates the use of the Glauber dynamics to approximate the number of (Δ + 1)-colourings of a graph. In contrast to the conjectured mixing time of O(nlog n) for $k\geq \Delta+2$ colours, we show that the mixing time of the Glauber dynamics for (Δ + 1)-colourings restricted to non-frozen colourings can be Ω(n2). Finally, we prove some results about the existence of graphs with large girth and frozen colourings, and study frozen colourings in random regular graphs.

Two Extremal Problems in Graph Theory

10.37236/1182 ◽  
1994 ◽  
Vol 1 (1) ◽  
Author(s):  
Richard A. Brualdi ◽  
Stephen Mellendorf
Keyword(s):  
Graph Theory ◽  
Extremal Problems ◽  
Turan's Theorem ◽  
Turán’S Theorem ◽  
Positive Integers

We consider the following two problems. (1) Let $t$ and $n$ be positive integers with $n\geq t\geq 2$. Determine the maximum number of edges of a graph of order $n$ that contains neither $K_t$ nor $K_{t,t}$ as a subgraph. (2) Let $r$, $t$ and $n$ be positive integers with $n\geq rt$ and $t\geq 2$. Determine the maximum number of edges of a graph of order $n$ that does not contain $r$ disjoint copies of $K_t$. Problem 1 for $n < 2t$ is solved by Turán's theorem and we solve it for $n=2t$. We also solve Problem 2 for $n=rt$.

scholarly journals Homomorphisms from sparse graphs with large girth

2004 ◽  
Vol 90 (1) ◽  
pp. 147-159 ◽  
Author(s):  
O.V. Borodin ◽  
S.-J. Kim ◽  
A.V. Kostochka ◽  
D.B. West
Keyword(s):  
Sparse Graphs ◽  
Large Girth

scholarly journals Application of Reverse Vertex Magic Labeling of a Graph

2019 ◽  
Vol 8 (11) ◽  
pp. 12-14
Keyword(s):  
Graph Theory ◽  
Graph Labeling ◽  
The Given ◽  
Positive Integers ◽  
A Company

Graph labeling is a currently emerging area in the research of graph theory. A graph labeling is an assignment of integers to the vertices or edges or both subject to certain conditions. If the labels of edges are distinct positive integers and for each vertex the sum of the labels of all edges incident with is the same for every vertex in the given graph, then the labeling of the graph is called magic labeling. There are several types of magic labeling defined on graphs. In this paper we consider vertex magic labeling and group magic labeling of graphs as an application of magic labeling. We solve a problem of finding number of computers/workstations to be allocated to each department in a company under certain conditions.

scholarly journals Supersaturated Sparse Graphs and Hypergraphs

2018 ◽  
Vol 2020 (2) ◽  
pp. 378-402 ◽  
Author(s):  
Asaf Ferber ◽  
Gweneth McKinley ◽  
Wojciech Samotij
Keyword(s):  
Growth Rate ◽  
Graph Theory ◽  
Extremal Graph Theory ◽  
Central Problem ◽  
Uniform Hypergraph ◽  
Natural Assumption ◽  
Full Generality ◽  
Sparse Graphs ◽  
Analogous Statement ◽  
Bipartite Case

Abstract A central problem in extremal graph theory is to estimate, for a given graph H, the number of H-free graphs on a given set of n vertices. In the case when H is not bipartite, Erd̋s, Frankl, and Rödl proved that there are 2(1+o(1))ex(n, H) such graphs. In the bipartite case, however, bounds of the form 2O(ex(n, H)) have been proven only for relatively few special graphs H. As a 1st attempt at addressing this problem in full generality, we show that such a bound follows merely from a rather natural assumption on the growth rate of n ↦ ex(n, H); an analogous statement remains true when H is a uniform hypergraph. Subsequently, we derive several new results, along with most previously known estimates, as simple corollaries of our theorem. At the heart of our proof lies a general supersaturation statement that extends the seminal work of Erd̋s and Simonovits. The bounds on the number of H-free hypergraphs are derived from it using the method of hypergraph containers.

The Blow-up Lemma

1999 ◽  
Vol 8 (1-2) ◽  
pp. 161-176 ◽  
Author(s):  
JÁNOS KOMLÓS
Keyword(s):  
Graph Theory ◽  
Blow Up ◽  
Extremal Graph ◽  
Extremal Graph Theory ◽  
Regularity Lemma ◽  
Sparse Graphs ◽  
Dense Graphs ◽  
Recent Developments ◽  
Szemeredi's Regularity Lemma

Extremal graph theory has a great number of conjectures concerning the embedding of large sparse graphs into dense graphs. Szemerédi's Regularity Lemma is a valuable tool in finding embeddings of small graphs. The Blow-up Lemma, proved recently by Komlós, Sárközy and Szemerédi, can be applied to obtain approximate versions of many of the embedding conjectures. In this paper we review recent developments in the area.

scholarly journals Ramsey-type numbers involving graphs and hypergraphs with large girth

2021 ◽  
pp. 1-19
Author(s):  
Hiêp Hàn ◽  
Troy Retter ◽  
Vojtêch Rödl ◽  
Mathias Schacht
Keyword(s):  
Arithmetic Progressions ◽  
Ramsey Number ◽  
Ramsey Property ◽  
Numerical Problem ◽  
The Third ◽  
Ramsey Type ◽  
Large Girth ◽  
Positive Integers

Abstract Erdős asked if, for every pair of positive integers g and k, there exists a graph H having girth (H) = k and the property that every r-colouring of the edges of H yields a monochromatic cycle C k . The existence of such graphs H was confirmed by the third author and Ruciński. We consider the related numerical problem of estimating the order of the smallest graph H with this property for given integers r and k. We show that there exists a graph H on R10k2; k15k3 vertices (where R = R(C k ; r) is the r-colour Ramsey number for the cycle C k ) having girth (H) = k and the Ramsey property that every r-colouring of the edges of H yields a monochromatic C k Two related numerical problems regarding arithmetic progressions in subsets of the integers and cliques in graphs are also considered.

scholarly journals On the Cartesian product of of an arbitrarily partitionable graph and a traceable graph

2014 ◽  
Vol Vol. 16 no. 1 (Graph Theory) ◽  
Author(s):  
Olivier Baudon ◽  
Julien Bensmail ◽  
Rafał Kalinowski ◽  
Antoni Marczyk ◽  
Jakub Przybyło ◽  
...  
Keyword(s):  
Graph Theory ◽  
Cartesian Product ◽  
Connected Subgraph ◽  
Vertex Set ◽  
International Audience ◽  
Positive Integers

Graph Theory International audience A graph G of order n is called arbitrarily partitionable (AP, for short) if, for every sequence τ=(n1,\textellipsis,nk) of positive integers that sum up to n, there exists a partition (V1,\textellipsis,Vk) of the vertex set V(G) such that each set Vi induces a connected subgraph of order ni. A graph G is called AP+1 if, given a vertex u∈V(G) and an index q∈ &#x007b;1,\textellipsis,k&#x007d;, such a partition exists with u∈Vq. We consider the Cartesian product of AP graphs. We prove that if G is AP+1 and H is traceable, then the Cartesian product G□ H is AP+1. We also prove that G□H is AP, whenever G and H are AP and the order of one of them is not greater than four.

scholarly journals Defective and clustered choosability of sparse graphs

2019 ◽  
Vol 28 (5) ◽  
pp. 791-810 ◽  
Author(s):  
Kevin Hendrey ◽  
David R. Wood
Keyword(s):  
Graph Theory ◽  
Maximum Degree ◽  
Discrete Math ◽  
Average Degree ◽  
Graph Colouring ◽  
Sparse Graphs ◽  
Trade Off ◽  
Maximum Average Degree ◽  
The Earth

AbstractAn (improper) graph colouring hasdefect dif each monochromatic subgraph has maximum degree at mostd, and hasclustering cif each monochromatic component has at mostcvertices. This paper studies defective and clustered list-colourings for graphs with given maximum average degree. We prove that every graph with maximum average degree less than (2d+2)/(d+2)kisk-choosable with defectd. This improves upon a similar result by Havet and Sereni (J. Graph Theory, 2006). For clustered choosability of graphs with maximum average degreem, no (1-ɛ)mbound on the number of colours was previously known. The above result withd=1 solves this problem. It implies that every graph with maximum average degreemis$\lfloor{\frac{3}{4}m+1}\rfloor$-choosable with clustering 2. This extends a result of Kopreski and Yu (Discrete Math., 2017) to the setting of choosability. We then prove two results about clustered choosability that explore the trade-off between the number of colours and the clustering. In particular, we prove that every graph with maximum average degreemis$\lfloor{\frac{7}{10}m+1}\rfloor$-choosable with clustering 9, and is$\lfloor{\frac{2}{3}m+1}\rfloor$-choosable with clusteringO(m). As an example, the later result implies that every biplanar graph is 8-choosable with bounded clustering. This is the best known result for the clustered version of the earth–moon problem. The results extend to the setting where we only consider the maximum average degree of subgraphs with at least some number of vertices. Several applications are presented.

Vertex Selection Heuristics in Branch-and-Bound Algorithms for the Maximum k-Plex Problem

2019 ◽  
Vol 28 (05) ◽  
pp. 1950015
Author(s):  
Kuixian Wu ◽  
Jian Gao ◽  
Rong Chen ◽  
Xianji Cui
Keyword(s):  
Social Networks ◽  
Graph Theory ◽  
Branch And Bound ◽  
State Of The Art ◽  
The State ◽  
Branch And Bound Algorithm ◽  
Sparse Graphs ◽  
Historical Information ◽  
Branch And Bound Algorithms ◽  
Heuristic Strategies

As a relaxation of clique in graph theory, k-plex is a powerful tool for analyzing social networks and identifying cohesive structures in graphs. Recently, more and more researchers have concentrated on the algorithms for the maximum k-plex problem. Among those algorithms, a branch-and-bound algorithm proposed very recently shows a good performance on solving large sparse graphs, but does not work well on social networks. In this paper, we propose two novel vertex selection heuristic strategies for branching. The first one employs historical information of vertex reduction, and the second one is a combination of the first heuristic and the degree-based approach. Intensive experiments on Facebook benchmark show that the algorithm combining our heuristics outperforms the state-of-the-art algorithms.

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