Turán problems in graphs and hypergraphs

Mantel's theorem says that among all triangle-free graphs of a given order the balanced complete bipartite graph is the unique graph of maximum size. In Chapter 2, we prove an analogue of this result for 3-graphs (3-uniform hy¬pergraphs) together with an associated stability result. Let K− 4 , F5 and F6 be 3-graphs with vertex sets {1, 2,3, 4}, {1, 2,3,4, 5} and {1, 2,3,4, 5, 6} re¬spectively and edge sets E(K−4 ) = {123, 124, 134}, E(F5) = {123, 124, 345}, E(F6) = {123, 124,345, 156} and F = {K4, F6}. For n =6 5 the unique F-free 3-graph of order n and maximum size is the balanced complete tri¬partite 3-graph S3(n). This extends an old result of Bollobas that S3(n) is the unique 3-graph of maximum size with no copy of K− 4 or F5. In 1941, Turán generalised Mantel's theorem to cliques of arbitrary size and then asked whether similar results could be obtained for cliques on hyper-graphs. This has become one of the central unsolved problems in the field of extremal combinatorics. In Chapter 3, we prove that the Turán density of K(3) 5 together with six other induced subgraphs is 3/4. This is analogous to a similar result obtained for K(3) 4 by Razborov. In Chapter 4, we consider various generalisations of the Turán density. For example, we prove that, if the density in C of ¯P3 is x and C is K3-free, then |E(C)| /(n ) ≤ 1/4+(1/4)J1 − (8/3)x. This is motivated by the observation 2 that the extremal graph for K3 is ¯P3-free, so that the upper bound is a natural extension of a stability result for K3. The question how many edges can be deleted from a blow-up of H before it is H-free subject to the constraint that the same proportion of edges are deleted from each connected pair of vertex sets has become known as the Turán density problem. In Chapter 5, using entropy compression supplemented with some analytic methods, we derive an upper bound of 1 − 1/('y(Δ(H) − /3)), where Δ(H) is the maximum degree of H, 3 ≤ 'y < 4 and /3 ≤ 1. The new bound asymptotically approaches the existing best upper bound despite being derived in a completely different way. The techniques used in these results, illustrating their breadth and connec¬tions between them, are set out in Chapter 1

On the structure of graphs with forbidden induced substructures

One of the central goals in extremal combinatorics is to understand how the global structure of a combinatorial object, e.g. a graph, hypergraph or set system, is affected by local constraints. In this thesis we are concerned with structural properties of graphs and hypergraphs which locally do not look like some type of forbidden induced pattern. Patterns can be single subgraphs, families of subgraphs, or in the multicolour version colourings or families of colourings of subgraphs. Erdős and Szekeres\u27s quantitative version of Ramsey\u27s theorem asserts that in every $2$-edge-colouring of the complete graph on $n$ vertices there is a monochromatic clique on at least $\frac{1}{2}\log n$ vertices. The famous Erdős-Hajnal conjecture asserts that forbidding fixed colourings on subgraphs ensures much larger monochromatic cliques. The conjecture is open in general, though a few partial results are known. The first part of this thesis will be concerned with different variants of this conjecture: A bipartite variant, a multicolour variant, and an order-size variant for hypergraphs. In the second part of this thesis we focus more on order-size pairs; an order-size pair $(n,e)$ is the family consisting of all graphs of order $n$ and size $e$, i.e. on $n$ vertices with $e$ edges. We consider order-size pairs in different settings: The graph setting, the bipartite setting and the hypergraph setting. In all these settings we investigate the existence of absolutely avoidable pairs, i.e. fixed pairs that are avoided by all order-size pairs with sufficiently large order, and also forcing densities of order-size pairs $(m,f)$, i.e. for $n$ approaching infinity, the limit superior of the fraction of all possible sizes $e$, such that the order-size pair $(n,e)$ does not avoid the pair $(m,f)$

Probabilistic and extremal studies in additive combinatorics

The results in this thesis concern extremal and probabilistic topics in number theoretic settings. We prove sufficient conditions on when certain types of integer solutions to linear systems of equations in binomial random sets are distributed normally, results on the typical approximate structure of pairs of integer subsets with a given sumset cardinality, as well as upper bounds on how large a family of integer sets defining pairwise distinct sumsets can be. In order to prove the typical structural result on pairs of integer sets, we also establish a new multipartite version of the method of hypergraph containers, generalizing earlier work by Morris, Saxton and Samotij.L'objectiu de la combinatòria additiva “històricament també anomenada teoria combinatòria de nombres” és la d’estudiar l'estructura additiva de conjunts en determinats grups ambient. La combinatòria extremal estudia quant de gran pot ser una col·lecció d'objectes finits abans d'exhibir determinats requisits estructurals. La combinatòria probabilística analitza estructures combinatòries aleatòries, identificant en particular l'estructura dels objectes combinatoris típics. Entre els estudis més celebrats hi ha el treball de grafs aleatoris iniciat per Erdös i Rényi. Un exemple especialment rellevant de com aquestes tres àrees s'entrellacen és el desenvolupament per Erdös del mètode probabilístic en teoria de nombres i en combinatòria, que mostra l'existència de moltes estructures extremes en configuracions additives utilitzant tècniques probabilistes. Tots els temes d'aquesta tesi es troben en la intersecció d'aquestes tres àrees, i apareixen en els problemes següents. Solucions enteres de sistemes d'equacions lineals. Els darrers anys s'han obtingut resultats pel que fa a l’existència de llindars per a determinades solucions enteres a un sistema arbitrari d'equacions lineals donat, responent a la pregunta de quan s'espera que el subconjunt aleatori binomial d'un conjunt inicial de nombres enters contingui solucions gairebé sempre. La següent pregunta lògica és la següent. Suposem que estem en la zona en que hi haurà solucions enteres en el conjunt aleatori binomial, com es distribueixen aleshores aquestes solucions? Al capítol 1, avançarem per respondre aquesta pregunta proporcionant condicions suficients per a quan una gran varietat de solucions segueixen una distribució normal. També parlarem de com, en determinats casos, aquestes condicions suficients també són necessàries. Conjunts amb suma acotada. Què es pot dir de l'estructura de dos conjunts finits en un grup abelià si la seva suma de Minkowski no és molt més gran que la dels conjunts? Un resultat clàssic de Kneser diu que això pot passar si i només si la suma de Minkowski és periòdica respecte a un subgrup propi. En el capítol 3 establirem dos tipus de resultats. En primer lloc, establirem les anomenades versions robustes dels teoremes clàssics de Kneser i Freiman. Robust aquí es refereix al fet que en comptes de demanar informació estructural sobre els conjunts constituents amb el coneixement que la seva suma és petita, només necessitem que això sigui vàlid per a un subconjunt gran passa si només volem donar una informació estructural per a gairebé tots els parells de conjunts amb una suma d'una mida determinada? Donem un teorema d'estructura aproximat que s'aplica a gairebé la majoria dels rangs possibles per la mida dels conjunts suma. Sistemes de conjunts de Sidon. Les preguntes clàssiques sobre els conjunts de Sidon són determinar la seva mida màxima o saber quan un conjunt aleatori és un conjunt de Sidon. Al capítol 4 generalitzem la noció de conjunts de Sidon per establir sistemes i establim els límits corresponents per a aquestes dues preguntes. També demostrem un resultat de densitat relativa, resultat condicionat a una conjectura sobre l'estructura específica dels sistemes màxims de Sidon. Conjunts independents en hipergrafs. El mètode dels contenidors d'hipergrafs és una eina general que es pot utilitzar per obtenir resultats sobre el nombre i l'estructura de conjunts independents en els hipergrafs. La connexió amb la combinatòria additiva apareix perquè molts problemes additius es poden codificar com l'estudi de conjunts independents en hipergrafs.Postprint (published version

Advances in Discrete Applied Mathematics and Graph Theory

The present reprint contains twelve papers published in the Special Issue “Advances in Discrete Applied Mathematics and Graph Theory, 2021” of the MDPI Mathematics journal, which cover a wide range of topics connected to the theory and applications of Graph Theory and Discrete Applied Mathematics. The focus of the majority of papers is on recent advances in graph theory and applications in chemical graph theory. In particular, the topics studied include bipartite and multipartite Ramsey numbers, graph coloring and chromatic numbers, several varieties of domination (Double Roman, Quasi-Total Roman, Total 3-Roman) and two graph indices of interest in chemical graph theory (Sombor index, generalized ABC index), as well as hyperspaces of graphs and local inclusive distance vertex irregular graphs

Discrete Geometry and Convexity in Honour of Imre Bárány

This special volume is contributed by the speakers of the Discrete Geometry and Convexity conference, held in Budapest, June 19–23, 2017. The aim of the conference is to celebrate the 70th birthday and the scientific achievements of professor Imre Bárány, a pioneering researcher of discrete and convex geometry, topological methods, and combinatorics. The extended abstracts presented here are written by prominent mathematicians whose work has special connections to that of professor Bárány. Topics that are covered include: discrete and combinatorial geometry, convex geometry and general convexity, topological and combinatorial methods. The research papers are presented here in two sections. After this preface and a short overview of Imre Bárány’s works, the main part consists of 20 short but very high level surveys and/or original results (at least an extended abstract of them) by the invited speakers. Then in the second part there are 13 short summaries of further contributed talks. We would like to dedicate this volume to Imre, our great teacher, inspiring colleague, and warm-hearted friend

Detection and Evaluation of Clusters within Sequential Data

Motivated by theoretical advancements in dimensionality reduction techniques we use a recent model, called Block Markov Chains, to conduct a practical study of clustering in real-world sequential data. Clustering algorithms for Block Markov Chains possess theoretical optimality guarantees and can be deployed in sparse data regimes. Despite these favorable theoretical properties, a thorough evaluation of these algorithms in realistic settings has been lacking. We address this issue and investigate the suitability of these clustering algorithms in exploratory data analysis of real-world sequential data. In particular, our sequential data is derived from human DNA, written text, animal movement data and financial markets. In order to evaluate the determined clusters, and the associated Block Markov Chain model, we further develop a set of evaluation tools. These tools include benchmarking, spectral noise analysis and statistical model selection tools. An efficient implementation of the clustering algorithm and the new evaluation tools is made available together with this paper. Practical challenges associated to real-world data are encountered and discussed. It is ultimately found that the Block Markov Chain model assumption, together with the tools developed here, can indeed produce meaningful insights in exploratory data analyses despite the complexity and sparsity of real-world data.Comment: 37 pages, 12 figure

Alte und sehr alte Landkarten

Neuss: Bruno Buike 2020, 822p. - E52 - full title: Alte und sehr alte Landkarten - 800 Seiten Materialien - Reste eines Computercrashs mit Datenverlust - ENGLISH title: Old and vey old maps, 800p. material