13 research outputs found
The Chromatic Structure of Dense Graphs
This thesis focusses on extremal graph theory, the study of how local constraints on a graph affect its macroscopic structure. We primarily consider the chromatic structure: whether a graph has or is close to having some (low) chromatic number.
Chapter 2 is the slight exception. We consider an induced version of the classical Turán problem. Introduced by Loh, Tait, Timmons, and Zhou, the induced Turán number ex(n, {H, F-ind}) is the greatest number of edges in an n-vertex graph with no copy of H and no induced copy of F. We asymptotically determine ex(n, {H, F-ind}) for H not bipartite and F neither an independent set nor a complete bipartite graph. We also improve the upper bound for ex(n, {H, K_{2, t}-ind}) as well as the lower bound for the clique number of graphs that have some fixed edge density and no induced K_{2, t}.
The next three chapters form the heart of the thesis. Chapters 3 and 4 consider the Erdős-Simonovits question for locally r-colourable graphs: what are the structure and chromatic number of graphs with large minimum degree and where every neighbourhood is r-colourable? Chapter 3 deals with the locally bipartite case and Chapter 4 with the general case.
While the subject of Chapters 3 and 4 is a natural local to global colouring question, it is also essential for determining the minimum degree stability of H-free graphs, the focus of Chapter 5. Given a graph H of chromatic number r + 1, this asks for the minimum degree that guarantees that an H-free graph is close to r-partite. This is analogous to the classical edge stability of Erdős and Simonovits. We also consider the question for the family of graphs to which H is not homomorphic, showing that it has the same answer.
Chapter 6 considers sparse analogues of the results of Chapters 3 to 5 obtaining the thresholds at which the sparse problem degenerates away from the dense one.
Finally, Chapter 7 considers a chromatic Ramsey problem first posed by Erdős: what is the greatest chromatic number of a triangle-free graph on vertices or with m edges? We improve the best known bounds and obtain tight (up to a constant factor) bounds for the list chromatic number, answering a question of Cames van Batenburg, de Joannis de Verclos, Kang, and Pirot
Global hypercontractivity and its applications
The hypercontractive inequality on the discrete cube plays a crucial role in
many fundamental results in the Analysis of Boolean functions, such as the KKL
theorem, Friedgut's junta theorem and the invariance principle. In these
results the cube is equipped with the uniform measure, but it is desirable,
particularly for applications to the theory of sharp thresholds, to also obtain
such results for general -biased measures. However, simple examples show
that when , there is no hypercontractive inequality that is strong
enough.
In this paper, we establish an effective hypercontractive inequality for
general that applies to `global functions', i.e. functions that are not
significantly affected by a restriction of a small set of coordinates. This
class of functions appears naturally, e.g. in Bourgain's sharp threshold
theorem, which states that such functions exhibit a sharp threshold. We
demonstrate the power of our tool by strengthening Bourgain's theorem, thereby
making progress on a conjecture of Kahn and Kalai and by establishing a
-biased analog of the invariance principle.
Our results have significant applications in Extremal Combinatorics. Here we
obtain new results on the Tur\'an number of any bounded degree uniform
hypergraph obtained as the expansion of a hypergraph of bounded uniformity.
These are asymptotically sharp over an essentially optimal regime for both the
uniformity and the number of edges and solve a number of open problems in the
area. In particular, we give general conditions under which the crosscut
parameter asymptotically determines the Tur\'an number, answering a question of
Mubayi and Verstra\"ete. We also apply the Junta Method to refine our
asymptotic results and obtain several exact results, including proofs of the
Huang--Loh--Sudakov conjecture on cross matchings and the
F\"uredi--Jiang--Seiver conjecture on path expansions.Comment: Subsumes arXiv:1906.0556
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 tambĂ© anomenada teoria combinatòria de nombres” Ă©s la d’estudiar l'estructura additiva de conjunts en determinats grups ambient. La combinatòria extremal estudia quant de gran pot ser una col·lecciĂł d'objectes finits abans d'exhibir determinats requisits estructurals. La combinatòria probabilĂstica analitza estructures combinatòries aleatòries, identificant en particular l'estructura dels objectes combinatoris tĂpics. Entre els estudis mĂ©s celebrats hi ha el treball de grafs aleatoris iniciat per Erdös i RĂ©nyi. Un exemple especialment rellevant de com aquestes tres Ă rees s'entrellacen Ă©s el desenvolupament per Erdös del mètode probabilĂstic 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 intersecciĂł d'aquestes tres Ă rees, i apareixen en els problemes segĂĽents. 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 gairebĂ© sempre. La segĂĽent pregunta lògica Ă©s la segĂĽent. Suposem que estem en la zona en que hi haurĂ solucions enteres en el conjunt aleatori binomial, com es distribueixen aleshores aquestes solucions? Al capĂtol 1, avançarem per respondre aquesta pregunta proporcionant condicions suficients per a quan una gran varietat de solucions segueixen una distribuciĂł normal. TambĂ© parlarem de com, en determinats casos, aquestes condicions suficients tambĂ© sĂłn necessĂ ries. Conjunts amb suma acotada. Què es pot dir de l'estructura de dos conjunts finits en un grup abeliĂ si la seva suma de Minkowski no Ă©s molt mĂ©s gran que la dels conjunts? Un resultat clĂ ssic de Kneser diu que això pot passar si i nomĂ©s si la suma de Minkowski Ă©s periòdica respecte a un subgrup propi. En el capĂtol 3 establirem dos tipus de resultats. En primer lloc, establirem les anomenades versions robustes dels teoremes clĂ ssics de Kneser i Freiman. Robust aquĂ es refereix al fet que en comptes de demanar informaciĂł estructural sobre els conjunts constituents amb el coneixement que la seva suma Ă©s petita, nomĂ©s necessitem que això sigui vĂ lid per a un subconjunt gran passa si nomĂ©s volem donar una informaciĂł estructural per a gairebĂ© tots els parells de conjunts amb una suma d'una mida determinada? Donem un teorema d'estructura aproximat que s'aplica a gairebĂ© la majoria dels rangs possibles per la mida dels conjunts suma. Sistemes de conjunts de Sidon. Les preguntes clĂ ssiques sobre els conjunts de Sidon sĂłn determinar la seva mida mĂ xima o saber quan un conjunt aleatori Ă©s un conjunt de Sidon. Al capĂtol 4 generalitzem la nociĂł de conjunts de Sidon per establir sistemes i establim els lĂmits corresponents per a aquestes dues preguntes. TambĂ© demostrem un resultat de densitat relativa, resultat condicionat a una conjectura sobre l'estructura especĂfica dels sistemes mĂ xims de Sidon. Conjunts independents en hipergrafs. El mètode dels contenidors d'hipergrafs Ă©s una eina general que es pot utilitzar per obtenir resultats sobre el nombre i l'estructura de conjunts independents en els hipergrafs. La connexiĂł amb la combinatòria additiva apareix perquè molts problemes additius es poden codificar com l'estudi de conjunts independents en hipergrafs.Postprint (published version