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Title:  Extremal problems in disjoint cycles and graph saturation 
Author(s):  Yeager, Elyse Christine 
Director of Research:  Kostochka, Alexandr V. 
Doctoral Committee Chair(s):  Balogh, Jozsef 
Doctoral Committee Member(s):  Reznick, Bruce A.; Molla, Theodore 
Department / Program:  Mathematics 
Discipline:  Mathematics 
Degree Granting Institution:  University of Illinois at UrbanaChampaign 
Degree:  Ph.D. 
Genre:  Dissertation 
Subject(s):  equitable coloring
cycles CorradiHajnal HajnalSzemeredi ChenLihWu graph saturation cocriticality edgecolored saturation 
Abstract:  In this thesis, we tackle two main themes: sufficient conditions for the existence of particular subgraphs in a graph, and variations on graph saturation. Determining whether a graph contains a certain subgraph is a computationally difficult problem; as such, sufficient conditions for the existence of a given subgraph are prized. In Chapter 2, we offer a significant refinement of the CorradiHajnal Theorem, which gives sufficient conditions for the existence of a given number of disjoint cycles in a graph. Further, our refined theorem leads to an answer for a question posed by G. Dirac in 1963 regarding the existence of disjoint cycles in graphs with a certain connectivity. This answer comprises Chapter 3. In Chapter 4 we prove a result about equitable coloring: that is, a proper coloring whose color classes all have the same size. Our equitablecoloring result confirms a partial case of a generalized version of the muchstudied ChenLihWu conjecture on equitable coloring. In addition, the equitablecoloring result is equivalent to a statement about the existence of disjoint cycles, contributing to our refinement of the CorradiHajnal Theorem. In Chapters 5 and 6, we move to the topic of graph saturation, which is related to the Turan problem. One imagines a set of n vertices, to which edges are added onebyone so that a forbidden subgraph never appears. At some point, no more edges can be added. The Turan problem asks the maximum number of edges in such a graph; the saturation number, on the other hand, asks the minimum number of edges. Two variations of this parameter are studied. In Chapter 5, we study the saturation of Ramseyminimal families. Ramsey theory deals with partitioning the edges of graphs so that each partition avoids the particular forbidden subgraph assigned to it. Our motivation for studying these families is that they provide a convincing edgecolored (Ramsey) version of graph saturation. We develop a method, called iterated recoloring, for using results from graph saturation to understand this Ramsey version of saturation. As a proof of concept, we use iterated recoloring to determine the saturation number of the Ramseyminimal families of matchings and describe the assiociated extremal graphs. An induced version of graph saturation was suggested by Martin and Smith. In order to offer a parameter that is defined for all forbidden graphs, Martin and Smith consider generalized graphs, called trigraphs. Of particular interest is the case when the inducedsaturated trigraphs in question are equivalent to graphs. In Chapter 6, we show that a surprisingly large number of families fall into this case. Further, we define and investigate another parameter that is a version of induced saturation that is closer in spirit to the original version of graph saturation, but that is not defined for all forbidden subgraphs. 
Issue Date:  20150423 
Type:  Text 
URI:  http://hdl.handle.net/2142/78444 
Rights Information:  Copyright 2015 by Elyse C. Yeager 
Date Available in IDEALS:  20150722 
Date Deposited:  May 2015 
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Dissertations and Theses  Mathematics

Graduate Dissertations and Theses at Illinois
Graduate Theses and Dissertations at Illinois