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 Title: Extremal problems on variations of graph colorings Author(s): Choi, Ilkyoo Director of Research: Kostochka, Alexandr V. Doctoral Committee Chair(s): Reznick, Bruce Doctoral Committee Member(s): Kostochka, Alexandr V.; West, Douglas B.; Lidicky, Bernard Department / Program: Mathematics Discipline: Mathematics Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): Graph Theory Graph Coloring Choosability List Chromatic Number Discharging Abstract: This thesis investigates various coloring problems in graph theory. Graph coloring is an essential part of combinatorics and discrete mathematics, as it deals with the fundamental problem of partitioning objects so that each part satisfies a certain condition. In particular, we study how forbidding certain structures (subgraphs) affects a given coloring parameter. Open since 1977, the Borodin--Kostochka Conjecture states that given a graph $G$ with maximum degree $\Delta(G)$ at least 9, if $G$ has no clique of size $\Delta(G)$, then $G$ is $(\Delta(G)-1)$-colorable. The current best result by Reed shows that the statement of the Borodin--Kostochka Conjecture is true for graphs with maximum degree at least $10^{14}$. We produce a result of this type for the list chromatic number; namely, we prove that given a graph $G$ with maximum degree at least $10^{20}$, if $G$ has no clique of size $\Delta(G)$, then $G$ is $(\Delta(G)-1)$-choosable. Cai, Wang, and Zhu proved that a toroidal graph with no 6-cycles is 5-choosable, and they conjectured that the only case when it is not 4-choosable is when the graph contains $K_5$. We disprove this conjecture by constructing a family of graphs containing neither 6-cycles nor $K_5$ that are not even 4-colorable. This family is embeddable not only on a torus, but also on any surface except the plane and the projective plane. We prove a slightly weaker statement suggested by Zhu that toroidal graphs containing neither 6-cycles nor $K^-_5$ are 4-choosable. We also study questions regarding variants of coloring. We provide additional positive support for a question by \v Skrekovski regarding choosability with separation for planar graphs, and we completely answer a question by Raspaud and Wang regarding vertex arboricity for toroidal graphs. We also improve results regarding improper coloring of planar graphs, responding to a question of Montassier and Ochem. Issue Date: 2014-05-30 URI: http://hdl.handle.net/2142/49578 Rights Information: Copyright 2014 Ilkyoo Choi Date Available in IDEALS: 2014-05-30 Date Deposited: 2014-05
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