Louisiana State University LSU Digital Commons LSU Doctoral Dissertations Graduate School 4-5-2018 On Structures of Large Rooted Graphs Shilin Wang Louisiana State University and Agricultural and Mechanical College, [email protected] Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_dissertations Part of the Discrete Mathematics and Combinatorics Commons Recommended Citation Wang, Shilin, "On Structures of Large Rooted Graphs" (2018). LSU Doctoral Dissertations. 4568. https://digitalcommons.lsu.edu/gradschool_dissertations/4568 This Dissertation is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Doctoral Dissertations by an authorized graduate school editor of LSU Digital Commons. For more information, please [email protected]. ON STRUCTURES OF LARGE ROOTED GRAPHS A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy in The Department of Mathematics by Shilin Wang B.S. in Applied Math, South China Normal University, 2009 M.S. in Operational Research and Cybernetics, South China Normal University, 2012 M.S. in Math, Louisiana State University, 2014 M.S. in Applied Statistics, Louisiana State University, 2017 May 2018 Acknowledgments This dissertation would not be possible without several contributions. First and most, it is a pleasure to express my sincere gratitude and thanks to my advisor Guoli Ding for the continuous support and encouragement, the useful comments, the immense knowledge of my Ph.D. study and related research. As my advisor and mentor, Prof. Ding has taught me more than I could ever give him credit for here. No words can express how much I thank him. His advice on both research as well as on my career have been priceless. Besides my advisor, I would like to thank the rest of my thesis committee: Prof. Li, Prof. Litherland, Prof. Olafsson, Prof. Oporowski, Prof. Vertigan, for their insightful comments and encouragement. It is a pleasure also to thank my life-coach, my mother Yuhua Zhang: because she encourages me to pursue my dream. This dissertation is specially dedicated to my family: my parents for their support an encouragement. I would also like to express how grateful I am to all my dear friends: for their understanding and motivations to fulfill my dream. Finally, I am also grateful to myself. Thank you for working so hard every day during the last thirteen years; thank you for never giving up even in the face of poverty, failure, loneliness, social pressure; thank you for being a braver, kindhearted, and brighter girl. ii Table of Contents Acknowledgments . ii Abstract . iv Chapter 1: Introduction . 1 1.1 Preliminaries . .1 1.2 Minor and Topological Minor . .6 1.3 Unavoidable large graphs . .9 1.4 Excluding large graphs . 11 1.5 Basic lemmas . 15 1.6 Rooted graphs . 20 Chapter 2: Unavoidable Large Rooted Connected Graphs . 26 2.1 Large rooted graphs . 26 2.2 Large generalized rooted graphs . 34 Chapter 3: Unavoidable Large Rooted 2-connected Graphs . 43 3.1 Large rooted graphs . 43 3.2 Large generalized rooted graphs . 51 Chapter 4: Excluding a Large Comb . 59 4.1 Excluding a nicely confined comb . 59 4.2 Excluding a confined comb . 64 Chapter 5: Excluding a Heavy Path . 73 5.1 Packing ABA-paths . 73 5.2 Excluding a heavy path . 76 5.3 Excluding a heavy path rooted minor . 83 Chapter 6: Excluding a Large Star . 87 6.1 One conjecture . 87 6.2 Excluding a nicely confined K1;4 .................... 88 6.3 Excluding a nicely confined star K1;5 ................. 95 6.4 Excluding a large confined star rooted minor . 98 References . 101 Vita ........................................................................102 iii Abstract A rooted graph is a pair (G; R), where G is a graph and R ⊆ V (G). There are two research topics in this thesis. One is about unavoidable substructures in suffi- ciently large rooted graphs. The other is about characterizations of rooted graphs excluding specific large graphs. The first topic of this thesis is motivated by Ramsey Theorem, which states that Kn and Kn are unavoidable induced subgraphs in every sufficiently large graph. It is also motivated by a classical result of Oporowski, Oxley, and Thomas, which determines unavoidable large 3-connected minors. We first determine unavoidable induced subgraphs, and unavoidable subgraphs in connected graphs with suffi- ciently many roots. We also extend this result to generalized rooted connected graphs. Secondly, we extend these results to rooted graphs of higher connectivity. In particular, we determine unavoidable subgraphs of sufficiently large rooted 2- connected graphs. Again, this result is extended to generalized rooted 2-connected graphs. The second topic of this dissertation is motivated by two results of Robertson and Seymour, let's only talk about path and star. In the first result they established that graphs without a long path subgraph are precisely those that can be constructed using a specific operation within a bounded number of iterations, starting from the trivial graph. In the second result they showed that graphs without a large star minor are those that are subdivisions of graphs with bounded number vertices. We consider similar problems for path, star and comb. We have some theorems on characterizations of rooted connected graphs excluding a heavy path, a large (nicely) confined comb, a large (nicely) confined star, which are similar to those of Robertson and Seymour. Moreover, our results strengthen their related results. iv 1 Introduction This dissertation is about structures of large rooted graphs. Before stating our main results, we need some necessary definitions. In this chapter, we begin with a brief summary of basic definitions on graph minor. Then we introduce basic lemmas which are used in the proof of later theorems in this thesis. Finally we list some related results on unavoidable substructures in large graphs, and on characterizations of excluding certain large graphs. An outline of this dissertation will be given in section 1.6. 1.1 Preliminaries In this section, we provide some basic definitions and lemmas in graph theory, which are used in the thesis. Undefined terminology can be found in [2]. Let N denote the set of positive integers. We will use symbols ;; [; \; 2; ⊆; n; + for set operations in the usual sense. We also remark that A [ fvg is abbreviated to A + v. A graph G consists of an ordered pair (V; E) of disjoint finite sets, where V is not empty, and an incidence relation such that each member of E is incident with one or two members of V . Members of E and V are called edges and vertices, respectively. The number of vertices of G is its order, and is denoted by jGj, which is exactly jV j. If e is an edge of G or v is a vertex of G, instead of writing e 2 E or v 2 V we may simply write e 2 G or v 2 G. If an edge e is incident with a vertex x, then x is called an end of e. If two distinct vertices x; y are incident with a common edge, or if two distinct edges x; y are incident with a common vertex, then we say that x; y are adjacent. An edge with only one end is a loop. If two non-loop edges have the same ends, then they are parallel and each is a parallel edge in G. 1 A graph G is simple if it has neither loops nor parallel edges. A graph with only one vertex is called trivial. All graphs considered in this thesis are simple, except where otherwise noted. For a simple graph G = (V; E), each edge e 2 E consists of two distinct ends x and y, and is denoted by e = xy. For a vertex v 2 V , a neighbor of v is a vertex adjacent with v. The set of neighbors of v is denoted by NG(v). We also remark that NG(v) is abbreviated to N(v) when the dependence on G is clear. The number of edges incident with v is called the degree of v, and is denoted by dG(v), which is exactly jNG(v)j. The maximal degree of all vertices in G is denoted by 4(G). When X ⊆ V , we denote by NG(X) the set of vertices outside X that S are adjacent to at least one vertex in X. That is to say, NG(X) = v2X NG(v)nX. The complement G of G is the graph on V such that two distinct vertices of G are adjacent if and only if they are not adjacent in G. Let G = (V; E) be a graph. Deleting an edge e from G means deleting e from E, and is denoted by Gne. Deleting a vertex v from G means deleting v from V and deleting all edges incident with v from E, and is denoted by Gnv. For any two distinct e; f 2 E, it is clear that Gnenf = Gnfne. Thus we can define the deletion of a set of edges. For any X ⊆ E, deleting X from G means deleting every edge of X from G, and is denoted by GnX. Similarly, for any two distinct vertices x; y 2 V , we also have Gnxny = Gnynx. Naturally, for any X ⊆ V , deleting X from G means deleting every vertex of X from G, and is denoted by GnX. For non-adjacent vertices x; y 2 V , let G + xy = (V; E [ fxyg). A subgraph of a graph G = (V; E) is a graph obtained from G by deleting a vertex set and an edge set.