Ph.D. / In this thesis, our primary objective is to investigate the effects that various graph modifications have on the total domination number of a graph. In Chapter 1, we introduce basic graph theory concepts and preliminary definitions. In Chapters 2 and 3, we investigate the graph modification of edge removal. In Chapter 2, we characterize graphs for which the removal of any arbitrary edge increases the total domination number. We also begin the investigation of graphs for which the removal of any arbitrary edge has no effect on the total domination number. In Chapter 3, we continue this investigation and determine the minimum number of edges required for these graphs. The contents of Chapters 2 and 3 have been published in Discrete Applied Mathematics [15] and [16]. In Chapter 4, we investigate the graph modification of edge addition. In particular, we focus our attention on graphs for which adding an edge between any pair of nonadjacent vertices has no effect on the total domination number. We characterize these graphs, determine a sharp upper bound on their total domination number and determine which combinations of order and total domination number are attainable. 10 11 We also study claw-free graphs which have this property. The contents of this chapter were published in Discrete Mathematics [20]. In Chapter 5, we investigate the graph modification of vertex removal. We characterize graphs for which the removal of any vertex changes the total domination number and find sharp upper and lower bounds on the total domination number of these graphs. We also characterize graphs for which the removal of an arbitrary vertex has no effect on the total domination number and we further show that they have no forbidden subgraphs. The contents of this chapter were published in Discrete Applied Mathematics [14]. In Chapters 6 and 7, we investigate the graph modification of edge lifting. In Chapter 6, we show that there are no trees for which every possible edge lift decreases the domination number, and we characterize trees for which every possible edge lift increases the domination number. The contents of Chapter 6 were published in the journal Quaestiones Mathematicae [17]. In Chapter 7, we show that there are no trees for which every possible edge lift decreases the total domination number and that there are no trees for which every possible edge lift leaves the total domination number unchanged. We characterize trees for which every possible edge lift increases the total domination number. At the time of the writing of this thesis, the contents of Chapter 7 have been published online in the Journal of Combinatorial Optimization [18] and will appear in print in a future issue.

D. Phil (Mathematics) / In this thesis we investigate generalized chromatic numbers in the context of hereditary graph properties. We also investigate the general topic of invariants of graphs as well as graph properties. In Chapter 1 we give relevant definitions and terminology pertaining to graph properties. In Chapter 2 we investigate generalized chromatic numbers of some well-known additive hereditary graph properties. This problem necessitates the investigation of reducible bounds. One of the results here is an improvement on a known upper bound for the path partition number of the property Wk. We also look at the generalized chromatic number of infinite graphs and hereby establish the connection between the generalized chromatic number of properties and infinite graphs. In Chapter 3 the analogous question of the generalized edge-chromatic number of some well-known additive hereditary properties is investigated. Similarly we find decomposable bounds and are also able to find generalized edge-chromatic numbers of properties using some well-known decomposable bounds. In Chapter 4 we investigate the more general topic of graph invariants and the role they play in chains of graph properties and then conversely the invariants that arise from chains of graph properties. Moreover we investigate the effects on monotonicity of the invariants versus heredity and additivity of graph properties. In Chapter 5 the general topic of invariants of graph properties defined in terms of the set of minimal forbidden subgraphs of the properties is studied. This enables us to investigate invariants so defined on binary operations between graph properties. In Chapter 6 the notion of natural and near-natural invariants are introduced and are also studied on binary operations of graph properties. The set of minimal forbidden subgraphs again plays a role in the definition of invariants here and this then leads us to study the completion number of a property.

D.Phil. / Domination in graphs is now well studied in graph theory and the literature on this subject has been surveyed and detailed in the two books by Haynes, Hedetniemi, and Slater [45, 46]. In this thesis, we continue the study of domination, by adding to the theory; improving a number of known bounds and solving two previously published conjectures. With the exception of the introduction, each chapter in this thesis corresponds to a single paper already published or submitted as a journal article. Despite the seeming disparity in the content of some of these articles, there are two overarching goals achieved in this thesis. The rst is an attempt to partition the vertex set of a graph into two sets, each holding a speci c domination-type property. The second is simply to improve known bounds for various domination parameters. In particular, an edge weighting function is presented which has been useful in providing some of these bounds. Although the research began as two separate areas of focus, there has been a fair degree of overlap and a number of the results contained in this thesis bridge the gap quite pleasingly. Specially, Chapter 11 uses the edge weighting function to prove a bound on one of the sets in our most fundamental partitions, while the improvement on a known bound presented in Chapter 7 was inspired by considering the possible existence of another partition. This latter proof relies implicitly on the `almost' existence of such a partition.

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The concept of a tree has been used in various areas of mathematics for over a century. In particular, trees appear to be one of the most fundamental notions in computer science. Sequential algorithms for trees are generally well studied. Unfortunately many of these sequential algorithms use methods which seem to be inherently sequential. One of the contributions of this thesis is the introduction of several parallel techniques for the construction of various types of trees and the presentation of new parallel tree construction algorithms using these methods. Along with the parallel tree construction techniques presented here, we develop techniques which have broader applications. We use the Parallel Random Access Machine as our model of computation. We consider two basic methods of constructing trees:tree expansion and tree synthesis. In the tree expansion method, we start with a single vertex and construct a tree by adding nodes of degree one and/or by subdividing edges. We use the parallel tree expansion technique to construct the tree representation for graphs in the family of graphs known as cographs. In the tree synthesis method, we start with a forest of single node subtrees and construct a tree by adding edges or (for rooted trees) by creating parent nodes for some roots of the trees in the forest. We present a family of parallel and sequential algorithms to construct various approximations to the Huffman tree. All these algorithms apply the tree synthesis method by constructing a tree in a level-by-level fashion. To support one of the algorithms in the family we develop a technique which we call the cascading sampling technique. One might suspect that the parallel tree synthesis method can be applied only to trees of polylogarithmic height, but this is not the case.We present a technique which we call the valley filling technique and develop its accelerated version called the accelerated valley filling technique. We present an application of this technique to an optimal parallel algorithm for construction of minimax trees. / Science, Faculty of / Computer Science, Department of / Graduate

An acyclic orientation of a graph is an assignment of a direction to each edge in a way that does not form any directed cycles. Acyclic orientations of a complete bipartite graph are in bijection with a class of matrices called lonesum matrices, which can be uniquely reconstructed from their row and column sums. We utilize this connection and other properties of lonesum matrices to determine an analytic form of the generating function for the length of the longest path in an acyclic orientation on a complete bipartite graph, and then study the distribution of the length of the longest path when the acyclic orientation is random. We use methods of analytic combinatorics, including analytic combinatorics in several variables (ACSV), to determine asymptotics for lonesum matrices and other related classes. / Includes bibliography. / Dissertation (PhD)--Florida Atlantic University, 2021. / FAU Electronic Theses and Dissertations Collection

Lattices of hereditary properties of nite graphs have been extensively studied. We investigate the lattice L of induced-hereditary properties of countable graphs. Of interest to us will be some of the members of L. Much of our focus will be on hom-properties. We analyze their behaviour and consider their link to solving the long standing Hedetniemi Conjecture. We then discuss universal graphs and construct a universal graph for hom-properties. We then use these universal graphs to prove a theorem by Szekeres and Wilf. Lastly we off er a new proof of a theorem by Du ffus, Sands and Woodrow. / Dissertation (MSc)--University of Pretoria, 2013. / Mathematics and Applied Mathematics / Unrestricted

In this dissertation several results in Steinhaus graphs are investigated. First under some further conditions imposed on the induced cycles in steinhaus graphs, the order of induced cycles in Steinhaus graphs is at most [(n+3)/2]. Next the results of maximum clique size in Steinhaus graphs are used to enumerate the Steinhaus graphs having maximal cliques. Finally the concept of jumbled graphs and Posa's Lemma are used to show that almost all Steinhaus graphs are Hamiltonian.

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