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Milicevic, Luka. "Topics in metric geometry, combinatorial geometry, extremal combinatorics and additive combinatorics." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/273375.
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Narayanan, Bhargav. "Problems in Ramsey theory, probabilistic combinatorics and extremal graph theory." Thesis, University of Cambridge, 2015. https://www.repository.cam.ac.uk/handle/1810/252850.
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Krohne, Edward. "Continuous Combinatorics of a Lattice Graph in the Cantor Space." Thesis, University of North Texas, 2016. https://digital.library.unt.edu/ark:/67531/metadc849680/.
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We present a novel theorem of Borel Combinatorics that sheds light on the types of continuous functions that can be defined on the Cantor space. We specifically consider the part X=F(2ᴳ) from the Cantor space, where the group G is the additive group of integer pairs ℤ². That is, X is the set of aperiodic {0,1} labelings of the two-dimensional infinite lattice graph. We give X the Bernoulli shift action, and this action induces a graph on X in which each connected component is again a two-dimensional lattice graph. It is folklore that no continuous (indeed, Borel) function provides a two-coloring of the graph on X, despite the fact that any finite subgraph of X is bipartite. Our main result offers a much more complete analysis of continuous functions on this space. We construct a countable collection of finite graphs, each consisting of twelve "tiles", such that for any property P (such as "two-coloring") that is locally recognizable in the proper sense, a continuous function with property P exists on X if and only if a function with a corresponding property P' exists on one of the graphs in the collection. We present the theorem, and give several applications.
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Weller, Kerstin B. "Connectivity and related properties for graph classes." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:667a139e-6d2c-4f67-8487-04c3a0136226.
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There has been much recent interest in random graphs sampled uniformly from the set of (labelled) graphs on n vertices in a suitably structured class A. An important and well-studied example of such a suitable structure is bridge-addability, introduced in 2005 by McDiarmid et al. in the course of studying random planar graphs. A class A is bridge-addable when the following holds: if we take any graph G in A and any pair u,v of vertices that are in different components in G, then the graph G′ obtained by adding the edge uv to G is also in A. It was shown that for a random graph sampled from a bridge-addable class, the probability that it is connected is always bounded away from 0, and the number of components is bounded above by a Poisson law. What happens if ’bridge-addable’ is replaced by something weaker? In this thesis, this question is explored in several different directions. After an introductory chapter and a chapter on generating function methods presenting standard techniques as well as some new technical results needed later, we look at minor-closed, labelled classes of graphs. The excluded minors are always assumed to be connected, which is equivalent to the class A being decomposable - a graph is in A if and only if every component of the graph is in A. When A is minor-closed, decomposable and bridge-addable various properties are known (McDiarmid 2010), generalizing results for planar graphs. A minor-closed class is decomposable and bridge-addable if and only if all excluded minors are 2-connected. Chapter 3 presents a series of examples where the excluded minors are not 2-connected, analysed using generating functions as well as techniques from graph theory. This is a step towards a classification of connectivity behaviour for minor-closed classes of graphs. In contrast to the bridge-addable case, different types of behaviours are observed. Chapter 4 deals with a new, more general vari- ant of bridge-addability related to edge-expander graphs. We will see that as long as we are allowed to introduce ’sufficiently many’ edges between components, the number of components of a random graph can still be bounded above by a Pois- son law. In this context, random forests in Kn,n are studied in detail. Chapter 5 takes a different approach, and studies the class of labelled forests where some vertices belong to a specified stable set. A weighting parameter y for the vertices belonging to the stable set is introduced, and a graph is sampled with probability proportional to y*s where s is the size of its stable set. The behaviour of this class is studied for y tending to ∞. Chapters 6 concerns random graphs sampled from general decomposable classes. We investigate the minimum size of a component, in both the labelled and the unlabelled case.
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Lin, Matthew. "Graph Cohomology." Scholarship @ Claremont, 2016. https://scholarship.claremont.edu/hmc_theses/82.
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What is the cohomology of a graph? Cohomology is a topological invariant and encodes such information as genus and euler characteristic. Graphs are combinatorial objects which may not a priori admit a natural and isomorphism invariant cohomology ring. In this project, given any finite graph G, we constructively define a cohomology ring H*(G) of G. Our method uses graph associahedra and toric varieties. Given a graph, there is a canonically associated convex polytope, called the graph associahedron, constructed from G. In turn, a convex polytope uniquely determines a toric variety. We synthesize these results, and describe the cohomology of the associated variety directly in terms of the graph G itself.
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Carroll, Christina C. "Enumerative combinatorics of posets." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/22659.
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Thesis (Ph. D.)--Mathematics, Georgia Institute of Technology, 2008.<br>Committee Chair: Tetali, Prasad; Committee Member: Duke, Richard; Committee Member: Heitsch, Christine; Committee Member: Randall, Dana; Committee Member: Trotter, William T.
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Ferra, Gomes de Almeida Girão António José. "Extremal and structural problems of graphs." Thesis, University of Cambridge, 2019. https://www.repository.cam.ac.uk/handle/1810/285427.
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In this dissertation, we are interested in studying several parameters of graphs and understanding their extreme values. We begin in Chapter~$2$ with a question on edge colouring. When can a partial proper edge colouring of a graph of maximum degree $\Delta$ be extended to a proper colouring of the entire graph using an `optimal' set of colours? Albertson and Moore conjectured this is always possible provided no two precoloured edges are within distance $2$. The main result of Chapter~$2$ comes close to proving this conjecture. Moreover, in Chapter~$3$, we completely answer the previous question for the class of planar graphs. Next, in Chapter~$4$, we investigate some Ramsey theoretical problems. We determine exactly what minimum degree a graph $G$ must have to guarantee that, for any two-colouring of $E(G)$, we can partition $V(G)$ into two parts where each part induces a connected monochromatic subgraph. This completely resolves a conjecture of Bal and Debiasio. We also prove a `covering' version of this result. Finally, we study another variant of these problems which deals with coverings of a graph by monochromatic components of distinct colours. The following saturation problem proposed by Barrus, Ferrara, Vandenbussche, and Wenger is considered in Chapter~$5$. Given a graph $H$ and a set of colours $\{1,2,\ldots,t\}$ (for some integer $t\geq |E(H)|$), we define $sat_{t}(n, R(H))$ to be the minimum number of $t$-coloured edges in a graph on $n$ vertices which does not contain a rainbow copy of $H$ but the addition of any non-edge in any colour from $\{1,2,\ldots,t\}$ creates such a copy. We prove several results concerning these extremal numbers. In particular, we determine the correct order of $sat_{t}(n, R(H))$, as a function of $n$, for every connected graph $H$ of minimum degree greater than $1$ and for every integer $t\geq e(H)$. In Chapter~$6$, we consider the following question: under what conditions does a Hamiltonian graph on $n$ vertices possess a second cycle of length at least $n-o(n)$? We prove that the `weak' assumption of a minimum degree greater or equal to $3$ guarantees the existence of such a long cycle. We solve two problems related to majority colouring in Chapter~$7$. This topic was recently studied by Kreutzer, Oum, Seymour, van der Zypen and Wood. They raised the problem of determining, for a natural number $k$, the smallest positive integer $m = m(k)$ such that every digraph can be coloured with $m$ colours, where each vertex has the same colour as at most a proportion of $\frac{1}{k}$ of its out-neighbours. Our main theorem states that $m(k) \in \{2k-1, 2k\}$. We study the following problem, raised by Caro and Yuster, in Chapter~$8$. Does every graph $G$ contain a `large' induced subgraph $H$ which has $k$ vertices of degree exactly $\Delta(H)$? We answer in the affirmative an approximate version of this question. Indeed, we prove that, for every $k$, there exists $g(k)$ such that any $n$ vertex graph $G$ with maximum degree $\Delta$ contains an induced subgraph $H$ with at least $n-g(k)\sqrt{\Delta}$ vertices such that $V(H)$ contains at least $k$ vertices of the same degree $d \ge \Delta(H)-g(k)$. This result is sharp up to the order of $g(k)$. %Subsequently, we investigate a concept called $\textit{path-pairability}$. A graph is said to be path-pairable if for any pairing of its vertices there exist a collection of edge-disjoint paths routing the the vertices of each pair. A question we are concerned here asks whether every planar path pairable graph on $n$ vertices must possess a vertex of degree linear in $n$. Indeed, we answer this question in the affirmative. We also sketch a proof resolving an analogous question for graphs embeddable on surfaces of bounded genus. Finally, in Chapter~$9$, we move on to examine $k$-linked tournaments. A tournament $T$ is said to be $k$-linked if for any two disjoint sets of vertices $\{x_1,\ldots ,x_k\}$ and $\{y_1,\dots,y_k\}$ there are directed vertex disjoint paths $P_1,\dots, P_k$ such that $P_i$ joins $x_i$ to $y_i$ for $i = 1,\ldots, k$. We prove that any $4k$ strongly-connected tournament with sufficiently large minimum out-degree is $k$-linked. This result comes close to proving a conjecture of Pokrovskiy.
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Noel, Jonathan A. "Extremal combinatorics, graph limits and computational complexity." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:8743ff27-b5e9-403a-a52a-3d6299792c7b.
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This thesis is primarily focused on problems in extremal combinatorics, although we will also consider some questions of analytic and algorithmic nature. The d-dimensional hypercube is the graph with vertex set {0,1}<sup>d</sup> where two vertices are adjacent if they differ in exactly one coordinate. In Chapter 2 we obtain an upper bound on the 'saturation number' of Q<sub>m</sub> in Q<sub>d</sub>. Specifically, we show that for m ≥ 2 fixed and d large there exists a subgraph G of Q<sub>d</sub> of bounded average degree such that G does not contain a copy of Q<sub>m</sub> but, for every G' such that G ⊊ G' ⊆ Q<sub>d</sub>, the graph G' contains a copy of Q<sub>m</sub>. This result answers a question of Johnson and Pinto and is best possible up to a factor of O(m). In Chapter 3, we show that there exists ε > 0 such that for all k and for n sufficiently large there is a collection of at most 2<sup>(1-ε)k</sup> subsets of [n] which does not contain a chain of length k+1 under inclusion and is maximal subject to this property. This disproves a conjecture of Gerbner, Keszegh, Lemons, Palmer, Pálvölgyi and Patkós. We also prove that there exists a constant c ∈ (0,1) such that the smallest such collection is of cardinality 2<sup>(1+o(1))<sup>ck</sup> </sup> for all k. In Chapter 4, we obtain an exact expression for the 'weak saturation number' of Q<sub>m</sub> in Q<sub>d</sub>. That is, we determine the minimum number of edges in a spanning subgraph G of Q<sub>d</sub> such that the edges of E(Q<sub>d</sub>)\E(G) can be added to G, one edge at a time, such that each new edge completes a copy of Q<sub>m</sub>. This answers another question of Johnson and Pinto. We also obtain a more general result for the weak saturation of 'axis aligned' copies of a multidimensional grid in a larger grid. In the r-neighbour bootstrap process, one begins with a set A<sub>0</sub> of 'infected' vertices in a graph G and, at each step, a 'healthy' vertex becomes infected if it has at least r infected neighbours. If every vertex of G is eventually infected, then we say that A<sub>0</sub> percolates. In Chapter 5, we apply ideas from weak saturation to prove that, for fixed r ≥ 2, every percolating set in Q<sub>d</sub> has cardinality at least (1+o(1))(d choose r-1)/r. This confirms a conjecture of Balogh and Bollobás and is asymptotically best possible. In addition, we determine the minimum cardinality exactly in the case r=3 (the minimum cardinality in the case r=2 was already known). In Chapter 6, we provide a framework for proving lower bounds on the number of comparable pairs in a subset S of a partially ordered set (poset) of prescribed size. We apply this framework to obtain an explicit bound of this type for the poset 𝒱(q,n) consisting of all subspaces of 𝔽<sub>q</sub><sup>n</sup>ordered by inclusion which is best possible when S is not too large. In Chapter 7, we apply the result from Chapter 6 along with the recently developed 'container method,' to obtain an upper bound on the number of antichains in 𝒱(q,n) and a bound on the size of the largest antichain in a p-random subset of 𝒱(q,n) which holds with high probability for p in a certain range. In Chapter 8, we construct a 'finitely forcible graphon' W for which there exists a sequence (ε<sub>i</sub>)<sup>∞</sup><sub>i=1</sub> tending to zero such that, for all i ≥ 1, every weak ε<sub>i</sub>-regular partition of W has at least exp(ε<sub>i</sub><sup>-2</sup>/2<sup>5log∗ε<sub>i</sub><sup>-2</sup></sup>) parts. This result shows that the structure of a finitely forcible graphon can be much more complex than was anticipated in a paper of Lovász and Szegedy. For positive integers p,q with p/q ❘≥ 2, a circular (p,q)-colouring of a graph G is a mapping V(G) → ℤ<sub>p</sub> such that any two adjacent vertices are mapped to elements of ℤ<sub>p</sub> at distance at least q from one another. The reconfiguration problem for circular colourings asks, given two (p,q)-colourings f and g of G, is it possible to transform f into g by recolouring one vertex at a time so that every intermediate mapping is a p,q-colouring? In Chapter 9, we show that this question can be answered in polynomial time for 2 ≤ p/q < 4 and is PSPACE-complete for p/q ≥ 4.
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Krzywkowski, Marcin Piotr. "Hat problem on a graph." Thesis, University of Exeter, 2012. http://hdl.handle.net/10036/4019.
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The topic of this thesis is the hat problem. In this problem, a team of n players enters a room, and a blue or red hat is randomly placed on the head of each player. Every player can see the hats of all of the other players but not his own. Then each player must simultaneously guess the color of his own hat or pass. The team wins if at least one player guesses his hat color correctly and no one guesses his hat color wrong, otherwise the team loses. The aim is to maximize the probability of winning. This thesis is based on publications, which form the second chapter. In the first chapter we give an overview of the published results. In Section 1.1 we introduce to the hat problem and the hat problem on a graph, where vertices correspond to players, and a player can see the adjacent players. To the hat problem on a graph we devote the next few sections. First, we give some fundamental theorems about the problem. Then we solve the hat problem on trees, cycles, and unicyclic graphs. Next we consider the hat problem on graphs with a universal vertex. We also investigate the problem on graphs with a neighborhood-dominated vertex. In addition, we consider the hat problem on disconnected graphs. Next we investigate the problem on graphs such that the only known information are degrees of vertices. We also present Nordhaus-Gaddum type inequalities for the hat problem on a graph. In Section 1.6 we investigate the hat problem on directed graphs. The topic of Section 1.7 is the generalized hat problem with q >= 2 colors. A modified hat problem is considered in Section 1.8. In this problem there are n >= 3 players and two colors. The players do not have to guess their hat colors simultaneously and we modify the way of making a guess. We give an optimal strategy for this problem which guarantees the win. Applications of the hat problem and its connections to different areas of science are presented in Section 1.9. We also give there a comprehensive list of variations of the hat problem considered in the literature.
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Dickson, James Odziemiec. "An Introduction to Ramsey Theory on Graphs." Thesis, Virginia Tech, 2011. http://hdl.handle.net/10919/32873.
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Ross, Bailey Ann. "Combinatorics and topology of curves and knots." [Boise, Idaho] : Boise State University, 2010. http://scholarworks.boisestate.edu/td/89/.
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Gruslys, Vytautas. "Tilings and other combinatorial results." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/271311.
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In this dissertation we treat three tiling problems and three problems in combinatorial geometry, extremal graph theory and sparse Ramsey theory. We first consider tilings of $\mathbb{Z}^n$. In this setting a tile $T$ is just a finite subset of $\mathbb{Z}^n$. We say that $T$ tiles $\mathbb{Z}^n$ if the latter set admits a partition into isometric copies of $T$. Chalcraft observed that there exist $T$ that do not tile $\mathbb{Z}^n$ but tile $\mathbb{Z}^{d}$ for some $d > n$. He conjectured that such $d$ exists for any given tile. We prove this conjecture in Chapter 2. In Chapter 3 we prove a conjecture of Lonc, stating that for any poset $P$ of size a power of $2$, if $P$ has a greatest and a least element, then there is a positive integer $k$ such that $[2]^k$ can be partitioned into copies of $P$. The third tiling problem is about vertex-partitions of the hypercube graph $Q_n$. Offner asked: if $G$ is a subgraph of $Q_n$ such $|G|$ is a power of $2$, must $V(Q_d)$, for some $d$, admit a partition into isomorphic copies of $G$? In Chapter 4 we answer this question in the affirmative. We follow up with a question in combinatorial geometry. A line in a planar set $P$ is a maximal collinear subset of $P$. P\'or and Wood considered colourings of finite $P$ without large lines with a bounded number of colours. In particular, they examined whether monochromatic lines always appear in such colourings provided that $|P|$ is large. They conjectured that for all $k,l \ge 2$ there exists an $n \ge 2$ such that if $|P| \ge n$ and $P$ does not contain a line of cardinality larger than $l$, then every colouring of $P$ with $k$ colours produces a monochromatic line. In Chapter 5 we construct arbitrarily large counterexamples for the case $k=l=3$. We follow up with a problem in extremal graph theory. For any graph, we say that a given edge is triangular if it forms a triangle with two other edges. How few triangular edges can there be in a graph with $n$ vertices and $m$ edges? For sufficiently large $n$ we prove a conjecture of F\"uredi and Maleki that gives an exact formula for this minimum. This proof is given in Chapter 6. Finally, Chapter 7 is concerned with degrees of vertices in directed hypergraphs. One way to prescribe an orientation to an $r$-uniform graph $H$ is to assign for each of its edges one of the $r!$ possible orderings of its elements. Then, for any $p$-set of vertices $A$ and any $p$-set of indices $I \subset [r]$, we define the $I$-degree of $A$ to be the number of edges containing vertices $A$ in precisely the positions labelled by $I$. Caro and Hansberg were interested in determining whether a given $r$-uniform hypergraph admits an orientation where every set of $p$ vertices has some $I$-degree equal to $0$. They conjectured that a certain Hall-type condition is sufficient. We show that this is true for $r$ large, but false in general.
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Gray, Aaron D. "Extremal Results for Peg Solitaire on Graphs." Digital Commons @ East Tennessee State University, 2013. https://dc.etsu.edu/etd/2274.
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In a 2011 paper by Beeler and Hoilman, the game of peg solitaire is generalized to arbitrary boards. These boards are treated as graphs in the combinatorial sense. An open problem from that paper is to determine the minimum number of edges necessary for a graph with a fixed number of vertices to be solvable. This thesis provides new bounds on this number. It also provides necessary and sufficient conditions for two families of graphs to be solvable, along with criticality results, and the maximum number of pegs that can be left in each of the two graph families.
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Hill, Alan. "Self-Dual Graphs." Thesis, University of Waterloo, 2002. http://hdl.handle.net/10012/1014.
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The study of self-duality has attracted some attention over the past decade. A good deal of research in that time has been done on constructing and classifying all self-dual graphs and in particular polyhedra. We will give an overview of the recent research in the first two chapters. In the third chapter, we will show the necessary condition that a self-complementary self-dual graph have <i>n</i> ≡ 0, 1 (mod 8) vertices and we will review White's infinite class (the Paley graphs, for which <i>n</i> ≡ 1 (mod 8)). Finally, we will construct a new infinite class of self-complementary self-dual graphs for which <i>n</i> ≡ 0 (mod 8).
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Durig, Rebekah Libby. "The Game of Light: A Graph Theoretical Approach." OpenSIUC, 2017. https://opensiuc.lib.siu.edu/theses/2199.
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In the Game of Light, as formulated in "Harmonic Evolutions" (J. Kocik, 2007), there is a definition of dynamic graphs and a thorough explanation of how to find the structure of the digraph that shows the changing of states during the game. This thesis furthers this research in two directions: first, by exploring what happens when there are more than two vertex states, by expanding the state space to any cyclic group. Secondly, the research attempted to identify families of graphs and describe their graph states using only the number of vertex states. To further both of these goals, two programs were written, one as a calculator to compute the digraph structure, and one as a visualization tool that automates the game of light, allowing users to input graphs with simple point and click commands, and to easily see how graphs evolve. Finally, about one hundred graphs were evaluated using the calculator, and the resulting structures are recorded.
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Walvoort, Clayton A. "Peg Solitaire on Trees with Diameter Four." Digital Commons @ East Tennessee State University, 2013. https://dc.etsu.edu/etd/1113.
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In a paper by Beeler and Hoilman, the traditional game of peg solitaire is generalized to graphs in the combinatorial sense. One of the important open problems in this paper was to classify solvable trees. In this thesis, we will give necessary and sufficient conditions for the solvability for all trees with diameter four. We also give the maximum number of pegs that can be left on such a graph under the restriction that we jump whenever possible.
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Warnke, Lutz. "Random graph processes with dependencies." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:71b48e5f-a192-4684-a864-ea9059a25d74.
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Random graph processes are basic mathematical models for large-scale networks evolving over time. Their systematic study was pioneered by Erdös and Rényi around 1960, and one key feature of many 'classical' models is that the edges appear independently. While this makes them amenable to a rigorous analysis, it is desirable, both mathematically and in terms of applications, to understand more complicated situations. In this thesis the main goal is to improve our rigorous understanding of evolving random graphs with significant dependencies. The first model we consider is known as an Achlioptas process: in each step two random edges are chosen, and using a given rule only one of them is selected and added to the evolving graph. Since 2000 a large class of 'complex' rules has eluded a rigorous analysis, and it was widely believed that these could give rise to a striking and unusual phenomenon. Making this explicit, Achlioptas, D'Souza and Spencer conjectured in Science that one such rule yields a very abrupt (discontinuous) percolation phase transition. We disprove this, showing that the transition is in fact continuous for all Achlioptas process. In addition, we give the first rigorous analysis of the more 'complex' rules, proving that certain key statistics are tightly concentrated (i) in the subcritical evolution, and (ii) also later on if an associated system of differential equations has a unique solution. The second model we study is the H-free process, where random edges are added subject to the constraint that they do not complete a copy of some fixed graph H. The most important open question for such 'constrained' processes is due to Erdös, Suen and Winkler: in 1995 they asked what the typical final number of edges is. While Osthus and Taraz answered this in 2000 up to logarithmic factors for a large class of graphs H, more precise bounds are only known for a few special graphs. We close this gap for the cases where a cycle of fixed length is forbidden, determining the final number of edges up to constants. Our result not only establishes several conjectures, it is also the first which answers the more than 15-year old question of Erdös et. al. for a class of forbidden graphs H.
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Green, Hannah E. "Differentiating Between a Protein and its Decoy Using Nested Graph Models and Weighted Graph Theoretical Invariants." Digital Commons @ East Tennessee State University, 2017. https://dc.etsu.edu/etd/3248.
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To determine the function of a protein, we must know its 3-dimensional structure, which can be difficult to ascertain. Currently, predictive models are used to determine the structure of a protein from its sequence, but these models do not always predict the correct structure. To this end we use a nested graph model along with weighted invariants to minimize the errors and improve the accuracy of a predictive model to determine if we have the correct structure for a protein.
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Pensaert, William. "Hamilton Paths in Generalized Petersen Graphs." Thesis, University of Waterloo, 2002. http://hdl.handle.net/10012/1198.
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This thesis puts forward the conjecture that for <i>n</i> > 3<i>k</i> with <i>k</i> > 2, the generalized Petersen graph, <i>GP</i>(<i>n,k</i>) is Hamilton-laceable if <i>n</i> is even and <i>k</i> is odd, and it is Hamilton-connected otherwise. We take the first step in the proof of this conjecture by proving the case <i>n</i> = 3<i>k</i> + 1 and <i>k</i> greater than or equal to 1. We do this mainly by means of an induction which takes us from <i>GP</i>(3<i>k</i> + 1, <i>k</i>) to <i>GP</i>(3(<i>k</i> + 2) + 1, <i>k</i> + 2). The induction takes the form of mapping a Hamilton path in the smaller graph piecewise to the larger graph an inserting subpaths we call <i>rotors</i> to obtain a Hamilton path in the larger graph.
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Meyer, Marie. "Polytopes Associated to Graph Laplacians." UKnowledge, 2018. https://uknowledge.uky.edu/math_etds/54.
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Graphs provide interesting ways to generate families of lattice polytopes. In particular, one can use matrices encoding the information of a finite graph to define vertices of a polytope. This dissertation initiates the study of the Laplacian simplex, PG, obtained from a finite graph G by taking the convex hull of the columns of the Laplacian matrix for G. The Laplacian simplex is extended through the use of a parallel construction with a finite digraph D to obtain the Laplacian polytope, PD.
Basic properties of both families of simplices, PG and PD, are established using techniques from Ehrhart theory. Motivated by a well-known conjecture in the field, our investigation focuses on reflexivity, the integer decomposition property, and unimodality of Ehrhart h*-vectors of these polytopes. A systematic investigation of PG for trees, cycles, and complete graphs is provided, which is enhanced by an investigation of PD for cyclic digraphs. We form intriguing connections with other families of simplices and produce G and D such that the h*-vectors of PG and PD exhibit extremal behavior.
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Aftene, Florin. "Vertex-Relaxed Graceful Labelings of Graphs and Congruences." TopSCHOLAR®, 2018. https://digitalcommons.wku.edu/theses/2664.
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A labeling of a graph is an assignment of a natural number to each vertex
of a graph. Graceful labelings are very important types of labelings. The study of graceful labelings is very difficult and little has been shown about such labelings. Vertex-relaxed graceful labelings of graphs are a class of labelings that include graceful labelings, and their study gives an approach to the study of graceful labelings. In this thesis we generalize the congruence approach of Rosa to obtain new criteria for vertex-relaxed graceful labelings of graphs. To do this, we generalize Faulhaber’s Formula, which is a famous result about sums of powers of integers.
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Okoth, Isaac Owino. "Combinatorics of oriented trees and tree-like structures." Thesis, Stellenbosch : Stellenbosch University, 2015. http://hdl.handle.net/10019.1/96860.
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Thesis (PhD)--Stellenbosch University, 2015.<br>ENGLISH ABSTRACT : In this thesis, a number of combinatorial objects are enumerated. Du and
Yin as well as Shin and Zeng (by a different approach) proved an elegant
formula for the number of labelled trees with respect to a given in degree
sequence, where each edge is oriented from a vertex of lower label towards
a vertex of higher label. We refine their result to also take the number of
sources (vertices of in degree 0) or sinks (vertices of out degree 0) into account.
We find formulas for the mean and variance of the number of sinks
or sources in these trees. We also obtain a differential equation and a functional
equation satisfied by the generating function for these trees. Analogous
results for labelled trees with two marked vertices, related to functional
digraphs, are also established.
We extend the work to count reachable vertices, sinks and leaf sinks in
these trees. Among other results, we obtain a counting formula for the number
of labelled trees on n vertices in which exactly k vertices are reachable
from a given vertex v and also the average number of vertices that are reachable
from a specified vertex in labelled trees of order n.
In this dissertation, we also enumerate certain families of set partitions
and related tree-like structures. We provide a proof for a formula that counts
connected cycle-free families of k set partitions of {1, . . . , n} satisfying a certain
coherence condition and then establish a bijection between these families and the set of labelled free k-ary cacti with a given vertex-degree distribution.
We then show that the formula also counts coloured Husimi graphs
in which there are no blocks of the same colour that are incident to one another.
We extend the work to count coloured oriented cacti and coloured
cacti.
Noncrossing trees and related tree-like structures are also considered in
this thesis. Specifically, we establish formulas for locally oriented noncrossing
trees with a given number of sources and sinks, and also with given
indegree and outdegree sequences. The work is extended to obtain the average
number of reachable vertices in these trees. We then generalise the
concept of noncrossing trees to find formulas for the number of noncrossing
Husimi graphs, cacti and oriented cacti. The study is further extended
to find formulas for the number of bicoloured noncrossing Husimi graphs
and the number of noncrossing connected cycle-free pairs of set partitions.<br>AFRIKAANSE OPSOMMING : In hierdie tesis word ’n aantal kombinatoriese objekte geenumereer. Du
en Yin asook Shin en Zeng (deur middel van ’n ander benadering) het ’n
elegante formule vir die aantal geëtiketteerde bome met betrekking tot ’n
gegewe ingangsgraadry, waar elke lyn van die nodus met die kleiner etiket
na die nodus met die groter etiket toe georiënteer word. Ons verfyn hul
resultaat deur ook die aantal bronne (nodusse met ingangsgraad 0) en putte
(nodusse met uitgangsgraad 0) in ag te neem. Ons vind formules vir die gemiddelde
en variansie van die aantal putte of bronne in hierdie bome. Ons
bepaal verder ’n differensiaalvergelyking en ’n funksionaalvergelyking wat
deur die voortbringende funksie van hierdie bome bevredig word. Analoë
resultate vir geëtiketteerde bome met twee gemerkte nodusse (wat verwant
is aan funksionele digrafieke), is ook gevind.
Ons gaan verder voort deur ook bereikbare nodusse, bronne en putte in
hierdie bome at te tel. Onder andere verkry ons ’n formule vir die aantal geëtiketteerde
bome met n nodusse waarin presies k nodusse vanaf ’n gegewe
nodus v bereikbaar is asook die gemiddelde aantal nodusse wat bereikbaar
is vanaf ’n gegewe nodus.
Ons enumereer in hierdie tesis verder sekere families van versamelingsverdelings
en soortgelyke boom-vormige strukture. Ons gee ’n bewys vir ’n
formule wat die aantal van samehangende siklus-vrye families van k versamelingsverdelings
op {1, . . . , n} wat ’n sekere koherensie-vereiste bevredig,
en ons beskryf ’n bijeksie tussen hierdie familie en die versameling van
geëtiketteerde vrye k-êre kaktusse met ’n gegewe nodus-graad-verdeling.
Ons toon ook dat hierdie formule ook gekleurde Husimi-grafieke tel waar
blokke van dieselfde kleur nie insident met mekaar mag wees nie. Ons tel
verder ook gekleurde georiënteerde kaktusse en gekleurde kaktusse.
Nie-kruisende bome en soortgelyke boom-vormige strukture word in
hierdie tesis ook beskou. On bepaal spesifiek formules vir lokaal georiënteerde
nie-kruisende bome wat ’n gegewe aantal bronne en putte het asook
nie-kruisende bome met gegewe ingangs- en uitgangsgraadrye. Ons
gaan voort deur die gemiddelde aantal bereikbare nodusse in hierdie bome
te bepaal. Ons veralgemeen dan die konsep van nie-kruisende bome en
vind formules vir die aantal nie-kruisende Husimi-grafieke, kaktusse en
georiënteerde kaktusse. Laastens vind ons ’n formule vir die aantaal tweegekleurde
nie-kruisende Husimi-grafieke en die aantal nie-kruisende samehangende
siklus-vrye pare van versamelingsverdelings.
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Butler, Steven Kay. "Bounding the Number of Graphs Containing Very Long Induced Paths." Diss., CLICK HERE for online access, 2003. http://contentdm.lib.byu.edu/ETD/image/etd158.pdf.
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Gardner, Bradley. "Italian Domination on Ladders and Related Products." Digital Commons @ East Tennessee State University, 2018. https://dc.etsu.edu/etd/3509.
Full textAbstract:
An Italian dominating function on a graph $G = (V,E)$ is a function such that $f : V \to \{0,1,2\}$, and for each vertex $v \in V$ for which $f(v) = 0$, we have $\sum_{u\in N(v)}f(u) \geq 2$. The weight of an Italian dominating function is $f(V) = \sum_{v\in V(G)}f(v)$. The minimum weight of all such functions on a graph $G$ is called the Italian domination number of $G$. In this thesis, we will consider Italian domination in various types of products of a graph $G$ with the complete graph $K_2$. We will find the value of the Italian domination number for ladders, specific families of prisms, mobius ladders and related products including categorical products $G\times K_2$ and lexicographic products $G\cdot K_2$. Finally, we will conclude with open problems.
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Mayhew, Dillon. "Matroids and complexity." Thesis, University of Oxford, 2005. http://ora.ox.ac.uk/objects/uuid:23640923-17c3-4ad8-9845-320e3b662910.
Full textAbstract:
We consider different ways of describing a matroid to a Turing machine by listing the members of various families of subsets, and we construct an order on these different methods of description. We show that, under this scheme, several natural matroid problems are complete in classes thought not to be equal to P. We list various results linking parameters of basis graphs to parameters of their associated matroids. For small values of k we determine which matroids have the clique number, chromatic number, or maximum degree of their basis graphs bounded above by k. If P is a class of graphs that is closed under isomorphism and induced subgraphs, then the set of matroids whose basis graphs belong to P is closed under minors. We characterise the minor-closed classes that arise in this way, and exhibit several examples. One way of choosing a basis of a matroid at random is to select a total ordering of the ground set uniformly at random and use the greedy algorithm. We consider the class of matroids having the property that this procedure chooses a basis uniformly at random. Finally we consider a problem mentioned by Oxley. He asked if, for every two elements and n - 2 cocircuits in an n-connected matroid, there is a circuit that contains both elements and that meets every cocircuit. We show that a slightly stronger property holds for regular matroids.
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Murphy, Kyle. "On t-Restricted Optimal Rubbling of Graphs." Digital Commons @ East Tennessee State University, 2017. https://dc.etsu.edu/etd/3251.
Full textAbstract:
For a graph G = (V;E), a pebble distribution is defined as a mapping of the vertex set in to the integers, where each vertex begins with f(v) pebbles. A pebbling move takes two pebbles from some vertex adjacent to v and places one pebble on v. A rubbling move takes one pebble from each of two vertices that are adjacent to v and places one pebble on v. A vertex x is reachable under a pebbling distribution f if there exists some sequence of rubbling and pebbling moves that places a pebble on x. A pebbling distribution where every vertex is reachable is called a rubbling configuration. The t-restricted optimal rubbling number of G is the minimum number of pebbles required for a rubbling configuration where no vertex is initially assigned more than t pebbles. Here we present results on the 1-restricted optimal rubbling number and the 2- restricted optimal rubbling number.
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Meeks, Kitty M. F. T. "Graph colourings and games." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:a805a379-f891-4250-9a7d-df109f9f52e2.
Full textAbstract:
Graph colourings and combinatorial games are two very widely studied topics in discrete mathematics. This thesis addresses the computational complexity of a range of problems falling within one or both of these subjects. Much of the thesis is concerned with the computational complexity of problems related to the combinatorial game (Free-)Flood-It, in which players aim to make a coloured graph monochromatic ("flood" the graph) with the minimum possible number of flooding operations; such problems are known to be computationally hard in many cases. We begin by proving some general structural results about the behaviour of the game, including a powerful characterisation of the number of moves required to flood a graph in terms of the number of moves required to flood its spanning trees; these structural results are then applied to prove tractability results about a number of flood-filling problems. We also consider the computational complexity of flood-filling problems when the game is played on a rectangular grid of fixed height (focussing in particular on 3xn and 2xn grids), answering an open question of Clifford, Jalsenius, Montanaro and Sach. The final chapter concerns the parameterised complexity of list problems on graphs of bounded treewidth. We prove structural results determining the list edge chromatic number and list total chromatic number of graphs with bounded treewidth and large maximum degree, which are special cases of the List (Edge) Colouring Conjecture and Total Colouring Conjecture respectively. Using these results, we show that the problem of determining either of these quantities is fixed parameter tractable, parameterised by the treewidth of the input graph. Finally, we analyse a list version of the Hamilton Path problem, and prove it to be W[1]-hard when parameterised by the pathwidth of the input graph. These results answer two open questions of Fellows, Fomin, Lokshtanov, Rosamond, Saurabh, Szeider and Thomassen.
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Mattern, Amelia. "Ordering and Reordering: Using Heffter Arrays to Biembed Complete Graphs." ScholarWorks @ UVM, 2015. http://scholarworks.uvm.edu/graddis/341.
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In this paper we extend the study of Heffter arrays and the biembedding of graphs on orientable surfaces first discussed by Archdeacon in 2014. We begin with the definitions of Heffter systems, Heffter arrays, and their relationship to orientable biembeddings through current graphs. We then focus on two specific cases. We first prove the existence of embeddings for every K_(6n+1) with every edge on a face of size 3 and a face of size n. We next present partial results for biembedding K_(10n+1) with every edge on a face of size 5 and a face of size n. Finally, we address the more general question of ordering subsets of Z_n take away {0}. We conclude with some open conjectures and further explorations.
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Riddle, Marcia Ling. "Sandwich Theorem and Calculation of the Theta Function for Several Graphs." Diss., CLICK HERE for online access, 2003. http://contentdm.lib.byu.edu/ETD/image/etd181.pdf.
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Whalen, Peter. "Pfaffian orientations, flat embeddings, and Steinberg's conjecture." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/52207.
Full textAbstract:
The first result of this thesis is a partial result in the direction of Steinberg's Conjecture. Steinberg's Conjecture states that any planar graph without cycles of length four or five is three colorable. Borodin, Glebov, Montassier, and Raspaud showed that planar graphs without cycles of length four, five, or seven are three colorable and Borodin and Glebov showed that planar graphs without five cycles or triangles at distance at most two apart are three colorable. We prove a statement that implies the first of these theorems and is incomparable with the second: that any planar graph with no cycles of length four through six or cycles of length seven with incident triangles distance exactly two apart are three colorable.
The third and fourth chapters of this thesis are concerned with the study of Pfaffian orientations. A theorem proved by William McCuaig and, independently, Neil Robertson, Paul Seymour, and Robin Thomas provides a good characterization for whether or not a bipartite graph has a Pfaffian orientation as well as a polynomial time algorithm for that problem. We reprove this characterization and provide a new algorithm for this problem. In Chapter 3, we generalize a preliminary result needed to reprove this theorem. Specifically, we show that any internally 4-connected, non-planar bipartite graph contains a subdivision of K3,3 in which each path has odd length. In Chapter 4, we make use of this result to provide a much shorter proof using elementary methods of this characterization.
In the fourth and fifth chapters we investigate flat embeddings. A piecewise-linear embedding of a graph in 3-space is flat if every cycle of the graph bounds a disk disjoint from the rest of the graph. We provide a structural theorem for flat embeddings that indicates how to build them from small pieces in Chapter 5. In Chapter 6, we present a class of flat graphs that are highly non-planar in the sense that, for any fixed k, there are an infinite number of members of the class such that deleting k vertices leaves the graph non-planar.
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McCall, Kevin J. "3-Maps And Their Generalizations." VCU Scholars Compass, 2018. https://scholarscompass.vcu.edu/etd/5581.
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A 3-map is a 3-region colorable map. They have been studied by Craft and White in their paper 3-maps. This thesis introduces topological graph theory and then investigates 3-maps in detail, including examples, special types of 3-maps, the use of 3-maps to find the genus of special graphs, and a generalization known as n-maps.
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Meadows, Adam M. "Decompositions of Mixed Graphs with Partial Orientations of the P4." Digital Commons @ East Tennessee State University, 2009. https://dc.etsu.edu/etd/1870.
Full textAbstract:
A decomposition D of a graph H by a graph G is a partition of the edge set of H such that the subgraph induced by the edges in each part of the partition is isomorphic to G. A mixed graph on V vertices is an ordered pair (V,C), where V is a set of vertices, |V| = v, and C is a set of ordered and unordered pairs, denoted (x, y) and [x, y] respectively, of elements of V [8]. An ordered pair (x, y) ∈ C is called an arc of (V,C) and an unordered pair [x, y] ∈ C is called an edge of graph (V,C). A path on n vertices is denoted as Pn. A partial orientation on G is obtained by replacing each edge [x, y] ∈ E(G) with either (x, y), (y, x), or [x, y] in such a way that there are twice as many arcs as edges. The complete mixed graph on v vertices, denoted Mv, is the mixed graph (V,C) where for every pair of distinct vertices v1, v2 ∈ V , we have {(v1, v2), (v2, v1), [v1, v2]} ⊂ C. The goal of this thesis is to establish necessary and sufficient conditions for decomposition of Mv by all possible partial orientations of P4.
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Yang, Joyce C. "Interval Graphs." Scholarship @ Claremont, 2016. https://scholarship.claremont.edu/hmc_theses/83.
Full textAbstract:
We examine the problem of counting interval graphs. We answer the question posed by Hanlon, of whether the formal power series generating function of the number of interval graphs on n vertices has a positive radius of convergence. We have found that it is zero. We have obtained a lower bound and an upper bound on the number of interval graphs on n vertices. We also study the application of interval graphs to the dynamic storage allocation problem. Dynamic storage allocation has been shown to be NP-complete by Stockmeyer. Coloring interval graphs on-line has applications to dynamic storage allocation. The most colors used by Kierstead's algorithm is 3 ω -2, where ω is the size of the largest clique in the graph. We determine a lower bound on the colors used. One such lower bound is 2 ω -1.
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Scott, Hamilton. "Zero Sets in Graphs." Digital Commons @ East Tennessee State University, 2010. https://dc.etsu.edu/etd/1705.
Full textAbstract:
Let S ⊆ V be an arbitrary subset of vertices of a graph G = (V,E). The differential ∂(S) equals the difference between the cardinality of the set of vertices not in S but adjacent to vertices in S, and the cardinality of the set S. The differential of a graph G equals the maximum differential of any subset S of V . A set S is called a zero set if ∂(S) = 0. In this thesis we introduce the study of zero sets in graphs. We give proofs of the existence of zero sets in various kinds of graphs such as even order graphs, bipartite graphs, and graphs of maximum degree 3. We also give proofs regarding the existence of graphs which contain no zero sets and the construction of zero-free graphs from graphs which contain zero sets.
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Holt, Tracy Lance. "On the Attainability of Upper Bounds for the Circular Chromatic Number of K4-Minor-Free Graphs." Digital Commons @ East Tennessee State University, 2008. https://dc.etsu.edu/etd/1916.
Full textAbstract:
Let G be a graph. For k ≥ d ≥ 1, a k/d -coloring of G is a coloring c of vertices of G with colors 0, 1, 2, . . ., k - 1, such that d ≤ | c(x) - c(y) | ≤ k - d, whenever xy is an edge of G. We say that the circular chromatic number of G, denoted χc(G), is equal to the smallest k/d where a k/d -coloring exists. In [6], Pan and Zhu have given a function μ(g) that gives an upper bound for the circular-chromatic number for every K4-minor-free graph Gg of odd girth at least g, g ≥ 3. In [7], they have shown that their upper bound in [6] can not be improved by constructing a sequence of graphs approaching μ(g) asymptotically. We prove that for every odd integer g = 2k + 1, there exists a graph Gg ∈ G/K4 of odd girth g such that χc(Gg) = μ(g) if and only if k is not divisible by 3. In other words, for any odd g, the question of attainability of μ(g) is answered for all g by our results. Furthermore, the proofs [6] and [7] are long and tedious. We give simpler proofs for both of their results.
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Moore, Christian G. "Global Supply Sets in Graphs." Digital Commons @ East Tennessee State University, 2016. https://dc.etsu.edu/etd/3025.
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For a graph G=(V,E), a set S⊆V is a global supply set if every vertex v∈V\S has at least one neighbor, say u, in S such that u has at least as many neighbors in S as v has in V \S. The global supply number is the minimum cardinality of a global supply set, denoted γgs (G). We introduce global supply sets and determine the global supply number for selected families of graphs. Also, we give bounds on the global supply number for general graphs, trees, and grid graphs.
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Djang, Claire. "Two-Coloring Cycles In Complete Graphs." Oberlin College Honors Theses / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=oberlin1370618319.
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Ball, Cory BH. "The Apprentices' Tower of Hanoi." Digital Commons @ East Tennessee State University, 2015. https://dc.etsu.edu/etd/2512.
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The Apprentices' Tower of Hanoi is introduced in this thesis. Several bounds are found in regards to optimal algorithms which solve the puzzle. Graph theoretic properties of the associated state graphs are explored. A brief summary of other Tower of Hanoi variants is also presented.
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Rockney, Alissa Ann. "A Predictive Model Which Uses Descriptors of RNA Secondary Structures Derived from Graph Theory." Digital Commons @ East Tennessee State University, 2011. https://dc.etsu.edu/etd/1300.
Full textAbstract:
The secondary structures of ribonucleic acid (RNA) have been successfully modeled with graph-theoretic structures. Often, simple graphs are used to represent secondary RNA structures; however, in this research, a multigraph representation of RNA is used, in which vertices represent stems and edges represent the internal motifs. Any type of RNA secondary structure may be represented by a graph in this manner. We define novel graphical invariants to quantify the multigraphs and obtain characteristic descriptors of the secondary structures. These descriptors are used to train an artificial neural network (ANN) to recognize the characteristics of secondary RNA structure. Using the ANN, we classify the multigraphs as either RNA-like or not RNA-like. This classification method produced results similar to other classification methods. Given the expanding library of secondary RNA motifs, this method may provide a tool to help identify new structures and to guide the rational design of RNA molecules.
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Glover, Cory. "The Non-Backtracking Spectrum of a Graph and Non-Bactracking PageRank." BYU ScholarsArchive, 2021. https://scholarsarchive.byu.edu/etd/9194.
Full textAbstract:
This thesis studies two problems centered around non-backtracking walks on graphs. First, we analyze the spectrum of the non-backtracking matrix of a graph. We show how to obtain the eigenvectors of the non-backtracking matrix using a smaller matrix and in doing so, create a block diagonal decomposition which more clearly expresses the non-backtracking matrix eigenvalues. Additionally, we develop upper and lower bounds on the matrix spectrum and use the spectrum to investigate properties of the graph. Second, we investigate the difference between PageRank and non-backtracking PageRank. We show some instances where there is no difference and develop an algorithm to compare PageRank and non-backtracking PageRank under certain conditions using $\mu$-PageRank.
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Koessler, Denise Renee. "A Predictive Model for Secondary RNA Structure Using Graph Theory and a Neural Network." Digital Commons @ East Tennessee State University, 2010. https://dc.etsu.edu/etd/1684.
Full textAbstract:
In this work we use a graph-theoretic representation of secondary RNA structure found in the database RAG: RNA-As-Graphs. We model the bonding of two RNA secondary structures to form a larger structure with a graph operation called merge. The resulting data from each tree merge operation is summarized and represented by a vector. We use these vectors as input values for a neural network and train the network to recognize a tree as RNA-like or not based on the merge data vector.
The network correctly assigned a high probability of RNA-likeness to trees identified as RNA-like in the RAG database, and a low probability of RNA-likeness to those classified as not RNA-like in the RAG database. We then used the neural network to predict the RNA-likeness of all the trees of order 9. The use of a graph operation to theoretically describe the bonding of secondary RNA is novel.
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Deren, Michael. "Universal Hypergraphs." Digital Commons @ East Tennessee State University, 2011. https://dc.etsu.edu/etd/1290.
Full textAbstract:
In this thesis, we study universal hypergraphs. What are these? Let us start with defining a universal graph as a graph on n vertices that contains each of the many possible graphs of a smaller size k < n as an induced subgraph. A hypergraph is a discrete structure on n vertices in which edges can be of any size, unlike graphs, where the edge size is always two. If all edges are of size three, then the hypergraph is said to be 3-uniform. If a 3-uniform hypergraph can have edges colored one of a colors, then it is called a 3-uniform hypergraph with a colors. Analogously with universal graphs, a universal, induced, 3-uniform, k-hypergraph, with a possible edge colors is then defined to be a 3-uniform a-colored hypergraph on n vertices that contains each of the many possible 3-uniform a-colored hypergraphs on k vertices, k < n. In this thesis, we study conditions for the existence of a such a universal hypergraph, and address the question of how large n must be, given a fixed k, so that hypergraphs on n vertices are universal with high probability. This extends the work of Alon, [2] who studied the case of a = 2, and that too for graphs (not hypergraphs).
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Rocha, Maria Margarita. "Tutte-Equivalent Matroids." CSUSB ScholarWorks, 2018. https://scholarworks.lib.csusb.edu/etd/759.
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We begin by introducing matroids in the context of finite collections of vectors from a vector space over a specified field, where the notion of independence is linear independence. Then we will introduce the concept of a matroid invariant. Specifically, we will look at the Tutte polynomial, which is a well-defined two-variable invariant that can be used to determine differences and similarities between a collection of given matroids. The Tutte polynomial can tell us certain properties of a given matroid (such as the number of bases, independent sets, etc.) without the need to manually solve for them. Although the Tutte polynomial gives us significant information about a matroid, it does not uniquely determine a matroid. This thesis will focus on non-isomorphic matroids that have the same Tutte polynomial. We call such matroids Tutte-equivalent, and we will study the characteristics needed for two matroids to be Tutte-equivalent. Finally, we will demonstrate methods to construct families of Tutte-equivalent matroids.
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Carney, Nicholas. "Roman Domination Cover Rubbling." Digital Commons @ East Tennessee State University, 2019. https://dc.etsu.edu/etd/3617.
Full textAbstract:
In this thesis, we introduce Roman domination cover rubbling as an extension of domination cover rubbling. We define a parameter on a graph $G$ called the \textit{Roman domination cover rubbling number}, denoted $\rho_{R}(G)$, as the smallest number of pebbles, so that from any initial configuration of those pebbles on $G$, it is possible to obtain a configuration which is Roman dominating after some sequence of pebbling and rubbling moves. We begin by characterizing graphs $G$ having small $\rho_{R}(G)$ value. Among other things, we also obtain the Roman domination cover rubbling number for paths and give an upper bound for the Roman domination cover rubbling number of a tree.
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Ortiz, Jazmin. "Chromatic Polynomials and Orbital Chromatic Polynomials and their Roots." Scholarship @ Claremont, 2015. http://scholarship.claremont.edu/hmc_theses/92.
Full textAbstract:
The chromatic polynomial of a graph, is a polynomial that when evaluated at a positive integer k, is the number of proper k colorings of the graph. We can then find the orbital chromatic polynomial of a graph and a group of automorphisms of the graph, which is a polynomial whose value at a positive integer k is the number of orbits of k-colorings of a graph when acted upon by the group. By considering the roots of the orbital chromatic and chromatic polynomials, the similarities and differences of these polynomials is studied. Specifically we work toward proving a conjecture concerning the gap between the real roots of the chromatic polynomial and the real roots of the orbital chromatic polynomial.
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Pham, Hong Phong. "Studies on Optimal Colorful Structures in Vertex-Colored Graphs." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLS528.
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Dans cette thèse, nous étudions des problèmes différents de coloration maximale dans les graphes sommet-colorés. Nous nous concentrons sur la recherche des structures avec le nombre maximal possible de couleurs par des algorithmes en temps polynomial, nous donnons aussi la preuve des problèmes NP-difficiles pour des graphes spécifiques. En particulier, nous étudions d’abord le problème de l’appariement coloré maximum. Nous montrons que ce problème peut être résolu efficacement en temps polynomial. En plus, nous considérons également une version spécifique de ce problème, à savoir l’appariement tropical, qui consiste à trouver un appariement contenant toutes les couleurs du graphe original. De même, un algorithme de temps polynomial est également fourni pour le problème de l’appariement tropical avec la cardinalité minimale et le problème de l’appariement tropical maximum avec la cardinalité minimale. Ensuite, nous étudions le problème des chemins colorés maximum. Il existe deux versions pour ce problème: le problème de plus court chemin tropical, c’est-à-dire de trouver un chemin tropical avec le poids total minimum et le problème de plus longue chemin coloré, à savoir, trouver un chemin avec un nombre maximum possible de couleurs. Nous montrons que les deux versions de ce problème sont NP-difficile pour un graphe orienté acyclique, graphes de cactus et graphes d'intervalles où le problème de plus long chemin est facile. De plus, nous fournissons également un algorithme de paramètre fixe pour le premier dans les graphes généraux et plusieurs algorithmes de temps polynomiaux pour le second dans les graphes spécifiques, y compris les graphes des chaîne bipartites, graphes de seuil, arborescences, graphes des blocs et graphes d'intervalles appropriés. Ensuite, nous considérons le problème des cycles colorés maximum. Nous montrons d'abord que le problème est NP-difficile même pour des graphes simples tels que des graphes divisés, des graphes bi-connecteurs et des graphes d'intervalles. Nous fournissons ensuite des algorithmes de temps polynomial pour les classes de graphes de seuil et graphes des chaîne bipartites et graphes d'intervalles appropriés. Plus tard, nous étudions le problème des cliques colorées maximum. Nous montrons tout d’abord que le problème est NP-difficile même pour plusieurs cas où le problème de clique maximum est facile, comme des graphes complémentaires des graphes de permutation bipartite, des graphes complémentaires de graphes convexes bipartites et des graphes de disques unitaires, et aussi pour des graphes sommet-colorées appropriés. Ensuite, nous proposons un algorithme paramétré XP et des algorithmes de temps polynomial pour les classes de graphes complémentaires de graphes en chaîne bipartites, des graphes multipartites complets et des graphes complémentaires de graphes cycles. Enfin, nous nous concentrons sur le problème des stables (ensembles indépendants) colorés maximum. Nous montrons d’abord que le problème est NP-difficile même dans certains cas où le problème de stable maximum est facile, tels que les co-graphes et les graphes des P₅-gratuit. Ensuite, nous fournissons des algorithmes de temps polynomial pour les graphes de grappes, et les arbres<br>In this thesis, we study different maximum colorful problems in vertex-colored graphs. We focus on finding structures with the possible maximum number of colors by efficient polynomial-time algorithms, or prove these problems as NP-hard for specific graphs. In particular, we first study the maximum colorful matching problem. We show that this problem can be efficiently solved in polynomial time. Moreover, we also consider a specific version of this problem, namely tropical matching, that is to find a matching containing all colors of the original graph, if any. Similarly, a polynomial time algorithm is also provided for the problem of tropical matching with the minimum cardinality and the problem of maximal tropical matching with the minimum cardinality. Then, we study the maximum colorful paths problem. There are two versions for this problem: the shortest tropical path problem, i.e., finding a tropical path with the minimum total weight, and the maximum colorful path problem, i.e., finding a path with the maximum number of colors possible. We show that both versions of this problem are NP-hard for directed acyclic graphs, cactus graphs and interval graphs where the longest path problem is easy. Moreover, we also provide a fixed parameter algorithm for the former in general graphs and several polynomial time algorithms for the latter in specific graphs, including bipartite chain graphs, threshold graphs, trees, block graphs, and proper interval graphs. Next we consider the maximum colorful cycles problem. We first show that the problem is NP-hard even for simple graphs such as split graphs, biconnected graphs, interval graphs. Then we provide polynomial-time algorithms for classes of threshold graphs and bipartite chain graphs and proper interval graphs. Later, we study the maximum colorful cliques problem. We first show that the problem is NP-hard even for several cases where the maximum clique problem is easy, such as complement graphs of bipartite permutation graphs, complement graphs of bipartite convex graphs, and unit disk graphs, and also for properly vertex-colored graphs. Next, we propose a XP parameterized algorithm and polynomial-time algorithms for classes of complement graphs of bipartite chain graphs, complete multipartite graphs and complement graphs of cycle graphs. Finally, we focus on the maximum colorful independent set problem. We first prove that the problem is NP-hard even for some cases where the maximum independent set problem is easy, such as cographs and P₅-free graphs. Next, we provide polynomial time algorithms for cluster graphs and trees
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Farley, Jerry Brent. "Chromatic Number of the Alphabet Overlap Graph, G(2, k , k-2)." Digital Commons @ East Tennessee State University, 2007. https://dc.etsu.edu/etd/2130.
Full textAbstract:
A graph G(a, k, t) is called an alphabet overlap graph where a, k, and t are positive integers such that 0 ≤ t < k and the vertex set V of G is defined as, V = {v : v = (v1v2...vk); vi ∊ {1, 2, ..., a}, (1 ≤ i ≤ k)}. That is, each vertex, v, is a word of length k over an alphabet of size a. There exists an edge between two vertices u, v if and only if the last t letters in u equal the first t letters in v or the first t letters in u equal the last t letters in v. We determine the chromatic number of G(a, k, t) for all k ≥ 3, t = k − 2, and a = 2; except when k = 7, 8, 9, and 11.
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Xia, Yan. "Packings and Coverings of Complete Graphs with a Hole with the 4-Cycle with a Pendant Edge." Digital Commons @ East Tennessee State University, 2013. https://dc.etsu.edu/etd/1173.
Full textAbstract:
In this thesis, we consider packings and coverings of various complete graphs with the 4-cycle with a pendant edge. We consider both restricted and unrestricted coverings. Necessary and sufficient conditions are given for such structures for (1) complete graphs Kv, (2) complete bipartite graphs Km,n, and (3) complete graphs with a hole K(v,w).
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Rahm, Ludwig. "Generating functions and regular languages of walks with modular restrictions in graphs." Thesis, Linköpings universitet, Matematik och tillämpad matematik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-138117.
Full textAbstract:
This thesis examines the problem of counting and describing walks in graphs, and the problem when such walks have modular restrictions on how many timesit visits each vertex. For the special cases of the path graph, the cycle graph, the grid graph and the cylinder graph, generating functions and regular languages for their walks and walks with modular restrictions are constructed. At the end of the thesis, a theorem is proved that connects the generating function for walks in a graph to the generating function for walks in a covering graph.
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Harris, Elizabeth Marie. "Global Domination Stable Graphs." Digital Commons @ East Tennessee State University, 2012. https://dc.etsu.edu/etd/1476.
Full textAbstract:
A set of vertices S in a graph G is a global dominating set (GDS) of G if S is a dominating set for both G and its complement G. The minimum cardinality of a global dominating set of G is the global domination number of G. We explore the effects of graph modifications on the global domination number. In particular, we explore edge removal, edge addition, and vertex removal.