Bard College Bard Digital Commons Senior Projects Spring 2019 Bard Undergraduate Senior Projects Spring 2019 Upper Bounds for the Number of Lattice Edges Needed to Represent 4-Regular Graphs as Lattice Graphs Shenze Li Bard College, [email protected] Follow this and additional works at: https://digitalcommons.bard.edu/senproj_s2019 Part of the Geometry and Topology Commons This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License. Recommended Citation Li, Shenze, "Upper Bounds for the Number of Lattice Edges Needed to Represent 4-Regular Graphs as Lattice Graphs" (2019). Senior Projects Spring 2019. 238. https://digitalcommons.bard.edu/senproj_s2019/238 This Open Access work is protected by copyright and/or related rights. It has been provided to you by Bard College's Stevenson Library with permission from the rights-holder(s). You are free to use this work in any way that is permitted by the copyright and related rights. For other uses you need to obtain permission from the rights- holder(s) directly, unless additional rights are indicated by a Creative Commons license in the record and/or on the work itself. For more information, please contact [email protected]. Upper Bounds for the Number of Lattice Edges Needed to Represent 4-Regular Graphs as Lattice Graphs ASeniorProjectsubmittedto The Division of Science, Mathematics, and Computing of Bard College by Shenze Li Annandale-on-Hudson, New York May, 2019 ii Abstract A lattice graph is a graph whose drawing, embedded in Euclidean space R2, has vertices that are the points with integer coefficients, and has edges that are unit length and are parallel to the coordinate axes. A 4-regular graph is a graph where each vertex has four edges containing it; a loop containing a vertex counts as two edges. The goal for my senior project is to find upper bounds for the number of lattice edges needed to represent 4-regular graphs as lattice graphs. iv Contents Abstract iii Dedication vii Acknowledgments ix 1 Background 1 1.1 IntroductiontoGraphTheory. 1 1.2 D and C Notation in 4-regular graph . 3 1.3 4-Regular Graphs in Lattice Graphs . 5 2 Counting 4-Regular Path Graphs 9 2.1 Symmetry in Counting 4-Regular Path Graphs . 9 2.2 Counting 4-Regular Path Graphs with Single Dn .................. 9 2.3 Counting 4-Regular Path Graphs with Two Dn ................... 10 2.3.1 Counting 4-Regular Path Graphs with Two Same Dn ........... 10 2.3.2 Counting 4-Regular Path Graphs with Two Di↵erent Dn ......... 11 3 Counting 4-Regular Cycle Graphs 13 3.1 Definition of 4-Regular Cycle Graphs . 13 3.2 Counting 4-Regular Cycle Graphs with Single Dn .................. 13 3.3 Counting 4-Regular Cycle Graphs with Two Dn ................... 14 3.3.1 Counting 4-Regular Cycle Graphs with Two Same Dn ........... 15 3.3.2 Counting 4-Regular Cycle Graphs with Two Di↵erent Dn ......... 15 4 Counting Lattice Edges in Lattice Graphs from 4-Regular Path Graphs 17 4.1 Finding the Lattice Graph that Requires the Minimum Number of Lattice Edges 17 4.2 Special Cases in Counting Lattice Edges in Lattice Graphs with Single Dn .... 18 4.3 Counting Lattice Edges in Lattice Graphs with Single Dn Without C ...... 19 4.4 Counting Lattice Edges in Lattice Graphs with Single Dn With C ........ 21 vi 4.5 Counting Lattice Edges in Lattice Graphs with Di and Dj Without C ...... 23 4.6 Counting Lattice Edges in Lattice Graphs with Di and Dj With C ........ 25 5 Counting Lattice Edges in Lattice Graph from 4-Regular Cycle Graphs 29 5.1 Counting Lattice Edges in Lattice Graphs with C .................. 29 5.2 Counting Lattice Edges in Lattice Graphs with Single Dn and C ......... 33 Bibliography 37 Dedication To my family. viii Acknowledgments First of all, I would like to thank my academic advisor and senior project advisor, Professor Ethan Bloch, for his guidance, help and valuable advise throughout this senior project. I would also like to thank Professor Stefan Mendez-Diez and Professor Japheth Wood for their suggestions to improve this senior project. I want to thank all my family members for their support and trust throughout my life, providing me with hidden but influential love in all their unique ways. x 1 Background 1.1 Introduction to Graph Theory The following definition is from [1]. Definition 1.1.1. A graph G =(V,E) is a mathematical structure consisting of two finite sets V and E. The elements of V are called vertices (or nodes), and the elements of E are called edges. Each edge has a set of one or two vertices associated to it, which are called its endpoints. 4 Definition 1.1.2. Degree is the number of edges incident to the vertex, with loops counted twice. 4 Definition 1.1.3. A loop is an edge that connects a vertex to itself. 4 Definition 1.1.4. Double edges are two edges that join the same two vertices. 4 Figure 1.1.1 is a graph that each vertices has degree 2. And the graph has 5 vertices and 5 edges. Definition 1.1.5. A simple graph has neither loops nor double edges. 4 Definition 1.1.6. A regular graph is a graph where all vertices have the same degree. A regular graph with vertices of degree k is called a k-regular graph. 4 2 1. BACKGROUND Figure 1.1.1. Definition 1.1.7. A 4-regular graph is a graph where each vertex has degree 4. A 4-regular graph can contain loops and double edges. 4 Figure 1.1.2 is a picture of a 4-regular graph that contains double edges. Figure 1.1.2. Figure 1.1.3 is a picture of a 4-regular graph that contains loops. Figure 1.1.3. The following two definitions are from [1]. Vp,Vc are the number of vertices of the simple graph. Ep,Ec are the number of edges of the simple graph. Definition 1.1.8. A path graph P is a simple graph with V = E + 1 that can be drawn | p| | p| so that all of its vertices and edges lie on a single straight line. 4 For example, Figure 1.2.7 is a path graph which has 4 vertices and 3 edges. 1.2. D AND C NOTATION IN 4-REGULAR GRAPH 3 Figure 1.1.4. Definition 1.1.9. A cycle graph is a single vertex with a self-loop or a simple graph C with V = E that can be drawn so that all of its vertices and edges lie on a single circle. | c| | c| 4 For example, Figure 3.1.1 is a cycle graph which has 6 vertices and 6 edges. Figure 1.1.5. 1.2 D and C Notation in 4-regular graph Definition 1.2.1. Let D1 be the graph in Figure 1.2.2 , let D2 be the graph in Figure 1.2.3 and let Dn be the graph in Figure 1.2.4, where the graph has n ellipses. A D-graph is a graph Di for some i N. Let C be the graph in Figure 1.2.1. 2 4 Figure 1.2.1. Figure 1.2.2. Figure 1.2.3. For example, we can use D and C notation to express Figure 1.2.5, we will get CD1D2C.The loops at the two ending vertices in Figure 1.2.5 are not being expressed in D and C notation, because we will always assume them to be there in this kind of graph. 4 1. BACKGROUND Figure 1.2.4. Figure 1.2.5. Definition 1.2.2. A4-regular path graph is a 4-regular graph that comes from removing all the edges of a path graph and replacing each edges removed either with a C graph or with a Dn graph that takes n + 2 adjacent vertices, and adding a loop at each end vertex. 4 For example, Figure 1.2.6 is a 4-regular path graph that comes from path graph Figure 1.2.7. The three edges of the path graph is removed and replaced by a D1 graph and a C graph with and a loop at each end. Figure 1.2.6. Definition 1.2.3. A4-regular cycle graph is a 4-regular graph that comes from removing all the edges of a cycle graph and replacing each edges removed either with a C graph or with a D graph that takes n + 2 adjacent vertices. n 4 1.3. 4-REGULAR GRAPHS IN LATTICE GRAPHS 5 Figure 1.2.7. For example, Figure 1.2.8 is a 4-regular cycle graph that comes from cycle graph Figure 1.2.9. The six edges of the cycle graph is removed and replaced by six C graph. Figure 1.2.8. Figure 1.2.9. 1.3 4-Regular Graphs in Lattice Graphs Definition 1.3.1. A Lattice graph is a graph whose drawing, embedded in Euclidean space R2 where the vertices are the points with integer coefficients, and the edges are unions of unit length edges that are parallel to the coordinate axes. 4 Figure 1.3.1 is a picture of a lattice. We want to represent 4-regular graphs in lattice graphs. For example, Figure 1.3.2 is a picture of a 4-regular graph with 2 vertices. When we represent Figure 1.3.2 in lattice graph, Figure 1.3.3 is one possible lattice graph.