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Perfect Colorings of a Design with 2-Dimensional Euclidean Perfect Colorings of a Design With 2-Dimensional Euclidean Crystallographic Symmetry Group BY Xi Xu A Study Presented to the Faculty of Wheaton College in Partial Fulfillment of the Requirements for Graduation with Departmental Honors in Mathematics Norton, Massachusetts May 2018 Acknowledgments First of all, I would like to offer my sincerest thanks to my amazing thesis advisor Professor Rachelle DeCoste. Rachelle's faith in me and her constant encouragements are the biggest motivations for this work. I cannot emphasize how grateful I am for her help and patience. I would like to thank Professor Shelly Leibowitz and Professor Chris Kotyk for being my committee members. They both have been very supportive and positive to me and my work. I would also like to thank my parents and friends for making my days at Wheaton fulfilling and exciting. Finally, I would like to offer my special thanks to my boyfriend Weiqi Feng for his love and support. I want to wish him the best of luck for his thesis next year. Hope he will have as much fun as I had! 2 Contents 1 Abstract 5 2 Background 6 2.1 Symmetry and Symmetry Group . .6 2.2 Metric Space and Isometry Group . .7 2.3 Crystallographic Groups . 10 3 Coloring and Perfect Coloring 12 4 Perfect Colorings of Square, Hexagon, and Pentagon 17 4.1 Perfect Colorings of Square . 17 4.2 Perfect Colorings of Hexagon . 20 4.3 Perfect Colorings of Pentagon . 23 5 Counting Perfect Colorings of a Design With Symmetry Group D2n 27 6 Frieze Patterns and Colorings 33 6.1 Frieze Groups and Frieze Patterns . 34 6.2 Colorings of Frieze Patterns . 40 6.3 Imposing D2n Symmetry on Motif of a Colored Frieze Pattern . 42 7 Wallpaper Patterns and Colorings 55 7.1 Wallpaper Groups and Wallpaper Patterns . 55 7.1.1 Wallpaper Patterns With Parallelogram Unit Cells . 58 7.1.2 Wallpaper Patterns With Rectangular Unit Cells . 59 7.1.3 Wallpaper Patterns With Rhombic Unit Cells . 63 7.1.4 Wallpaper Patterns With Square Unit Cells . 64 3 7.1.5 Wallpaper Patterns With Hexagonal Unit Cells . 67 7.2 Future Work: Colorings of Wallpaper Patterns . 70 4 1 Abstract This thesis introduces perfect colorings and the systematic way of generating perfect colorings. We study the perfect colorings for designs whose symmetry groups are 2-dimensional Euclidean crystallographic groups. Two-dimensional Euclidean crys- tallographic groups are discrete subgroups of the isometry group of the Euclidean plane. We classify crystallographic groups by their ranks. Crystallographic groups of rank 0, rank 1, and rank 2 are explained with details and examples. Theorems that related to the number of perfect colorings for designs with crystallographic symmetry groups are constructed and proved. 5 2 Background In this paper, we study colorings of objects with symmetry. We begin with objects with only rotation and reflection symmetries, and then expand our study to include translations in one direction and then two directions. In order to study symmetry with rigor, we begin with some background ideas. In this chapter, we introduce background and give several definitions that build the foundations for the reminder of this study. Examples are provided to explain the definitions as necessary. 2.1 Symmetry and Symmetry Group Definition 2.1 Symmetries are transformations, including reflection, rotation or translation, under which an object is invariant to itself. It is well known that the set of such transformations for an object forms a group. Thus, the symmetry group of an object is the group of all symmetries of the object. Example 2.2 A square has 8 symmetries: rotations around the center by, 0, π=2, π, and 3π=2 radians counter-clockwise, and reflections over the horizontal axis, the vertical axis, and the two diagonal axes. The set containing the 8 symmetries forms the symmetry group of the square. This group is the dihedral group D8 of order 8, a well understood group. Since it is a group, the composition of any two of these symmetries is again one of the 8 symmetries. Also, for every symmetry g, there is an inverse symmetry g−1. This means that the composition of any symmetry and its inverse symmetry is the identity symmetry, rotation by 0 radians. Clearly, for the reflections, each is its own inverse symmetry. Also the inverse symmetry of rotation by π=2 is rotation by 3π=2. 6 Figure 1: A square with its reflection axes. 2.2 Metric Space and Isometry Group Definition 2.3 [1] A metric is a function D that defines a distance between each pair of elements of a set. Given any set P, a function D : P × P ! R is a metric if the following properties are satisfied for every P; Q 2 P: (i) D(P; Q) = D(Q; P ); (ii) D(P; Q) ≥ 0; (iii) D(P; Q) = 0 if and only if P = Q. Example 2.4 In the Euclidean plane, denoted E2, the distance between 2 points p 2 2 (x1; y1) and (x2; y2) is defined by d((x1; y1); (x2; y2)) = (x2 − x1) + (y2 − y1) . The distance function d is a metric because d satisfies the three properties in the Definition 2.3. p 2 2 (i) For every (x1; y1) and (x2; y2), d((x1; y1); (x2; y2)) = (x2 − x1) + (y2 − y1) = p 2 2 (x1 − x2) + (y1 − y2) = d((x2; y2); (x1; y1)); 7 p 2 2 (ii) For every (x1; y1) and (x2; y2), d((x1; y1); (x2; y2)) = (x2 − x1) + (y2 − y1) ≥ 0; p 2 2 (iii) d((x1; y1); (x2; y2)) = (x2 − x1) + (y2 − y1) = 0 if and only if x2 −x1 = 0 and y2 − y1 = 0 if and only if x1 = x2 and y1 = y2 if and only if (x1; y1) = (x2; y2). This metric d on E2 is called the Euclidean metric. Definition 2.5 A metric space M is a set in which distances between all pairs of elements are defined by a metric. The Euclidean plane can be considered as a metric space with the Euclidean metric d as its metric. Definition 2.6 Given a metric space M with its metric D, an isometry is a one- to-one and onto mapping T : M ! M that preserves distance, i.e., D(x; y) = D(T (x);T (y)) 8 x; y 2 M. It is well known that the set of all isometries of a metric space M forms a group under composition.[2] The isometry group of M is denoted by Isom(M). It is proved in [1] and many other sources that the composition of two isometries is an isometry and the inverse of an isometry is an isometry. The identity mapping of Isom(M) is Te(x) = x 8 x 2 M. The isometry group of the Euclidean plane is denoted by 2 2 2 Isom(E ), and the identity mapping of Isom(E ) is Te(x; y) = (x; y) 8 (x; y) 2 E . Let ~v be a vector in the Euclidean plane and let (x; y) be a generic point in E2, then the mapping T (x; y) = (x; y) + ~v is a isometry that acts as a translation. In math, translation is an operation that shifts an object by a fixed distance. For the mapping T (x; y) = (x; y) + ~v, the point (x; y) is translated by distance jj~vjj in the direction of ~v. 8 It is shown in [2] that reflections across any line l 2 E2 and rotations about any point (x; y) 2 E2 are isometries of the Euclidean plane. The next two examples illustrate that reflections and rotations are elements of Isom(E2). Example 2.7 Let l be any line in the Euclidean plane, and P , Q be any two points in the Euclidean plane. Reflecting P and Q across the mirror line l gives points P 0 and Q0, respectively. The reflection across the line l is an isometry of the Euclidean plane. The distance between P and Q and the distance between the reflection points P 0 and Q0 are equal. Q Q0 d d P P 0 l Figure 2: Points P 0 and Q0 are reflections of points P and Q, respectively, across the mirror line l. Example 2.8 Let O, P , and Q be any points in the Euclidean plane. Rotate P and Q by a fixed angle with O as the rotation center. Let the end results be points P 0 and Q0, respectively. Rotation by a fixed angle with a fixed rotation center is an isometry of the Euclidean plane. The distance between P and Q and the distance between the rotation points P 0 and Q0 are equal. 9 P 0 Q0 d O P d Q Figure 3: Points P and Q are rotated by a fixed angle with O as the rotation center. The end results are points P 0 and Q0, respectively. 2.3 Crystallographic Groups Definition 2.9 The orthogonal group O(2) is defined as O(2) = fM : M is a 2 × 2 matrix with real entries such that MM T = Ig, where I is the identity matrix. Given any isometry A : E2 ! E2, it is proved in [2] that there exists a vector ~w 2 R2 and an orthogonal matrix B such that A(x) = Bx + ~w 8 x 2 R2. Let φ be a mapping from Isom(E2) to O(2) defined by φ(A) = B. Then it is straightforward to show that φ is a group homomorphism, and its kernel is the group of all translations of E2, Trans(E2).
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