Notes for Math 198: Mathematics of Symmetry
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Reflection Invariant and Symmetry Detection
1 Reflection Invariant and Symmetry Detection Erbo Li and Hua Li Abstract—Symmetry detection and discrimination are of fundamental meaning in science, technology, and engineering. This paper introduces reflection invariants and defines the directional moments(DMs) to detect symmetry for shape analysis and object recognition. And it demonstrates that detection of reflection symmetry can be done in a simple way by solving a trigonometric system derived from the DMs, and discrimination of reflection symmetry can be achieved by application of the reflection invariants in 2D and 3D. Rotation symmetry can also be determined based on that. Also, if none of reflection invariants is equal to zero, then there is no symmetry. And the experiments in 2D and 3D show that all the reflection lines or planes can be deterministically found using DMs up to order six. This result can be used to simplify the efforts of symmetry detection in research areas,such as protein structure, model retrieval, reverse engineering, and machine vision etc. Index Terms—symmetry detection, shape analysis, object recognition, directional moment, moment invariant, isometry, congruent, reflection, chirality, rotation F 1 INTRODUCTION Kazhdan et al. [1] developed a continuous measure and dis- The essence of geometric symmetry is self-evident, which cussed the properties of the reflective symmetry descriptor, can be found everywhere in nature and social lives, as which was expanded to 3D by [2] and was augmented in shown in Figure 1. It is true that we are living in a spatial distribution of the objects asymmetry by [3] . For symmetric world. Pursuing the explanation of symmetry symmetry discrimination [4] defined a symmetry distance will provide better understanding to the surrounding world of shapes. -
Molecular Symmetry
Molecular Symmetry Symmetry helps us understand molecular structure, some chemical properties, and characteristics of physical properties (spectroscopy) – used with group theory to predict vibrational spectra for the identification of molecular shape, and as a tool for understanding electronic structure and bonding. Symmetrical : implies the species possesses a number of indistinguishable configurations. 1 Group Theory : mathematical treatment of symmetry. symmetry operation – an operation performed on an object which leaves it in a configuration that is indistinguishable from, and superimposable on, the original configuration. symmetry elements – the points, lines, or planes to which a symmetry operation is carried out. Element Operation Symbol Identity Identity E Symmetry plane Reflection in the plane σ Inversion center Inversion of a point x,y,z to -x,-y,-z i Proper axis Rotation by (360/n)° Cn 1. Rotation by (360/n)° Improper axis S 2. Reflection in plane perpendicular to rotation axis n Proper axes of rotation (C n) Rotation with respect to a line (axis of rotation). •Cn is a rotation of (360/n)°. •C2 = 180° rotation, C 3 = 120° rotation, C 4 = 90° rotation, C 5 = 72° rotation, C 6 = 60° rotation… •Each rotation brings you to an indistinguishable state from the original. However, rotation by 90° about the same axis does not give back the identical molecule. XeF 4 is square planar. Therefore H 2O does NOT possess It has four different C 2 axes. a C 4 symmetry axis. A C 4 axis out of the page is called the principle axis because it has the largest n . By convention, the principle axis is in the z-direction 2 3 Reflection through a planes of symmetry (mirror plane) If reflection of all parts of a molecule through a plane produced an indistinguishable configuration, the symmetry element is called a mirror plane or plane of symmetry . -
Symmetry and Beauty in the Living World I Thank the Governing Body and the Director of the G.B
SYMMETRY AND BEAUTY IN THE LIVING WORLD I thank the Governing Body and the Director of the G.B. Pant Institute of Himalayan Environment & Development for providing me this opportunity to deliver the 17th Govind Ballabh Pant Memorial Lecture. Pt. Pant, as I have understood, was amongst those who contributed in multiple ways to shape and nurture the nation in general and the Himalayan area in particular. Established to honour this great ‘Son of the Mountains’, the Institute carries enormous responsibilities and expectations from millions of people across the region and outside. Undoubtedly the multidisciplinary skills and interdisciplinary approach of the Institute and the zeal of its members to work in remote areas and harsh Himalayan conditions will succeed in achieving the long term vision of Pt. Pant for the overall development of the region. My talk ‘Symmetry and Beauty in the Living World’ attempts to discuss aspects of symmetry and beauty in nature and their evolutionary explanations. I shall explain how these elements have helped developmental and evolutionary biologists to frame and answer research questions. INTRODUCTION Symmetry is an objective feature of the living world and also of some non-living entities. It forms an essential element of the laws of nature; it is often sought by human beings when they create artefacts. Beauty has to do with a subjective assessment of the extent to which something or someone has a pleasing appearance. It is something that people aspire to, whether in ideas, creations or people. Evolutionary biology tells us that it is useful to look for an evolutionary explanation of anything to do with life. -
Chapter 1 – Symmetry of Molecules – P. 1
Chapter 1 – Symmetry of Molecules – p. 1 - 1. Symmetry of Molecules 1.1 Symmetry Elements · Symmetry operation: Operation that transforms a molecule to an equivalent position and orientation, i.e. after the operation every point of the molecule is coincident with an equivalent point. · Symmetry element: Geometrical entity (line, plane or point) which respect to which one or more symmetry operations can be carried out. In molecules there are only four types of symmetry elements or operations: · Mirror planes: reflection with respect to plane; notation: s · Center of inversion: inversion of all atom positions with respect to inversion center, notation i · Proper axis: Rotation by 2p/n with respect to the axis, notation Cn · Improper axis: Rotation by 2p/n with respect to the axis, followed by reflection with respect to plane, perpendicular to axis, notation Sn Formally, this classification can be further simplified by expressing the inversion i as an improper rotation S2 and the reflection s as an improper rotation S1. Thus, the only symmetry elements in molecules are Cn and Sn. Important: Successive execution of two symmetry operation corresponds to another symmetry operation of the molecule. In order to make this statement a general rule, we require one more symmetry operation, the identity E. (1.1: Symmetry elements in CH4, successive execution of symmetry operations) 1.2. Systematic classification by symmetry groups According to their inherent symmetry elements, molecules can be classified systematically in so called symmetry groups. We use the so-called Schönfliess notation to name the groups, Chapter 1 – Symmetry of Molecules – p. 2 - which is the usual notation for molecules. -
Symmetry of Graphs. Circles
Symmetry of graphs. Circles Symmetry of graphs. Circles 1 / 10 Today we will be interested in reflection across the x-axis, reflection across the y-axis and reflection across the origin. Reflection across y reflection across x reflection across (0; 0) Sends (x,y) to (-x,y) Sends (x,y) to (x,-y) Sends (x,y) to (-x,-y) Examples with Symmetry What is Symmetry? Take some geometrical object. It is called symmetric if some geometric move preserves it Symmetry of graphs. Circles 2 / 10 Reflection across y reflection across x reflection across (0; 0) Sends (x,y) to (-x,y) Sends (x,y) to (x,-y) Sends (x,y) to (-x,-y) Examples with Symmetry What is Symmetry? Take some geometrical object. It is called symmetric if some geometric move preserves it Today we will be interested in reflection across the x-axis, reflection across the y-axis and reflection across the origin. Symmetry of graphs. Circles 2 / 10 Sends (x,y) to (-x,y) Sends (x,y) to (x,-y) Sends (x,y) to (-x,-y) Examples with Symmetry What is Symmetry? Take some geometrical object. It is called symmetric if some geometric move preserves it Today we will be interested in reflection across the x-axis, reflection across the y-axis and reflection across the origin. Reflection across y reflection across x reflection across (0; 0) Symmetry of graphs. Circles 2 / 10 Sends (x,y) to (-x,y) Sends (x,y) to (x,-y) Sends (x,y) to (-x,-y) Examples with Symmetry What is Symmetry? Take some geometrical object. -
Radial Symmetry Or Bilateral Symmetry Or "Spherical Symmetry"
Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or "spherical symmetry". A small minority exhibit no symmetry (are asymmetric). In nature and biology, symmetry is approximate. For example, plant leaves, while considered symmetric, will rarely match up exactly when folded in half. Radial symmetry These organisms resemble a pie where several cutting planes produce roughly identical pieces. An organism with radial symmetry exhibits no left or right sides. They have a top and a bottom (dorsal and ventral surface) only. Animals Symmetry is important in the taxonomy of animals; animals with bilateral symmetry are classified in the taxon Bilateria, which is generally accepted to be a clade of the kingdom Animalia. Bilateral symmetry means capable of being split into two equal parts so that one part is a mirror image of the other. The line of symmetry lies dorso-ventrally and anterior-posteriorly. Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, or aboral, end. This type of symmetry is especially suitable for sessile animals such as the sea anemone, floating animals such as jellyfish, and slow moving organisms such as sea stars (see special forms of radial symmetry). Animals in the phyla cnidaria and echinodermata exhibit radial symmetry (although many sea anemones and some corals exhibit bilateral symmetry defined by a single structure, the siphonoglyph) (see Willmer, 1990). -
In Square Units)
INVESTIGATIONS and EXPLORATIONS 1.1.1 – 1.1.5 By asking questions such as “What happens if…?” and “What if I change this…?” and answering them by trying different things, we can find out quite a lot of information about different shapes. In the first five sections of this first chapter, we explore symmetry, making predictions, perimeter, area, logical arguments, and angles by investigating each of them with interesting problems. These five sections are introductory and help the teacher determine students’ prior knowledge and preview some of the ideas that will be studied in this course. The following examples illustrate the geometry ideas in this section as well as some of the algebra review topics. See the Math Notes Boxes on pages 5, 10, 15, 19, and 24. Example 1 Suppose the rug in Figure 1 is enlarged as shown. one unit one square unit Figure 1 Figure 2 Figure 3 Fill in the table below to show how the perimeter and the area of the rug change as it is enlarged. Figure Number 1 2 3 4 5 20 Perimeter (in units) Area (in square units) The perimeter of a figure is the distance (length) around the outside of the figure while the area measures the surface within the figure. The area is measured in square units while the perimeter is simply a unit of length, such as inches or centimeters. Counting the units around the outside of Figure 1, we get a perimeter of 16 units. By counting the number of square units within Figure 1, we find the area is 12 square units. -
Geometry Topics
GEOMETRY TOPICS Transformations Rotation Turn! Reflection Flip! Translation Slide! After any of these transformations (turn, flip or slide), the shape still has the same size so the shapes are congruent. Rotations Rotation means turning around a center. The distance from the center to any point on the shape stays the same. The rotation has the same size as the original shape. Here a triangle is rotated around the point marked with a "+" Translations In Geometry, "Translation" simply means Moving. The translation has the same size of the original shape. To Translate a shape: Every point of the shape must move: the same distance in the same direction. Reflections A reflection is a flip over a line. In a Reflection every point is the same distance from a central line. The reflection has the same size as the original image. The central line is called the Mirror Line ... Mirror Lines can be in any direction. Reflection Symmetry Reflection Symmetry (sometimes called Line Symmetry or Mirror Symmetry) is easy to see, because one half is the reflection of the other half. Here my dog "Flame" has her face made perfectly symmetrical with a bit of photo magic. The white line down the center is the Line of Symmetry (also called the "Mirror Line") The reflection in this lake also has symmetry, but in this case: -The Line of Symmetry runs left-to-right (horizontally) -It is not perfect symmetry, because of the lake surface. Line of Symmetry The Line of Symmetry (also called the Mirror Line) can be in any direction. But there are four common directions, and they are named for the line they make on the standard XY graph. -
Symmetry in Chemistry - Group Theory
Symmetry in Chemistry - Group Theory Group Theory is one of the most powerful mathematical tools used in Quantum Chemistry and Spectroscopy. It allows the user to predict, interpret, rationalize, and often simplify complex theory and data. At its heart is the fact that the Set of Operations associated with the Symmetry Elements of a molecule constitute a mathematical set called a Group. This allows the application of the mathematical theorems associated with such groups to the Symmetry Operations. All Symmetry Operations associated with isolated molecules can be characterized as Rotations: k (a) Proper Rotations: Cn ; k = 1,......, n k When k = n, Cn = E, the Identity Operation n indicates a rotation of 360/n where n = 1,.... k (b) Improper Rotations: Sn , k = 1,....., n k When k = 1, n = 1 Sn = , Reflection Operation k When k = 1, n = 2 Sn = i , Inversion Operation In general practice we distinguish Five types of operation: (i) E, Identity Operation k (ii) Cn , Proper Rotation about an axis (iii) , Reflection through a plane (iv) i, Inversion through a center k (v) Sn , Rotation about an an axis followed by reflection through a plane perpendicular to that axis. Each of these Symmetry Operations is associated with a Symmetry Element which is a point, a line, or a plane about which the operation is performed such that the molecule's orientation and position before and after the operation are indistinguishable. The Symmetry Elements associated with a molecule are: (i) A Proper Axis of Rotation: Cn where n = 1,.... This implies n-fold rotational symmetry about the axis. -
Symmetry Operations and Symmetry Oparators
Dr. Lokesh Chandra Pati Dept. of Chemistry, J. K. College, Purulia Symmetry Operations and Symmetry Oparators: On order to study the symmetry of a molecule, certain operations such as rotation and reflection are performed and if by so doing, an arrangement is obtained which is indistinguishable from (superposable on) the original one, the operations is called a symmetry operation and the molecule is said to possess an element of symmetry defined by the operation performed. The symmetry operations are the ways of interchanging parts of a molecule. The symmetry operation and symmetry element are thus inseparably linked and often represented by the same symbols. Symmetry based solely on simple rotation is called symmetry of the first kind whereas symmetry on reflection or rotation- reflection is known as symmetry of second kind. Four fundamental elements of symmetry are present in the organic molecules. They are (i) Rotational axis of symmetry (Cn) (ii) Plane of symmetry (v) (iii) Centre of symmetry (i) (iv) Alternating of symmetry (Sn). Different manipulations of elements of symmetry that transform molecules into indistinguishable and identical structures are called symmetry operations and operation of identity respectively. The element of identity is designated as E or I. (i) Rotational axis of symmetry or proper axis of symmetry (Cn, where C stands for circulate and n is the fold):- If a molecule is rotated around an appropriate imaginary axis by an angle of 360/n and arrives at an arrangement indistinguishable from the original, the axis is called an n-fold simple or proper axis of symmetry or a rotational axis of symmetry of order n. -
Symmetry Groups
Symmetry Groups Symmetry plays an essential role in particle theory. If a theory is invariant under transformations by a symmetry group one obtains a conservation law and quantum numbers. For example, invariance under rotation in space leads to angular momentum conservation and to the quantum numbers j, mj for the total angular momentum. Gauge symmetries: These are local symmetries that act differently at each space-time point xµ = (t,x) . They create particularly stringent constraints on the structure of a theory. For example, gauge symmetry automatically determines the interaction between particles by introducing bosons that mediate the interaction. All current models of elementary particles incorporate gauge symmetry. (Details on p. 4) Groups: A group G consists of elements a , inverse elements a−1, a unit element 1, and a multiplication rule “ ⋅ ” with the properties: 1. If a,b are in G, then c = a ⋅ b is in G. 2. a ⋅ (b ⋅ c) = (a ⋅ b) ⋅ c 3. a ⋅ 1 = 1 ⋅ a = a 4. a ⋅ a−1 = a−1 ⋅ a = 1 The group is called Abelian if it obeys the additional commutation law: 5. a ⋅ b = b ⋅ a Product of Groups: The product group G×H of two groups G, H is defined by pairs of elements with the multiplication rule (g1,h1) ⋅ (g2,h2) = (g1 ⋅ g2 , h1 ⋅ h2) . A group is called simple if it cannot be decomposed into a product of two other groups. (Compare a prime number as analog.) If a group is not simple, consider the factor groups. Examples: n×n matrices of complex numbers with matrix multiplication. -
Symmetry in 2D
Symmetry in 2D 4/24/2013 L. Viciu| AC II | Symmetry in 2D 1 Outlook • Symmetry: definitions, unit cell choice • Symmetry operations in 2D • Symmetry combinations • Plane Point groups • Plane (space) groups • Finding the plane group: examples 4/24/2013 L. Viciu| AC II | Symmetry in 2D 2 Symmetry Symmetry is the preservation of form and configuration across a point, a line, or a plane. The techniques that are used to "take a shape and match it exactly to another” are called transformations Inorganic crystals usually have the shape which reflects their internal symmetry 4/24/2013 L. Viciu| AC II | Symmetry in 2D 3 Lattice = an array of points repeating periodically in space (2D or 3D). Motif/Basis = the repeating unit of a pattern (ex. an atom, a group of atoms, a molecule etc.) Unit cell = The smallest repetitive volume of the crystal, which when stacked together with replication reproduces the whole crystal 4/24/2013 L. Viciu| AC II | Symmetry in 2D 4 Unit cell convention By convention the unit cell is chosen so that it is as small as possible while reflecting the full symmetry of the lattice (b) to (e) correct unit cell: choice of origin is arbitrary but the cells should be identical; (f) incorrect unit cell: not permissible to isolate unit cells from each other (1 and 2 are not identical)4/24/2013 L. Viciu| AC II | Symmetry in 2D 5 A. West: Solid state chemistry and its applications Some Definitions • Symmetry element: An imaginary geometric entity (line, point, plane) about which a symmetry operation takes place • Symmetry Operation: a permutation of atoms such that an object (molecule or crystal) is transformed into a state indistinguishable from the starting state • Invariant point: point that maps onto itself • Asymmetric unit: The minimum unit from which the structure can be generated by symmetry operations 4/24/2013 L.