WikiMili The Free Encyclopedia

This article needs additional citations for verification .(August 2018) (Learn how and when to remove this template message) |

Given a topological space and a group acting on it, the images of a single point under the group action form an orbit of the action. A **fundamental domain** or **fundamental region** is a subset of the space which contains exactly one point from each of these orbits. It serves as a geometric realization for the abstract set of representatives of the orbits.

- Hints at a general definition
- Examples
- Fundamental domain for the modular group
- See also
- External links

There are many ways to choose a fundamental domain. Typically, a fundamental domain is required to be a connected subset with some restrictions on its boundary, for example, smooth or polyhedral. The images of a chosen fundamental domain under the group action then tile the space. One general construction of fundamental domains uses Voronoi cells.

Given an action of a group *G* on a topological space *X* by homeomorphisms, a fundamental domain for this action is a set *D* of representatives for the orbits. It is usually required to be a reasonably nice set topologically, in one of several precisely defined ways. One typical condition is that *D* is *almost* an open set, in the sense that *D* is the symmetric difference of an open set in *G* with a set of measure zero, for a certain (quasi)invariant measure on *X*. A fundamental domain always contains a free regular set *U*, an open set moved around by *G* into disjoint copies, and nearly as good as *D* in representing the orbits. Frequently *D* is required to be a complete set of coset representatives with some repetitions, but the repeated part has measure zero. This is a typical situation in ergodic theory. If a fundamental domain is used to calculate an integral on *X*/*G*, sets of measure zero do not matter.

For example, when *X* is Euclidean space **R**^{n} of dimension *n*, and *G* is the lattice **Z**^{n} acting on it by translations, the quotient *X*/*G* is the *n*-dimensional torus. A fundamental domain *D* here can be taken to be [0,1)^{n}, which differs from the open set (0,1)^{n} by a set of measure zero, or the closed unit cube [0,1]^{n}, whose boundary consists of the points whose orbit has more than one representative in *D*.

Examples in the three-dimensional Euclidean space **R**^{3}.

- for
*n*-fold rotation: an orbit is either a set of*n*points around the axis, or a single point on the axis; the fundamental domain is a sector - for reflection in a plane: an orbit is either a set of 2 points, one on each side of the plane, or a single point in the plane; the fundamental domain is a half-space bounded by that plane
- for reflection in a point: an orbit is a set of 2 points, one on each side of the center, except for one orbit, consisting of the center only; the fundamental domain is a half-space bounded by any plane through the center
- for 180° rotation about a line: an orbit is either a set of 2 points opposite to each other with respect to the axis, or a single point on the axis; the fundamental domain is a half-space bounded by any plane through the line
- for discrete translational symmetry in one direction: the orbits are translates of a 1D lattice in the direction of the translation vector; the fundamental domain is an infinite slab
- for discrete translational symmetry in two directions: the orbits are translates of a 2D lattice in the plane through the translation vectors; the fundamental domain is an infinite bar with parallelogrammatic cross section
- for discrete translational symmetry in three directions: the orbits are translates of the lattice; the fundamental domain is a primitive cell which is e.g. a parallelepiped, or a Wigner-Seitz cell, also called Voronoi cell/diagram.

In the case of translational symmetry combined with other symmetries, the fundamental domain is part of the primitive cell. For example, for wallpaper groups the fundamental domain is a factor 1, 2, 3, 4, 6, 8, or 12 smaller than the primitive cell.

The diagram to the right shows part of the construction of the fundamental domain for the action of the modular group Γ on the upper half-plane *H*.

This famous diagram appears in all classical books on modular functions. (It was probably well known to C. F. Gauss, who dealt with fundamental domains in the guise of the reduction theory of quadratic forms.) Here, each triangular region (bounded by the blue lines) is a free regular set of the action of Γ on *H*. The boundaries (the blue lines) are not a part of the free regular sets. To construct a fundamental domain of *H*/Γ, one must also consider how to assign points on the boundary, being careful not to double-count such points. Thus, the free regular set in this example is

The fundamental domain is built by adding the boundary on the left plus half the arc on the bottom including the point in the middle:

The choice of which points of the boundary to include as a part of the fundamental domain is arbitrary, and varies from author to author.

The core difficulty of defining the fundamental domain lies not so much with the definition of the set *per se*, but rather with how to treat integrals over the fundamental domain, when integrating functions with poles and zeros on the boundary of the domain.

In mathematics, a **group action** on a space is a group homomorphism of a given group into the group of transformations of the space. Similarly, a group action on a mathematical structure is a group homomorphism of a group into the automorphism group of the structure. One says that the group *acts* on the space or structure. If a group acts on a structure, it also acts on everything that is built on the structure. For example, the group of Euclidean isometries acts on Euclidean space and also on the figures drawn in it. In particular, it acts on the set of all triangles. Similarly, the group of symmetries of a polyhedron acts on the vertices, the edges, and the faces of the polyhedron.

In group theory, the **symmetry group** of a geometric object is the group of all transformations under which the object is invariant, endowed with the group operation of composition. Such a transformation is an invertible mapping of the ambient space which takes the object to itself, and which preserves all the relevant structure of the object. A frequent notation for the symmetry group of an object *X* is *G* = Sym(*X*).

In crystallography, **crystal structure** is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter.

In mathematics, and more specifically algebraic topology, a **covering map** is a continuous function from a topological space to a topological space such that each point in has an open neighbourhood **evenly covered** by . In this case, is called a **covering space** and the **base space** of the covering projection. The definition implies that every covering map is a local homeomorphism.

In mathematics, the **modular group** is the projective special linear group PSL(2, **Z**) of 2 × 2 matrices with integer coefficients and unit determinant. The matrices *A* and −*A* are identified. The modular group acts on the upper-half of the complex plane by fractional linear transformations, and the name "modular group" comes from the relation to moduli spaces and not from modular arithmetic.

A **wallpaper group** is a mathematical classification of a two-dimensional repetitive pattern, based on the symmetries in the pattern. Such patterns occur frequently in architecture and decorative art, especially in textiles and tiles as well as wallpaper.

In mathematics, the **real projective plane** is an example of a compact non-orientable two-dimensional manifold; in other words, a one-sided surface. It cannot be embedded in standard three-dimensional space without intersecting itself. It has basic applications to geometry, since the common construction of the real projective plane is as the space of lines in **R**^{3} passing through the origin.

In crystallography, the terms **crystal system**, **crystal family**, and **lattice system** each refer to one of several classes of space groups, lattices, point groups, or crystals. Informally, two crystals are in the same crystal system if they have similar symmetries, although there are many exceptions to this.

In mathematics, physics and chemistry, a **space group** is the symmetry group of a configuration in space, usually in three dimensions. In three dimensions, there are 219 distinct types, or 230 if chiral copies are considered distinct. Space groups are also studied in dimensions other than 3 where they are sometimes called **Bieberbach groups**, and are discrete cocompact groups of isometries of an oriented Euclidean space.

In geometry and group theory, a **lattice** in is a subgroup of the additive group which is isomorphic to the additive group , and which spans the real vector space . In other words, for any basis of , the subgroup of all linear combinations with integer coefficients of the basis vectors forms a lattice. A lattice may be viewed as a regular tiling of a space by a primitive cell.

In geometry and crystallography, a **Bravais lattice**, named after Auguste Bravais (1850), is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by:

**Rotational symmetry**, also known as **radial symmetry** in biology, is the property a shape has when it looks the same after some rotation by a partial turn. An object's degree of rotational symmetry is the number of distinct orientations in which it looks exactly the same for each rotation.

In geometry, to translate a geometric figure is to move it from one place to another without rotating it. A translation "slides" a thing by **a**: *T*_{a}(**p**) = **p** + **a**.

In geometry, biology, mineralogy, and solid state physics, a **primitive cell** is a minimum-volume cell corresponding to a single lattice point of a structure with discrete translational symmetry. The concept is used particularly in describing crystal structure in two and three dimensions, though it makes sense in all dimensions. A lattice can be characterized by the geometry of its primitive cell.

The **Wigner–Seitz cell**, named after Eugene Wigner and Frederick Seitz, is a primitive cell which has been constructed by applying Voronoi decomposition to a crystal lattice. It is used in the study of crystalline materials in solid-state physics.

In mathematics, a **fundamental pair of periods** is an ordered pair of complex numbers that define a lattice in the complex plane. This type of lattice is the underlying object with which elliptic functions and modular forms are defined.

In mathematics, a **fundamental polygon** can be defined for every compact Riemann surface of genus greater than 0. It encodes not only the topology of the surface through its fundamental group but also determines the Riemann surface up to conformal equivalence. By the uniformization theorem, every compact Riemann surface has simply connected universal covering surface given by exactly one of the following:

A **one-dimensional symmetry group** is a mathematical group that describes symmetries in one dimension (1D).

In geometry, a **point group in three dimensions** is an isometry group in three dimensions that leaves the origin fixed, or correspondingly, an isometry group of a sphere. It is a subgroup of the orthogonal group O(3), the group of all isometries that leave the origin fixed, or correspondingly, the group of orthogonal matrices. O(3) itself is a subgroup of the Euclidean group E(3) of all isometries.

In geometry, **Hermann–Mauguin notation** is used to represent the symmetry elements in point groups, plane groups and space groups. It is named after the German crystallographer Carl Hermann and the French mineralogist Charles-Victor Mauguin. This notation is sometimes called **international notation**, because it was adopted as standard by the *International Tables For Crystallography* since their first edition in 1935.

This page is based on this Wikipedia article

Text is available under the CC BY-SA 4.0 license; additional terms may apply.

Images, videos and audio are available under their respective licenses.

Text is available under the CC BY-SA 4.0 license; additional terms may apply.

Images, videos and audio are available under their respective licenses.