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Symmetry Group s of objects are isometry groups. Accordingly, analysis of isometry groups is analysis of possible Symmetries . All isometries of a bounded 3D object have one or more common fixed points. We choose the origin as one of them. The symmetry group of an object is sometimes also called full symmetry group, as opposed to its '''rotation group''' or '''proper symmetry group''', the intersection of its full symmetry group and the Rotation Group SO(3) of the 3D space itself. The rotation group of an object is equal to its full symmetry group Iff the object is Chiral . GROUP STRUCTURE SO(3) is a subgroup of ''E''+(3), which consists of ''direct'' isometries, i.e., isometries preserving Orientation ; it contains those which leave the origin fixed. O(3) is the Direct Product of SO(3) and the group generated by Inversion (denoted by its matrix −''I''): :O(3) = SO(3) × { ''I'' , −''I'' } Thus there is a 1-to-1 correspondence between all direct isometries and all indirect isometries, through inversion. Also there is a 1-to-1 correspondence between all groups of direct isometries ''H'' and all groups ''K'' of isometries which contain inversion: K H If a group of direct isometries ''H'' has a subgroup ''L'' of index 2, then, apart from the corresponding group containing inversion there is also a corresponding group that contains indirect isometries but no inversion: M where isometry ( ''A'' , ''I'' ) is identified with ''A''. Thus ''M'' is obtained from ''H'' by inverting the isometries in ''H'' \ ''L''. This group ''M'' is as abstract group isomorphic with ''H''. Conversely, for all isometry groups which contain indirect isometries but no inversion we can obtain a rotation group by inverting the indirect isometries. This is clarifying when categorizing isometry groups, see below. In 2D the Cyclic Group of ''k''-fold Rotation s ''Ck'' is for every positive integer ''k'' a normal subgroup of O(2,R) and SO(2,R). Accordingly, in 3D, for every axis the cyclic group of ''k''-fold rotations about that axis is a normal subgroup of the group of all rotations about that axis, and also of the group obtained by adding reflections in planes through the axis. 3D ISOMETRIES WHICH LEAVE THE ORIGIN FIXED The isometries of R''3'' which leave the origin fixed, forming the group O(''3'',R), can be categorized as follows:
The 4th and 5th in particular, and in a wider sense the 6th also, are called Improper Rotation s. See also the similar Overview Including Translations . CONJUGACY When comparing the symmetry type of two objects, the origin is chosen for each separately, i.e. they need not have the same center. Moreover, two objects are considered to be of the same symmetry type if their symmetry groups are conjugate subgroups of O(3) (two subgroups ''H''1, ''H''2 of a group ''G'' are ''conjugate'' , if there exists ''g'' ∈ ''G'' such that ''H''1=g-1''H''2''g'' ). Thus two 3D objects have the same symmetry type:
In the case of multiple mirror planes and/or axes of rotation, two symmetry groups or of the same symmetry type Iff there is a single rotation mapping this whole structure of the first symmetry group to that of the second. The conjugacy definition would also allow a mirror image of the structure, but this is not needed, the structure itself is achiral. For example, if a symmetry group contains a 3-fold axis of rotation, it contains rotations in two opposite directions. (The structure ''is'' chiral for 11 pairs of Space Group s with a screw axis.) INFINITE ISOMETRY GROUPS We restrict ourselves to isometry groups which are Closed as Topological Subgroup of O(3). This excludes for example the group of rotations by an irrational number of turns about an axis. The whole O(3) is the symmetry group of spherical symmetry; SO(3) is the corresponding Rotation Group . The other infinite isometry groups consist of all Rotation s about an axis through the origin, and those with additionally reflection in the planes through the axis, and/or reflection in the plane through the origin, perpendicular to the axis. Those with reflection in the planes through the axis, with or without reflection in the plane through the origin, perpendicular to the axis, are the symmetry groups for the two types of cylindrical symmetry. See also Rotational Symmetry With Respect To Any Angle . FINITE ISOMETRY GROUPS For point groups, being finite corresponds to being Discrete ; infinite discrete groups as in the case of Translational Symmetry and Glide Reflection al symmetry do not apply. Symmetries in 3D that leave the origin fixed are fully characterized by symmetries on a sphere centered at the origin. For finite 3D point groups, see also Spherical Symmetry Groups . Up to conjugacy the set of finite 3D point groups consists of:
A selection of point groups is compatible with discrete . THE SEVEN INFINITE SERIES The infinite series have an index ''n'', which can be any integer; in each series, the ''n''th symmetry group contains ''n''-fold Rotational Symmetry about an axis, i.e. symmetry with respect to a rotation by an angle 360°/''n''. ''n''=1 covers the cases of no rotational symmetry at all. There are four series with no other axes of rotational symmetry, see Cyclic Symmetries , and three with additional axes of 2-fold symmetry, see ''' Dihedral Symmetry '''. For ''n'' = they correspond to the Frieze Group s. Schönflies notation is used, and, in parentheses, Conway's Orbifold Notation ; the latter is not only conveniently related to its properties, but also to the order of the group, see below; it is a unified notation, also applicable for Wallpaper Group s and Frieze Group s. The 7 infinite series are:
The terms horizontal (h) and vertical (v) are used with respect to a vertical axis of rotation. Involution al symmetry (abstract group Z2 ):
The second of these is the first of the uniaxial groups ( Cyclic Group s) ''Cn'' of order ''n'' (also applicable in 2D), which are generated by a single rotation of angle 360°/''n''. In addition to this, one may add a mirror plane perpendicular to the axis, giving the group ''Cnh'' of order 2''n'', or a set of ''n'' mirror planes containing the axis, giving the group ''Cnv'', also of order 2''n''. The latter is the symmetry group for a regular ''n''-sided Pyramid . If both horizontal and vertical reflection planes are added, their intersections give ''n'' axes of rotation through 180°, so the group is no longer uniaxial. This new group of order 4''n'' is called ''Dnh''. Its subgroup of rotations is the Dihedral Group ''Dn'' of order 2''n'' which still has the 2-fold rotation axes perpendicular to the primary rotation axis, but no mirror planes. Note that in 2D ''Dn'' includes reflections, which can also be viewed as flipping over flat objects without distinction of front- and backside, but in 3D the two operations are distinguished: the group contains "flipping over", not reflections. There is one more group in this family, called ''Dnd'' (or ''Dnv''), which has vertical mirror planes containing the main rotation axis, but instead of having a horizontal mirror plane it has an isometry which is the combination of a reflection in the horizontal plane and a rotation by an angle 180°/''n''. ''Dnh'' is the symmetry group for a regular ''n''-sided Prisms and also for a regular n-sided Bipyramid . ''Dnd'' is the symmetry group for a regular ''n''-sided Antiprism , and also for a regular n-sided Trapezohedron . ''Dn'' is the symmetry group of a partially rotated prism. ''Sn'' is generated by the combination of a reflection in the horizontal plane and a rotation by an angle 360°/n. For ''n'' odd this is equal to the group generated by the two separately, ''Cnh'' of order 2''n'', and therefore the notation ''Sn'' is not needed; however, for ''n'' even it is distinct, and of order ''n''. Like ''Dnd'' it contains a number of Improper Rotation s without containing the corresponding rotations. All symmetry groups in the 7 infinite series are different, except for the following four pairs of mutually equal ones:
''S2'' is the group of order 2 with a single inversion (''Ci'' ) "Equal" is meant here as the same up to conjugacy in space. This is stronger than "up to algebraic isomorphism". For example, there are three different groups of order two in the first sense, but there is only one in the second sense. Similarly, e.g. ''S2n'' is algebraically isomorphic with Z''2n''. THE SEVEN REMAINING POINT GROUPS The remaining point groups are said to be of very high or Polyhedral symmetry because they have more than one rotation axis of order greater than 2. Using Cn to denote an axis of rotation through 360°/n and Sn to denote an axis of improper rotation through the same, the groups are:
RELATION BETWEEN ORBIFOLD NOTATION AND ORDER The order of each group is 2 divided by the Orbifold Euler Characteristic ; the latter is 2 minus the sum of the feature values, assigned as follows:
This can also be applied for ROTATION GROUPS The rotation groups, i.e. the finite subgroups of SO(3), are: the cyclic groups ''C''n (the rotation group of a regular Pyramid ), the dihedral groups ''D''n (the rotation group of a regular Prism , or regular Bipyramid ), and the rotation groups ''T'', ''O'' and ''I'' of a regular Tetrahedron , Octahedron / Cube and Icosahedron / Dodecahedron . In particular, the dihedral groups ''D''3, ''D''4 etc. are the rotation groups of plane regular polygons embedded in three-dimensional space, and such a figure may be considered as a degenerate regular prism. Therefore it is also called a ''dihedron'' (Greek: solid with two faces), which explains the name ''dihedral group''.
The rotation group of an object is equal to its full symmetry group Iff the object is Chiral . In other words, the chiral objects are those with their symmetry group in the list of rotation groups. CORRESPONDENCE BETWEEN ROTATION GROUPS AND OTHER GROUPS The following groups contain Inversion :
As explained above, there is a 1-to-1 correspondence between these groups and all rotation groups:
The other groups contain indirect isometries, but not inversion:
They all correspond to a rotation group ''H'' and a subgroup ''L'' of index 2 in the sense that they are obtained from ''H'' by inverting the isometries in ''H'' \ ''L'', as explained above:
MAXIMAL SYMMETRIES There are two discrete point groups with the property that no discrete point group has it as proper subgroup: ''Oh'' and ''Ih''. Their largest common subgroup is ''Th''. The two groups are obtained from it by changing 2-fold rotational symmetry to 4-fold, and adding 5-fold symmetry, respectively. Alternatively the two groups are generated by adding for each a reflection plane to ''Th''. There are two crystallographic point groups with the property that no crystallographic point group has it as proper subgroup: ''Oh'' and ''D6h''. Their maximal common subgroups, depending on orientation, are ''D3d'' and ''D2h''. THE GROUPS ARRANGED BY ABSTRACT GROUP TYPE Below the groups explained above are arranged by abstract group type. The smallest abstract groups which are ''not'' any symmetry group in 3D, are the Quaternion Group (of order 8), the Dicyclic Group Dic3 (of order 12), and 10 of the 14 groups of order 16. The column "# of order 2 elements" in the following tables shows the total number of isometry subgroups of types ''C2'' , ''Ci'' , ''Cs''. This total number is one of the characteristics helping to distinguish the various abstract group types, while their isometry type helps to distinguish the various isometry groups of the same abstract group. Within the possibilities of isometry groups in 3D, there are infinitely many abstract group types with 0, 1 and 3 elements of order 2, there are two with 2''n'' + 1 elements of order 2, and there are three with 2''n'' + 3 elements of order 2 (for each ''n'' ≥ 2 ). There is never a positive even number of elements of order 2. Symmetry groups in 3D which are cyclic as abstract group The Symmetry Group for ''n''-fold rotational Symmetry is ''Cn''; its abstract group type is Cyclic Group Zn , which is also denoted by ''Cn''. However, there are two more infinite series of symmetry groups with this abstract group type:
Thus we have, with bolding of the 10 cyclic crystallographic point groups, for which the Crystallographic Restriction applies: etc. Symmetry groups in 3D which are dihedral as abstract group In 2D Dihedral Group ''Dn'' includes reflections, which can also be viewed as flipping over flat objects without distinction of front- and backside. However, in 3D the two operations are distinguished: the symmetry group denoted by ''Dn'' contains ''n'' 2-fold axes perpendicular to the ''n''-fold axis, not reflections. ''Dn'' is the Rotation Group of the ''n''-sided Prism with regular base, and ''n''-sided Bipyramid with regular base, and also of a regular, ''n''-sided Antiprism and of a regular, ''n''-sided Trapezohedron . The group is also the full symmetry group of such objects after making them Chiral by e.g. an identical chiral marking on every face, or some modification in the shape. The abstract group type is Dihedral Group Dih''n'', which is also denoted by ''Dn''. However, there are three more infinite series of symmetry groups with this abstract group type:
Note the following property: :Dih''4n+2'' Dih''2n+1'' × Z2 Thus we have, with bolding of the 12 crystallographic point groups, and writing ''D1d'' as the equivalent ''C2h'': etc. Other ''C2n,h'' of order 4''n'' is of abstract group type Z2''n'' × Z2. For ''n'' = 1 we get Dih2 , already covered above, so ''n'' ≥ 2. Thus we have, with bolding of the 2 cyclic crystallographic point groups: etc. ''Dnh'' of order 4''n'' is of abstract group type Dihn × Z2. For odd ''n'' this is already covered above, so we have here ''D''2''nh'' of order 8''n'', which is of abstract group type Dih2''n'' × Z2 (''n''≥1). Thus we have, with bolding of the 3 dihedral crystallographic point groups: etc. The remaining seven are, with bolding of the 5 crystallographic point groups (see also above):
See also Icosahedral Symmetry . IMPOSSIBLE DISCRETE SYMMETRIES Since the overview is exhaustive, it also shows implicitly what is ''not'' possible as discrete symmetry group. For example:
etc. EXAMPLES A typical object with symmetry group ''C''n or ''D''n is a Propellor . FUNDAMENTAL DOMAIN The Fundamental Domain of a point group is a Conic Solid . An object with a given symmetry in a given orientation is characterized by the fundamental domain. If the object is a surface it is characterized by a surface in the fundamental domain continuing to its radial bordal faces or surface. If the copies of the surface do not fit, radial faces or surfaces can be added. They fit anyway if the fundamental domain is bounded by reflection planes. For a polyhedron this surface in the fundamental domain can be part of an arbitrary plane. For example, in the Disdyakis Triacontahedron one full face is a fundamental domain. Adjusting the orientation of the plane gives various possibilities of combining two or more adjacent faces to one, giving various other polyhedra with the same symmetry. The polyhedron is convex if the surface fits to its copies and the radial line perpendicular to the plane is in the fundamental domain. Also the surface in the fundamental domain may be composed of multiple faces. SEE ALSO
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