Cyclic symmetry in three dimensions

Template:3d point group navigator In three dimensional geometry, there are four infinite series of point groups in three dimensions (n≥1) with n-fold rotational or reflectional symmetry about one axis (by an angle of 360°/n) that does not change the object.

They are the finite symmetry groups on a cone. For n = ∞ they correspond to four frieze groups. Schönflies notation is used. The terms horizontal (h) and vertical (v) imply the existence and direction of reflections with respect to a vertical axis of symmetry. Also shown are Coxeter notation in brackets, and, in parentheses, orbifold notation.

File:Order 4 dihedral symmetry subgroup tree.png

Example symmetry subgroup tree for dihedral symmetry: D_4h, [4,2], (*224)

Types

Chiral

• C_n, [n]⁺, (nn) of order n - n-fold rotational symmetry - acro-n-gonal group (abstract group Z_n); for n=1: no symmetry (trivial group)

Achiral

File:S shaped packing.jpeg

Piece of loose-fill cushioning with C_2h symmetry

• C_nh, [n⁺,2], (n*) of order 2n - prismatic symmetry or ortho-n-gonal group (abstract group Z_n × Dih₁); for n=1 this is denoted by C_s (1*) and called reflection symmetry, also bilateral symmetry. It has reflection symmetry with respect to a plane perpendicular to the n-fold rotation axis.
• C_nv, [n], (*nn) of order 2n - pyramidal symmetry or full acro-n-gonal group (abstract group Dih_n); in biology C_2v is called biradial symmetry. For n=1 we have again C_s (1*). It has vertical mirror planes. This is the symmetry group for a regular n-sided pyramid.
• S_2n, [2⁺,2n⁺], (n×) of order 2n - gyro-n-gonal group (not to be confused with symmetric groups, for which the same notation is used; abstract group Z_2n); It has a 2n-fold rotoreflection axis, also called 2n-fold improper rotation axis, i.e., the symmetry group contains a combination of a reflection in the horizontal plane and a rotation by an angle 180°/n. Thus, like D_nd, it contains a number of improper rotations without containing the corresponding rotations.
  • for n=1 we have S₂ (1×), also denoted by C_i; this is inversion symmetry.

C_2h, [2,2⁺] (2*) and C_2v, [2], (*22) of order 4 are two of the three 3D symmetry group types with the Klein four-group as abstract group. C_2v applies e.g. for a rectangular tile with its top side different from its bottom side.

Frieze groups

In the limit these four groups represent Euclidean plane frieze groups as C_∞, C_∞h, C_∞v, and S_∞. Rotations become translations in the limit. Portions of the infinite plane can also be cut and connected into an infinite cylinder.

Frieze groups

Notations				Examples
IUC	Orbifold	Coxeter	Schönflies*	Euclidean plane	Cylindrical (n=6)
p1	∞∞	[∞]+	C∞	File:Frieze example p1.png	File:Uniaxial c6.png
p1m1	*∞∞	[∞]	C∞v	File:Frieze example p1m1.png	File:Uniaxial c6v.png
p11m	∞*	[∞+,2]	C∞h	File:Frieze example p11m.png	File:Uniaxial c6h.png
p11g	∞×	[∞+,2+]	S∞	File:Frieze example p11g.png	File:Uniaxial s6.png

Examples

S2/Ci (1x):	C4v (*44):		C5v (*55):
File:Parallelepiped.svg Parallelepiped	File:Square pyramid.png Square pyramid	File:Elongated square pyramid.png Elongated square pyramid	File:Pentagonal pyramid.png Pentagonal pyramid

See also

• Dihedral symmetry in three dimensions

References

• On Quaternions and Octonions, 2003, John Horton Conway and Derek A. Smith Template:ISBN
• The Symmetries of Things 2008, John H. Conway, Heidi Burgiel, Chaim Goodman-Strauss, Template:ISBN
• Kaleidoscopes: Selected Writings of H.S.M. Coxeter, edited by F. Arthur Sherk, Peter McMullen, Anthony C. Thompson, Asia Ivic Weiss, Wiley-Interscience Publication, 1995, Template:ISBN [1]
• N.W. Johnson: Geometries and Transformations, (2018) Template:ISBN Chapter 11: Finite symmetry groups, 11.5 Spherical Coxeter groups