Dihedral symmetry in three dimensions


Dihedral symmetry in three dimensions

This article deals with three infinite sequences of point groups in three dimensions which have a symmetry group that as abstract group is a dihedral group Dihn ( n ≥ 2 ).

See also point groups in two dimensions.

Chiral:

  • Dn (22n) of order 2ndihedral symmetry (abstract group Dn)

Achiral:

  • Dnh (*22n) of order 4nprismatic symmetry (abstract group Dn × C2)
  • Dnd (or Dnv) (2*n) of order 4nantiprismatic symmetry (abstract group D2n)

For a given n, all three have n-fold rotational symmetry about one axis (rotation by an angle of 360°/n does not change the object), and 2-fold about a perpendicular axis, hence about n of those. For n = ∞ they correspond to three frieze groups. Schönflies notation is used, and, in parentheses, Orbifold notation. The term horizontal (h) is used with respect to a vertical axis of rotation.

In 2D the symmetry group Dn includes reflections in lines. When the 2D plane is embedded horizontally in a 3D space, such a reflection can either be viewed as the restriction to that plane of a reflection in a vertical plane, or as the restriction to the plane of a rotation about the reflection line, by 180°. In 3D the two operations are distinguished: the group Dn contains rotations only, not reflections. The other group is pyramidal symmetry Cnv of the same order.

With reflection symmetry with respect to a plane perpendicular to the n-fold rotation axis we have Dnh (*22n).

Dnd (or Dnv) has vertical mirror planes between the horizontal rotation axes, not through them. As a result the vertical axis is a 2n-fold rotoreflection axis.

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.

n = 1 is not included because the three symmetries are equal to other ones:

  • D1 and C2: group of order 2 with a single 180° rotation
  • D1h and C2v: group of order 4 with a reflection in a plane and a 180° rotation through a line in that plane
  • D1d and C2h: group of order 4 with a reflection in a plane and a 180° rotation through a line perpendicular to that plane

For n = 2 there is not one main axes and two additional axes, but there are three equivalent ones.

  • D2 (222) of order 4 is one of the three symmetry group types with the Klein four-group as abstract group. It has three perpendicular 2-fold rotation axes. It is the symmetry group of a cuboid with an S written on two opposite faces, in the same orientation.
  • D2h (*222) of order 8 is the symmetry group of a cuboid
  • D2d (2*2) of order 8 is the symmetry group of e.g.:
    • a square cuboid with a diagonal drawn on one square face, and a perpendicular diagonal on the other one
    • a regular tetrahedron scaled in the direction of a line connecting the midpoints of two opposite edges (D2d is a subgroup of Td, by scaling we reduce the symmetry).

Subgroups

For Dnh

  • Cnh
  • Cnv
  • Dn

For Dnd

  • S2n
  • Cnv
  • Dn

Dnd is also subgroup of D2nh.

See also cyclic symmetries

Examples

Dnh (*22n):

Geometricprisms.gif
prisms

D5h (*225):

Pentagrammic prism.png
Pentagrammic prism
Pentagrammic antiprism.png
Pentagrammic antiprism

D4d (2*4):

Snub square antiprism.png
Snub square antiprism

D5d (2*5):

Antiprism5.jpg
Pentagonal antiprism
Pentagrammic crossed antiprism.png
Pentagrammic crossed-antiprism
Trapezohedron5.jpg
pentagonal trapezohedron

D17d (*22(17)):

Antiprism17.jpg
Heptadecagonal antiprism

See also


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