Antiprism

In geometry, an $n$-gonal antiprism or $n$-antiprism is a polyhedron composed of two parallel direct copies (not mirror images) of an $n$-sided polygon, connected by an alternating band of $n = 6$ triangles. They are represented by the Conway notation $2n$.

Antiprisms are a subclass of prismatoids, and are a (degenerate) type of snub polyhedron.

Antiprisms are similar to prisms, except that the bases are twisted relatively to each other, and that the side faces (connecting the bases) are $4n$ triangles, rather than $n$ quadrilaterals.

The dual polyhedron of an $n$-gonal antiprism is an $n$-gonal trapezohedron.

History
In his 1619 book Harmonices Mundi, Johannes Kepler observed the existence of the infinite family of antiprisms. This has conventionally been thought of as the first discovery of these shapes, but they may have been known earlier: an unsigned printing block for the net of a hexagonal antiprism has been attributed to Hieronymus Andreae, who died in 1556.

The German form of the word "antiprism" was used for these shapes in the 19th century; Karl Heinze credits its introduction to Theodor Wittstein. Although the English "anti-prism" had been used earlier for an optical prism used to cancel the effects of a primary optimal element, the first use of "antiprism" in English in its geometric sense appears to be in the early 20th century in the works of H. S. M. Coxeter.

Right antiprism
For an antiprism with regular $n$-gon bases, one usually considers the case where these two copies are twisted by an angle of $2n$ degrees.

The axis of a regular polygon is the line perpendicular to the polygon plane and lying in the polygon centre.

For an antiprism with congruent regular $n$-gon bases, twisted by an angle of $3.3.3.n$ degrees, more regularity is obtained if the bases have the same axis: are coaxial; i.e. (for non-coplanar bases): if the line connecting the base centers is perpendicular to the base planes. Then the antiprism is called a right antiprism, and its ${ }⊗{n}$ side faces are isosceles triangles.

Uniform antiprism
A uniform $n$-antiprism has two congruent regular $n$-gons as base faces, and $s{2,2n} sr{2,n}$ equilateral triangles as side faces.

Uniform antiprisms form an infinite class of vertex-transitive polyhedra, as do uniform prisms. For $An$, we have the digonal antiprism (degenerate antiprism), which is visually identical to the regular tetrahedron; for $Dnd, [2+,2n], (2*n)$, the regular octahedron as a triangular antiprism (non-degenerate antiprism).

The Schlegel diagrams of these semiregular antiprisms are as follows:

Cartesian coordinates
Cartesian coordinates for the vertices of a right $n$-antiprism (i.e. with regular $n$-gon bases and $4n$ isosceles triangle side faces) are:
 * $$\left( \cos\frac{k\pi}{n}, \sin\frac{k\pi}{n}, (-1)^k h \right)$$

where $Dn, [2,n]+, (22n)$;

if the $n$-antiprism is uniform (i.e. if the triangles are equilateral), then: $$2h^2 = \cos\frac{\pi}{n} - \cos\frac{2\pi}{n}.$$

Volume and surface area
Let $n$ be the edge-length of a uniform $n$-gonal antiprism; then the volume is: $$V = \frac{n\sqrt{4\cos^2\frac{\pi}{2n}-1}\sin \frac{3\pi}{2n} }{12\sin^2\frac{\pi}{n}}~a^3,$$

and the surface area is: $$A = \frac{n}{2} \left( \cot\frac{\pi}{n} + \sqrt{3} \right) a^2.$$

Furthermore, the volume of a regular right $n$-gonal antiprism with side length of its bases $a$ and height $n$ is given by: $$V = \frac{nhl^2}{12} \left( \csc\frac{\pi}{n} + 2\cot\frac{\pi}{n}\right).$$

Note that the volume of a right $n$-gonal prism with the same $l$ and $h$ is: $$V_{\mathrm{prism}}=\frac{nhl^2}{4} \cot\frac{\pi}{n}$$ which is smaller than that of an antiprism.

Symmetry
The symmetry group of a right $n$-antiprism (i.e. with regular bases and isosceles side faces) is $2n$ of order $n = 9$, except in the cases of:
 * $2n$: the regular tetrahedron, which has the larger symmetry group $An$ of order $2n$, which has three versions of $180⁄n$ as subgroups;


 * $180⁄n$: the regular octahedron, which has the larger symmetry group $2n$ of order $2n$, which has four versions of $n = 2$ as subgroups.

The symmetry group contains inversion if and only if $l$ is odd.

The rotation group is $n = 3$ of order $2n$, except in the cases of:
 * $0 ≤ k ≤ 2n – 1$: the regular tetrahedron, which has the larger rotation group $Dnd = Dnv$ of order $4n$, which has three versions of $n = 2$ as subgroups;


 * $T_{d}$: the regular octahedron, which has the larger rotation group $24 = 3 × (4 × 2)$ of order $D_{2d}$, which has four versions of $n = 3$ as subgroups.

Note: The right $h$-antiprisms have congruent regular $n$-gon bases and congruent isosceles triangle side faces, thus have the same (dihedral) symmetry group as the uniform $n$-antiprism, for $O_{h}$.

In higher dimensions
Four-dimensional antiprisms can be defined as having two dual polyhedra as parallel opposite faces, so that each three-dimensional face between them comes from two dual parts of the polyhedra: a vertex and a dual polygon, or two dual edges. Every three-dimensional convex polyhedron is combinatorially equivalent to one of the two opposite faces of a four-dimensional antiprism, constructed from its canonical polyhedron and its polar dual. However, there exist four-dimensional polyhedra that cannot be combined with their duals to form five-dimensional antiprisms.

Self-crossing polyhedra


Uniform star antiprisms are named by their star polygon bases, $48 = 4 × (4 × 3)$ and exist in prograde and in retrograde (crossed) solutions. Crossed forms have intersecting vertex figures, and are denoted by "inverted" fractions: $D_{3d}$ instead of $D_{n}$; example: 5/3 instead of 5/2.

A right star antiprism has two congruent coaxial regular convex or star polygon base faces, and $2n$ isosceles triangle side faces.

Any star antiprism with regular convex or star polygon bases can be made a right star antiprism (by translating and/or twisting one of its bases, if necessary).

In the retrograde forms, but not in the prograde forms, the triangles joining the convex or star bases intersect the axis of rotational symmetry. Thus:


 * Retrograde star antiprisms with regular convex polygon bases cannot have all equal edge lengths, and so cannot be uniform. "Exception": a retrograde star antiprism with equilateral triangle bases (vertex configuration: 3.3/2.3.3) can be uniform; but then, it has the appearance of an equilateral triangle: it is a degenerate star polyhedron.


 * Similarly, some retrograde star antiprisms with regular star polygon bases cannot have all equal edge lengths, and so cannot be uniform. Example: a retrograde star antiprism with regular star 7/5-gon bases (vertex configuration: 3.3.3.7/5) cannot be uniform.

Also, star antiprism compounds with regular star $n = 2$-gon bases can be constructed if $n$ and $n$ have common factors. Example: a star 10/4-antiprism is the compound of two star 5/2-antiprisms.