In geometry, an nsided antiprism is a polyhedron composed of two parallel copies of some particular nsided polygon, connected by an alternating band of triangles. Antiprisms are a subclass of the prismatoids.
Antiprisms are similar to prisms except the bases are twisted relative to each other, and that the side faces are triangles, rather than quadrilaterals.
In the case of a regular nsided base, one usually considers the case where its copy is twisted by an angle 180°/n. Extra regularity is obtained by the line connecting the base centers being perpendicular to the base planes, making it a right antiprism. It has, apart from the base faces, 2n isosceles triangles as faces.
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Uniform antiprism
A uniform antiprism has, apart from the base faces, 2n equilateral triangles as faces. They form an infinite series of vertexuniform polyhedra, as do the uniform prisms. For n=2 we have as degenerate case the regular tetrahedron, and for n=3 the nondegenerate regular octahedron.
The dual polyhedra of the antiprisms are the trapezohedra. Their existence was first discussed and their name was coined by Johannes Kepler.
Cartesian coordinates
Cartesian coordinates for the vertices of a right antiprism with ngonal bases and isosceles triangles are
with k ranging from 0 to 2n1; if the triangles are equilateral,
Volume and surface area
Let a be the edgelength of a uniform antiprism. Then the volume is
and the surface area is
Symmetry
The symmetry group of a right nsided antiprism with regular base and isosceles side faces is D_{nd} of order 4n, except in the case of a tetrahedron, which has the larger symmetry group T_{d} of order 24, which has three versions of D_{2d} as subgroups, and the octahedron, which has the larger symmetry group O_{h} of order 48, which has four versions of D_{3d} as subgroups.
The symmetry group contains inversion if and only if n is odd.
The rotation group is D_{n} of order 2n, except in the case of a tetrahedron, which has the larger rotation group T of order 12, which has three versions of D_{2} as subgroups, and the octahedron, which has the larger rotation group O of order 24, which has four versions of D_{3} as subgroups.
See also
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