Set of regular n-gonal hosohedra | |
---|---|

Type | regular polyhedron or spherical tiling |

Faces | n digons |

Edges | n |

Vertices | 2 |

Euler char. | 2 |

Vertex configuration | 2^{n} |

Wythoff symbol | n | 2 2 |

Schläfli symbol | {2,n} |

Coxeter diagram | |

Symmetry group | D_{nh}[2,n] (*22n) order 4 n |

Rotation group | D_{n}[2,n] ^{+}(22n) order 2 n |

Dual polyhedron | regular n-gonal dihedron |

In spherical geometry, an n-gonal **hosohedron** is a tessellation of lunes on a spherical surface, such that each lune shares the same two polar opposite vertices.

- Hosohedra as regular polyhedra
- Kaleidoscopic symmetry
- Relationship with the Steinmetz solid
- Derivative polyhedra
- Apeirogonal hosohedron
- Hosotopes
- Etymology
- See also
- References
- External links

A regular n-gonal hosohedron has Schläfli symbol {2,*n*}, with each spherical lune having internal angle 2π/*n* radians (360/*n* degrees).^{ [1] }^{ [2] }

For a regular polyhedron whose Schläfli symbol is {*m*, *n*}, the number of polygonal faces is :

The Platonic solids known to antiquity are the only integer solutions for *m* ≥ 3 and *n* ≥ 3. The restriction *m* ≥ 3 enforces that the polygonal faces must have at least three sides.

When considering polyhedra as a spherical tiling, this restriction may be relaxed, since digons (2-gons) can be represented as spherical lunes, having non-zero area.

Allowing *m* = 2 makes

and admits a new infinite class of regular polyhedra, which are the hosohedra. On a spherical surface, the polyhedron {2, *n*} is represented as *n* abutting lunes, with interior angles of 2π/*n*. All these spherical lunes share two common vertices.

Space | Spherical | Euclidean | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

Tiling name | (Monogonal) Henagonal hosohedron | Digonal hosohedron | (Triangular) Trigonal hosohedron | (Tetragonal) Square hosohedron | Pentagonal hosohedron | Hexagonal hosohedron | Heptagonal hosohedron | Octagonal hosohedron | Enneagonal hosohedron | Decagonal hosohedron | Hendecagonal hosohedron | Dodecagonal hosohedron | ... | Apeirogonal hosohedron |

Tiling image | ... | |||||||||||||

Schläfli symbol | {2,1} | {2,2} | {2,3} | {2,4} | {2,5} | {2,6} | {2,7} | {2,8} | {2,9} | {2,10} | {2,11} | {2,12} | ... | {2,∞} |

Coxeter diagram | ... | |||||||||||||

Faces and edges | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | ... | ∞ |

Vertices | 2 | ... | 2 | |||||||||||

Vertex config. | 2 | 2.2 | 2^{3} | 2^{4} | 2^{5} | 2^{6} | 2^{7} | 2^{8} | 2^{9} | 2^{10} | 2^{11} | 2^{12} | ... | 2^{∞} |

The digonal spherical lune faces of a -hosohedron, , represent the fundamental domains of dihedral symmetry in three dimensions: the cyclic symmetry , , , order . The reflection domains can be shown by alternately colored lunes as mirror images.

Bisecting each lune into two spherical triangles creates an -gonal bipyramid, which represents the dihedral symmetry , order .

Symmetry (order ) | Schönflies notation | |||||||
---|---|---|---|---|---|---|---|---|

Orbifold notation | ||||||||

Coxeter diagram | ||||||||

-gonal hosohedron | Schläfli symbol | |||||||

Alternately colored fundamental domains |

The tetragonal hosohedron is topologically equivalent to the bicylinder Steinmetz solid, the intersection of two cylinders at right-angles.^{ [3] }

The dual of the n-gonal hosohedron {2, *n*} is the *n*-gonal dihedron, {*n*, 2}. The polyhedron {2,2} is self-dual, and is both a hosohedron and a dihedron.

A hosohedron may be modified in the same manner as the other polyhedra to produce a truncated variation. The truncated *n*-gonal hosohedron is the n-gonal prism.

In the limit, the hosohedron becomes an apeirogonal hosohedron as a 2-dimensional tessellation:

Multidimensional analogues in general are called **hosotopes**. A regular hosotope with Schläfli symbol {2,*p*,...,*q*} has two vertices, each with a vertex figure {*p*,...,*q*}.

The two-dimensional hosotope, {2}, is a digon.

The term “hosohedron” appears to derive from the Greek ὅσος (*hosos*) “as many”, the idea being that a hosohedron can have “**as many** faces as desired”.^{ [4] } It was introduced by Vito Caravelli in the eighteenth century.^{ [5] }

Wikimedia Commons has media related to Hosohedra .

In geometry, a **regular icosahedron** is a convex polyhedron with 20 faces, 30 edges and 12 vertices. It is one of the five Platonic solids, and the one with the most faces.

In geometry, an **octahedron** is a polyhedron with eight faces. The term is most commonly used to refer to the **regular** octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex.

In elementary geometry, a **polytope** is a geometric object with flat sides (*faces*). Polytopes are the generalization of three-dimensional polyhedra to any number of dimensions. Polytopes may exist in any general number of dimensions n as an n-dimensional polytope or **n-polytope**. For example, a two-dimensional polygon is a 2-polytope and a three-dimensional polyhedron is a 3-polytope. In this context, "flat sides" means that the sides of a (*k* + 1)-polytope consist of k-polytopes that may have (*k* – 1)-polytopes in common.

In geometry, a **prism** is a polyhedron comprising an n-sided polygon base, a second base which is a translated copy of the first, and n other faces, necessarily all parallelograms, joining corresponding sides of the two bases. All cross-sections parallel to the bases are translations of the bases. Prisms are named after their bases, e.g. a prism with a pentagonal base is called a pentagonal prism. Prisms are a subclass of prismatoids.

A **regular polyhedron** is a polyhedron whose symmetry group acts transitively on its flags. A regular polyhedron is highly symmetrical, being all of edge-transitive, vertex-transitive and face-transitive. In classical contexts, many different equivalent definitions are used; a common one is that the faces are congruent regular polygons which are assembled in the same way around each vertex.

In Euclidean geometry, a **regular polygon** is a polygon that is direct equiangular and equilateral. Regular polygons may be either **convex**, **star** or **skew**. In the limit, a sequence of regular polygons with an increasing number of sides approximates a circle, if the perimeter or area is fixed, or a regular apeirogon, if the edge length is fixed.

In geometry, the **Schläfli symbol** is a notation of the form that defines regular polytopes and tessellations.

In geometry, a polytope or a tiling is **isogonal** or **vertex-transitive** if all its vertices are equivalent under the symmetries of the figure. This implies that each vertex is surrounded by the same kinds of face in the same or reverse order, and with the same angles between corresponding faces.

In geometry, a **uniform polyhedron** has regular polygons as faces and is vertex-transitive. It follows that all vertices are congruent.

In geometry, a **digon** is a polygon with two sides (edges) and two vertices. Its construction is degenerate in a Euclidean plane because either the two sides would coincide or one or both would have to be curved; however, it can be easily visualised in elliptic space.

A **dihedron** is a type of polyhedron, made of two polygon faces which share the same set of *n* edges. In three-dimensional Euclidean space, it is degenerate if its faces are flat, while in three-dimensional spherical space, a dihedron with flat faces can be thought of as a lens, an example of which is the fundamental domain of a lens space L(*p*,*q*). Dihedra have also been called **bihedra**, **flat polyhedra**, or **doubly covered polygons**.

In geometry, a **star polyhedron** is a polyhedron which has some repetitive quality of nonconvexity giving it a star-like visual quality.

In geometry, a **snub** is an operation applied to a polyhedron. The term originates from Kepler's names of two Archimedean solids, for the snub cube and snub dodecahedron. In general, snubs have chiral symmetry with two forms: with clockwise or counterclockwise orientation. By Kepler's names, a snub can be seen as an expansion of a regular polyhedron: moving the faces apart, twisting them about their centers, adding new polygons centered on the original vertices, and adding pairs of triangles fitting between the original edges.

In geometry, a **skew apeirohedron** is an infinite skew polyhedron consisting of nonplanar faces or nonplanar vertex figures, allowing the figure to extend indefinitely without folding round to form a closed surface.

In geometry, a **spherical polyhedron** or **spherical tiling** is a tiling of the sphere in which the surface is divided or partitioned by great arcs into bounded regions called spherical polygons. Much of the theory of symmetrical polyhedra is most conveniently derived in this way.

In mathematics, a **regular 4-polytope** is a regular four-dimensional polytope. They are the four-dimensional analogues of the regular polyhedra in three dimensions and the regular polygons in two dimensions.

In spherical geometry, a **spherical lune** is an area on a sphere bounded by two half great circles which meet at antipodal points. It is an example of a digon, {2}_{θ}, with dihedral angle θ. The word "lune" derives from *luna*, the Latin word for Moon.

In geometry, a (globally) **projective polyhedron** is a tessellation of the real projective plane. These are projective analogs of spherical polyhedra – tessellations of the sphere – and toroidal polyhedra – tessellations of the toroids.

In geometry, the **order-4 hexagonal tiling** is a regular tiling of the hyperbolic plane. It has Schläfli symbol of {6,4}.

- ↑ Coxeter,
*Regular polytopes*, p. 12 - ↑ Abstract Regular polytopes, p. 161
- ↑ Weisstein, Eric W. "Steinmetz Solid".
*MathWorld*. - ↑ Steven Schwartzman (1 January 1994).
*The Words of Mathematics: An Etymological Dictionary of Mathematical Terms Used in English*. MAA. pp. 108–109. ISBN 978-0-88385-511-9. - ↑ Coxeter, H.S.M. (1974).
*Regular Complex Polytopes*. London: Cambridge University Press. p. 20. ISBN 0-521-20125-X.The hosohedron {2,p} (in a slightly distorted form) was named by Vito Caravelli (1724–1800) …

- McMullen, Peter; Schulte, Egon (December 2002),
*Abstract Regular Polytopes*(1st ed.), Cambridge University Press, ISBN 0-521-81496-0 - Coxeter, H.S.M,
*Regular Polytopes*(third edition), Dover Publications Inc., ISBN 0-486-61480-8

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