Hosohedron

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

${\displaystyle N_{2}={\frac {4n}{2m+2n-mn}}.}$

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

${\displaystyle N_{2}={\frac {4n}{2\times 2+2n-2n}}=n,}$

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.

A regular trigonal hosohedron, {2,3}, represented as a tessellation of 3 spherical lunes on a sphere.

A regular tetragonal hosohedron, {2,4}, represented as a tessellation of 4 spherical lunes on a sphere.

Family of regular hosohedra · *n22 symmetry mutations of regular hosohedral tilings: nn

Space	Spherical						Euclidean
Tiling name	Henagonal hosohedron	Digonal hosohedron	Trigonal hosohedron	Square hosohedron	Pentagonal hosohedron	...	Apeirogonal hosohedron
Tiling image						...
Schläfli symbol	{2,1}	{2,2}	{2,3}	{2,4}	{2,5}	...	{2,∞}
Coxeter diagram						...
Faces and edges	1	2	3	4	5	...	∞
Vertices	2	2	2	2	2	...	2
Vertex config.	2	2.2	23	24	25	...	2∞

The ${\displaystyle 2n}$ digonal spherical lune faces of a ${\displaystyle 2n}$ -hosohedron, ${\displaystyle \{2,2n\}}$ , represent the fundamental domains of dihedral symmetry in three dimensions: the cyclic symmetry ${\displaystyle C_{nv}}$ , ${\displaystyle [n]}$ , ${\displaystyle (*nn)}$ , order ${\displaystyle 2n}$ . The reflection domains can be shown by alternately colored lunes as mirror images.

Bisecting each lune into two spherical triangles creates an ${\displaystyle n}$ -gonal bipyramid, which represents the dihedral symmetry ${\displaystyle D_{nh}}$ , order ${\displaystyle 4n}$ .

Different representations of the kaleidoscopic symmetry of certain small hosohedra

Symmetry (order ${\displaystyle 2n}$ )	Schönflies notation	${\displaystyle C_{nv}}$	${\displaystyle C_{1v}}$	${\displaystyle C_{2v}}$	${\displaystyle C_{3v}}$	${\displaystyle C_{4v}}$	${\displaystyle C_{5v}}$	${\displaystyle C_{6v}}$
	Orbifold notation	${\displaystyle (*nn)}$	${\displaystyle (*11)}$	${\displaystyle (*22)}$	${\displaystyle (*33)}$	${\displaystyle (*44)}$	${\displaystyle (*55)}$	${\displaystyle (*66)}$
	Coxeter diagram
		${\displaystyle [n]}$	${\displaystyle [\,\,]}$	${\displaystyle [2]}$	${\displaystyle [3]}$	${\displaystyle [4]}$	${\displaystyle [5]}$	${\displaystyle [6]}$
${\displaystyle 2n}$ -gonal hosohedron	Schläfli symbol	${\displaystyle \{2,2n\}}$	${\displaystyle \{2,2\}}$	${\displaystyle \{2,4\}}$	${\displaystyle \{2,6\}}$	${\displaystyle \{2,8\}}$	${\displaystyle \{2,10\}}$	${\displaystyle \{2,12\}}$
	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.

• Polyhedron
• Polytope

• 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

• Weisstein, Eric W. "Hosohedron". MathWorld.