Tetrahemihexahedron

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Tetrahemihexahedron
Tetrahemihexahedron
Type Uniform star polyhedron
Elements F = 7, E = 12
V = 6 (χ = 1)
Faces by sides 4{3}+3{4}
Wythoff symbol 3/2 3 | 2 (double-covering)
Symmetry group Td, [3,3], *332
Index references U04, C36, W67
Dual polyhedron Tetrahemihexacron
Vertex figure Tetrahemihexahedron
3.4.3/2.4
Bowers acronym Thah

In geometry, the tetrahemihexahedron or hemicuboctahedron is a uniform star polyhedron, indexed as U4. It has 6 vertices and 12 edges, and 7 faces: 4 triangular and 3 square. Its vertex figure is a crossed quadrilateral. Its Coxeter-Dynkin diagram is CDel node 1.pngCDel 3x.pngCDel rat.pngCDel d2.pngCDel node.pngCDel 3.pngCDel node 1.png (although this is a double covering of the tetrahemihexahedron).

It is the only non-prismatic uniform polyhedron with an odd number of faces. Its Wythoff symbol is 3/2 3 | 2, but actually that represents a double covering of the tetrahemihexahedron with 8 triangles and 6 squares, paired and coinciding in space. (It can more intuitively be seen as two coinciding tetrahemihexahedra.)

It is a hemipolyhedron. The "hemi" part of the name means some of the faces form a group with half as many members as some regular polyhedron—here, three square faces form a group with half as many faces as the regular hexahedron, better known as the cube—hence hemihexahedron. Hemi faces are also oriented in the same direction as the regular polyhedron's faces. The three square faces of the tetrahemihexahedron are, like the three facial orientations of the cube, mutually perpendicular.

The "half-as-many" characteristic also means that hemi faces must pass through the center of the polyhedron, where they all intersect each other. Visually, each square is divided into four right triangles, with two visible from each side.

It is the three-dimensional demicross polytope.

Related surfaces[edit]

It is a non-orientable surface. It is unique as the only uniform polyhedron with an Euler characteristic of 1 and is hence a projective polyhedron, yielding a representation of the real projective plane[1] very similar to the Roman surface.

RomanSurfaceTopView.PNG
Roman surface

Related polyhedra[edit]

It has the same vertices and edges as the regular octahedron. It also shares 4 of the 8 triangular faces of the octahedron, but has three additional square faces passing through the centre of the polyhedron.

Octahedron.png
Octahedron
Tetrahemihexahedron.png
Tetrahemihexahedron

The dual figure is the tetrahemihexacron.

It is 2-covered by the cuboctahedron,[1] which accordingly has the same abstract vertex figure (2 triangles and two squares: 3.4.3.4) and twice the vertices, edges, and faces.

Cuboctahedron.png
Cuboctahedron
Tetrahemihexahedron.png
Tetrahemihexahedron

It may also be constructed as a crossed triangular cuploid, being a reduced version of the {3/2}-cupola (retrograde triangular cupola, ratricu) by its {6/2}-gonal base.

Family of star-cupolae and star-cuploids
n / d 3 4 5 6 7 8
2 Tetrahemihexahedron.png
Crossed triangular cuploid
Pentagrammic cuploid.png
Pentagrammic cuploid
Heptagrammic cuploid.png
Heptagrammic cuploid
3 Crossed square cupola.png
Crossed square cupola
Crossed pentagrammic cupola.png
Crossed pentagrammic cupola
Heptagrammic cupola.png
Heptagrammic cupola
Octagrammic cupola.png
Octagrammic cupola
4 Crossed pentagonal cuploid.png
Crossed pentagonal cuploid
Crossed heptagrammic cuploid.png
Crossed heptagrammic cuploid
5 Crossed hexagonal cupola
(Flat)
Crossed heptagrammic cupola.png
Crossed heptagrammic cupola
Crossed octagrammic cupola.png
Crossed octagrammic cupola

Tetrahemihexacron[edit]

Tetrahemihexacron
Tetrahemihexahedron
Type Star polyhedron
Face 100px
Elements F = 6, E = 12
V = 7 (χ = 1)
Symmetry group Td, [3,3], *332
Index references DU04
dual polyhedron Tetrahemihexahedron

The tetrahemihexacron is the dual of the tetrahemihexahedron, and is one of nine dual hemipolyhedra.

Since the hemipolyhedra have faces passing through the center, the dual figures have corresponding vertices at infinity; properly, on the real projective plane at infinity.[2] In Magnus Wenninger's Dual Models, they are represented with intersecting prisms, each extending in both directions to the same vertex at infinity, in order to maintain symmetry. In practice the model prisms are cut off at a certain point that is convenient for the maker. Wenninger suggested these figures are members of a new class of stellation figures, called stellation to infinity. However, he also suggested that strictly speaking they are not polyhedra because their construction does not conform to the usual definitions.

Topologically it is considered to contain seven vertices. The three vertices considered at infinity (the real projective plane at infinity) correspond directionally to the three vertices of the hemi-octahedron, an abstract polyhedron. The other four vertices exist at alternate corners of a central cube (a demicube, in this case a tetrahedron).

References[edit]

External links[edit]