### Dual hemipolyhedra

In geometry, a hemipolyhedron is a uniform star polyhedron some of whose faces pass through its center. These "hemi" faces lie parallel to the faces of some other symmetrical polyhedron, and their count is half the number of faces of that other polyhedron - hence the "hemi" prefix.[1]

The prefix "hemi" is also used to refer to certain projective polyhedra, such as the hemi-cube, which are the image of a 2 to 1 map of a spherical polyhedron with central symmetry.

## Wythoff symbol and vertex figure

Their Wythoff symbols are of the form p/(p − q) p/q | r; their vertex figures are crossed quadrilaterals. The vertex configuration is p/q.2r.p/(p − q).2r. The 2r-gon faces pass through the center of the model. The p/(p − q) notation implies a {p/q} face turning backwards around the vertex figure.

The nine forms, listed with their Wythoff symbols and vertex configurations are:

 Tetrahemihexahedron3/2 3 | 2(3.4.3/2.4)(p/q = 3, r = 2) Octahemioctahedron3/2 3 | 3(3.6.3/2.6)(p/q = 3, r = 3) Small icosihemidodecahedron3/2 3 | 5(3.10.3/2.10)(p/q = 3, r = 5) Great icosihemidodecahedron3/2 3 | 5/3(3.10/3.3/2.10/3)(p/q = 3, r = 5/3) Small dodecahemicosahedron5/3 5/2 | 3(5/2.6.5/3.6)(p/q = 5/2, r = 3) Cubohemioctahedron4/3 4 | 3(4.6.4/3.6)(p/q = 4, r = 3) Small dodecahemidodecahedron5/4 5 | 5(5.10.5/4.10)(p/q = 5, r = 5) Great dodecahemidodecahedron5/3 5/2 | 5/3(5/2.10/3.5/3.10/3)(p/q = 5/2, r = 5/3) Great dodecahemicosahedron5/4 5 | 3(5.6.5/4.6)(p/q = 5, r = 3)

## Orientability

Only the octahemioctahedron represents an orientable surface; the remaining hemi polyhedra have non-orientable or single-sided surfaces.

## Duals of the 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.

There are 9 such duals, sharing only 5 distinct outward forms, four of them existing in outwardly identical pairs. The members of a given identical pair differ in their arrangements of true and false vertices (a false vertex is where two edges cross each other but do not join). The outwards forms are:

 Tetrahemihexacron Octahemioctacronand hexahemioctacron Small icosihemidodecacronand small dodecahemidodecacron Great dodecahemidodecacronand great icosihemidodecacron Great dodecahemicosacronand small dodecahemicosacron 3 intersecting infinite square prisms 4 intersecting infinite hexagonal prisms 6 intersecting infinite decagonal prisms 6 intersecting infinite decagrammic prisms 10 intersecting infinite hexagonal prisms

## Relationship with the quasiregular polyhedra

The hemipolyhedra occur in pairs as facetings of the quasiregular polyhedra with four faces at a vertex. These quasiregular polyhedra have vertex configuration m.n.m.n and their edges, in addition to forming the m- and n-gonal faces, also form hemi-faces of the hemipolyhedra. Thus, the hemipolyhedra can be derived from the quasiregular polyhedra by discarding either the m-gons or n-gons (to maintain two faces at an edge) and then inserting the hemi faces. Since either m-gons or n-gons may be discarded, either of two hemipolyhedra may be derived from each quasiregular polyhedron, except for the octahedron as a tetratetrahedron, where m = n = 3 and the two facetings are congruent. (This construction does not work for the quasiregular polyhedra with six faces at a vertex as their edges do not form any regular hemi-faces.)[1]

Since the hemipolyhedra, like the quasiregular polyhedra, also have two types of faces alternating around each vertex, they are sometimes also considered to be quasiregular.[1]

Quasiregular polyhedron
m.n.m.n
Hemi-faces (h-gons) Hemipolyhedron with m-gons discarded
m.h.m/m - 1.h
n.h.n/n - 1.h
Tetratetrahedron
3.3.3.3
m = 3, n = 3
squares Tetrahemihexahedron
3.4.3/2.4

Tetrahemihexahedron
3.4.3/2.4

Cuboctahedron
3.4.3.4
m = 3, n = 4
hexagons Cubohemioctahedron
4.6.4/3.6

Octahemioctahedron
3.6.3/2.6

Icosidodecahedron
3.5.3.5
m = 3, n = 5
decagons Small dodecahemidodecahedron
5.10.5/4.10

Small icosihemidodecahedron
3.10.3/2.10

5.5/2.5.5/2
m = 5, n = 5/2
hexagons Small dodecahemicosahedron
5/2.6.5/3.6

Great dodecahemicosahedron
5.6.5/4.6

Great icosidodecahedron
3.5/2.3.5/2
m = 3, n = 5/2
decagrams Great dodecahemidodecahedron
5/2.10/3.5/3.10/3

Great icosihemidodecahedron
3.10/3.3/2.10/3

Here m and n correspond to p/q above, and h corresponds to 2r above.

## References

• (Wenninger models: 67, 68, 78, 89, 91, 100, 102, 106, 107)
• Har'El, Z. Zvi Har’El (Page 10, 5.2. Hemi polyhedra p p'|r.)