Skew apeirohedron

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.

Skew apeirohedra have also been called polyhedral sponges.

Many are directly related to a convex uniform honeycomb, being the polygonal surface of a honeycomb with some of the cells removed. Characteristically, an infinite skew polyhedron divides 3-dimensional space into two halves. If one half is thought of as solid the figure is sometimes called a partial honeycomb.

Regular skew apeirohedra

According to Coxeter, in 1926 John Flinders Petrie generalized the concept of regular skew polygons (nonplanar polygons) to regular skew polyhedra (apeirohedra).

Coxeter and Petrie found three of these that filled 3-space:

Regular skew apeirohedra
File:Mucube external.png {4,6|4} mucube	File:Muoctahedron external.png {6,4|4} muoctahedron	File:Mutetrahedron external.png {6,6|3} mutetrahedron

There also exist chiral skew apeirohedra of types {4,6}, {6,4}, and {6,6}. These skew apeirohedra are vertex-transitive, edge-transitive, and face-transitive, but not mirror symmetric .

Beyond Euclidean 3-space, in 1967 C. W. L. Garner published a set of 31 regular skew polyhedra in hyperbolic 3-space.

Gott's regular pseudopolyhedrons

J. Richard Gott in 1967 published a larger set of seven infinite skew polyhedra which he called regular pseudopolyhedrons, including the three from Coxeter as {4,6}, {6,4}, and {6,6} and four new ones: {5,5}, {4,5}, {3,8}, {3,10}.

Gott relaxed the definition of regularity to allow his new figures. Where Coxeter and Petrie had required that the vertices be symmetrical, Gott required only that they be congruent. Thus, Gott's new examples are not regular by Coxeter and Petrie's definition.

Gott called the full set of regular polyhedra, regular tilings, and regular pseudopolyhedra as regular generalized polyhedra, representable by a {p,q} Schläfli symbol, with by p-gonal faces, q around each vertex. However neither the term "pseudopolyhedron" nor Gott's definition of regularity have achieved wide usage.

Crystallographer A.F. Wells in 1960's also published a list of skew apeirohedra. Melinda Green published many more in 1998.

{p,q}	Cells around a vertex	Vertex faces	Larger pattern	Space group			Related H2 orbifold notation
				Cubic space group	Coxeter notation	Fibrifold notation
{4,5}	3 cubes	File:Pseudo-platonic cubic polyhedron vertex.png	File:Pseudo-platonic cubic polyhedron.png	Im3m	[[4,3,4]]	8°:2	*4222
{4,5}	1 truncated octahedron 2 hexagonal prisms	File:Pseudo-platonic hexagonal prism truncated octahedral polyhedron vertex.png		I3	[[4,3+,4]]	8°:2	2*42
{3,7}	1 octahedron 1 icosahedron	File:Pseudo-platonic octa-icosahedral vertex.png	File:Pseudo-platonic octa-icosahedral.png	Fd3	[[3[4]]]+	2°−	3222
{3,8}	2 snub cubes	File:Pseudo-platonic snub cubic polyhedron vertex.png	File:Uniform apeirohedron snub cube 33333333.png	Fm3m	[4,(3,4)+]	2−−	32*
{3,9}	1 tetrahedron 3 octahedra	File:Pseudo-platonic tetra-octahedral polyhedron vertex.png	File:Pseudo-platonic tetra-octahedral polyhedron2.png	Fd3m	[[3[4]]]	2+:2	2*32
{3,9}	1 icosahedron 2 octahedra	File:Pseudo-platonic pyritohedral polyhedron vertex.png		I3	[[4,3+,4]]	8°:2	22*2
{3,12}	5 octahedra	File:Pseudo-platonic octahedral polyhedron vertex.png	File:Sk12x3.gif	Im3m	[[4,3,4]]	8°:2	2*32

Prismatic forms

File:Five-square skew polyhedron.png Prismatic form: {4,5}

There are two prismatic forms:

1. {4,5}: 5 squares on a vertex (Two parallel square tilings connected by cubic holes.)
2. {3,8}: 8 triangles on a vertex (Two parallel triangle tilings connected by octahedral holes.)

Other forms

{3,10} is also formed from parallel planes of triangular tilings, with alternating octahedral holes going both ways.

{5,5} is composed of 3 coplanar pentagons around a vertex and two perpendicular pentagons filling the gap.

Gott also acknowledged that there are other periodic forms of the regular planar tessellations. Both the square tiling {4,4} and triangular tiling {3,6} can be curved into approximating infinite cylinders in 3-space.

Theorems

He wrote some theorems:

1. For every regular polyhedron {p,q}: (p−2)*(q−2)<4. For Every regular tessellation: (p−2)*(q−2)=4. For every regular pseudopolyhedron: (p−2)*(q−2)>4.
2. The number of faces surrounding a given face is p*(q−2) in any regular generalized polyhedron.
3. Every regular pseudopolyhedron approximates a negatively curved surface.
4. The seven regular pseudopolyhedra are repeating structures.

Uniform skew apeirohedra

There are many other uniform (vertex-transitive) skew apeirohedra. Wachmann, Burt and Kleinmann (1974) discovered many examples but it is not known whether their list is complete.

A few are illustrated here. They can be named by their vertex configuration, although it is not a unique designation for skew forms.

Uniform skew apeirohedra related to uniform honeycombs

4.4.6.6		6.6.8.8
File:Cantitruncated cubic honeycomb apeirohedron 4466.png	File:Omnitruncated cubic honeycomb apeirohedron 4466.png	File:Runcicantic cubic honeycomb apeirohedron 6688.png
Related to cantitruncated cubic honeycomb, Template:CDD		Related to runcicantic cubic honeycomb, Template:CDD
4.4.4.6	4.8.4.8	3.3.3.3.3.3.3
File:Omnitruncated cubic honeycomb apeirohedron 4446.png	File:Skew polyhedron 4848.png	File:Icosahedron octahedron infinite skew pseudoregular polyhedron.png
Related to the omnitruncated cubic honeycomb: Template:CDD
4.4.4.6	4.4.4.8	3.4.4.4.4
File:Apeirohedron truncated octahedra and hexagonal prism 4446.png	File:Octagonal prism apeirohedron 4448.png	File:Skew polyhedron 34444.png Related to the runcitruncated cubic honeycomb. Template:CDD

Prismatic uniform skew apeirohedra

4.4.4.4.4	4.4.4.6
File:Pseudoregular apeirohedron prismatic 44444.png Related to Template:CDD	File:Skew polyhedron 4446a.png Related to Template:CDD

Others can be constructed as augmented chains of polyhedra:

File:Coxeter helix 3 colors.png File:Coxeter helix 3 colors cw.png	File:Cube stack diagonal-face helix apeirogon.png
Uniform Boerdijk–Coxeter helix	Stacks of cubes

See also

• Petrie polygon
• Regular skew polyhedron

References

• Coxeter, Regular Polytopes, Third edition, (1973), Dover edition, Template:Isbn
• Kaleidoscopes: Selected Writings of H.S.M. Coxeter, edited by F. Arthur Sherk, Peter McMullen, Anthony C. Thompson, Asia Ivic Weiss, Wiley-Interscience Publication, 1995, Template:Isbn [2]
  • (Paper 2) H.S.M. Coxeter, "The Regular Sponges, or Skew Polyhedra", Scripta Mathematica 6 (1939) 240-244.
• John H. Conway, Heidi Burgiel, Chaim Goodman-Strauss, (2008) The Symmetries of Things, Template:Isbn (Chapter 23, Objects with prime symmetry, pseudo-platonic polyhedra, p340-344)
• . [3] Template:Webarchive
• A. F. Wells, Three-Dimensional Nets and Polyhedra, Wiley, 1977. [4]
• A. Wachmann, M. Burt and M. Kleinmann, Infinite polyhedra, Technion, 1974. 2nd Edn. 2005.
• E. Schulte, J.M. Wills On Coxeter's regular skew polyhedra, Discrete Mathematics, Volume 60, June–July 1986, Pages 253–262

External links

• Template:GlossaryForHyperspace
• "Hyperbolic" Tessellations
• Infinite Regular Polyhedra [5] Template:Webarchive
• Infinite Repeating Polyhedra - Partial Honeycombs in 3-Space
• 18 SYMMETRY OF POLYTOPES AND POLYHEDRA, Egon Schulte: 18.3 REGULAR SKEW POLYHEDRA
• Infinite Polyhedra, T.E. Dorozinski