Skew apeirohedron

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In geometry, a skew apeirohedron is 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[edit]

According to Coxeter, in 1926 John Flinders Petrie generalized the concept of regular skew polygons (nonplanar polygons) to regular skew polyhedra (apeirohedra).[1]

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

Regular skew apeirohedra
Mucube.png
{4,6|4}
mucube
Muoctahedron.png
{6,4|4}
muoctahedron
Mutetrahedron.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 (Schulte 2004).

Beyond Euclidean 3-space, in 1967 C. W. L. Garner published a set of 31 regular skew polyhedra in hyperbolic 3-space.[2]

Gott's regular pseudopolyhedrons[edit]

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}.[3][4]

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.

A.F. Wells also published a list of pseudopolyhedra in the 1960s, including different forms with the same symbol:

{p,q} Cells Image Space group Related H2
orbifold
notation
Cubic
space
group
Coxeter
notation
Fibrifold
notation
{4,5} Cubes Im3m [[4,3,4]] 8o:2 *4222
{4,5} Truncated octahedra I3 [[4,3+,4]] 80:2 2*42
{3,7} Icosahedra Fd3 [[3[4]]]+ 2o− 3222
{3,8} Octahedra Fd3m [[3[4]]] 2+:2 2*32
{3,8} Snub cubes Uniform apeirohedron snub cube 33333333.png Fm3m [4,(3,4)+] 2−− 32*
{3,9} Icosahedra I3 [[4,3+,4]] 80:2 22*2
{3,12} Octahedra Im3m [[4,3,4]] 8o:2 2*32

However neither the term "pseudopolyhedron" nor Gott's definition of regularity have achieved wide usage.

Prismatic forms[edit]

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[edit]

{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[edit]

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 pseudopolyhedron are repeating structures.

Uniform skew apeirohedra[edit]

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
Cantitruncated cubic honeycomb apeirohedron 4466.png Omnitruncated cubic honeycomb apeirohedron 4466.png Runcicantic cubic honeycomb apeirohedron 6688.png
Related to cantitruncated cubic honeycomb, CDel node 1.pngCDel 4.pngCDel node 1.pngCDel 3.pngCDel node 1.pngCDel 4.pngCDel node.png Related to runcicantic cubic honeycomb, CDel node h1.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node 1.pngCDel 4.pngCDel node 1.png
4.4.4.6 4.8.4.8 3.3.3.3.3.3.3
Omnitruncated cubic honeycomb apeirohedron 4446.png Skew polyhedron 4848.png Icosahedron octahedron infinite skew pseudoregular polyhedron.png
Related to the omnitruncated cubic honeycomb: CDel node 1.pngCDel 4.pngCDel node 1.pngCDel 3.pngCDel node 1.pngCDel 4.pngCDel node 1.png
4.4.4.6 4.4.4.8 3.4.4.4.4
Apeirohedron truncated octahedra and hexagonal prism 4446.png Octagonal prism apeirohedron 4448.png Skew polyhedron 34444.png
Related to the runcitruncated cubic honeycomb.
CDel node 1.pngCDel 4.pngCDel node 1.pngCDel 3.pngCDel node.pngCDel 4.pngCDel node 1.png
Prismatic uniform skew apeirohedra
4.4.4.4.4 4.4.4.6
Pseudoregular apeirohedron prismatic 44444.png
Related to CDel node 1.pngCDel 4.pngCDel node.pngCDel 4.pngCDel node 1.pngCDel 2.pngCDel node 1.png
Skew polyhedron 4446a.png
Related to CDel node 1.pngCDel 6.pngCDel node 1.pngCDel 3.pngCDel node 1.pngCDel 2.pngCDel node 1.png

Others can be constructed as augmented chains of polyhedra:

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

See also[edit]

References[edit]

  1. ^ Coxeter, H. S. M. Regular Skew Polyhedra in Three and Four Dimensions. Proc. London Math. Soc. 43, 33-62, 1937.
  2. ^ Garner, C. W. L. Regular Skew Polyhedra in Hyperbolic Three-Space. Canad. J. Math. 19, 1179-1186, 1967. [1]
  3. ^ J. R. Gott, Pseudopolyhedrons, American Mathematical Monthly, Vol 74, p. 497-504, 1967.
  4. ^ The Symmetries of things, Pseudo-platonic polyhedra, p.340-344

External links[edit]