# Apeirotope

In geometry, an apeirotope or infinite polytope is a generalized polytope which has infinitely many facets.

## Definition

### Abstract apeirotope

An abstract n-polytope is a partially ordered set P (whose elements are called faces) such that P contains a least face and a greatest face, each maximal totally ordered subset (called a flag) contains exactly n + 2 faces, P is strongly connected, and there are exactly two faces that lie strictly between a and b are two faces whose ranks differ by two.[1][2] An abstract polytope is called an abstract apeirotope if it has infinitely many faces.[3]

An abstract polytope is called regular if its automorphism group Γ(P) acts transitively on all of the flags of P.[4]

## Classification

There are two main geometric classes of apeirotope:[5]

### Honeycombs

In general, a honeycomb in n dimensions is an infinite example of a polytope in n + 1 dimensions.

Tilings of the plane and close-packed space-fillings of polyhedra are examples of honeycombs in two and three dimensions respectively.

A line divided into infinitely many finite segments is an example of an apeirogon.

### Skew apeirotopes

#### Skew apeirogons

A skew apeirogon in two dimensions forms a zig-zag line in the plane. If the zig-zag is even and symmetrical, then the apeirogon is regular.

Skew apeirogons can be constructed in any number of dimensions. In three dimensions, a regular skew apeirogon traces out a helical spiral and may be either left- or right-handed.

#### Infinite skew polyhedra

There are three regular skew apeirohedra, which look rather like polyhedral sponges:

• 6 squares around each vertex, Coxeter symbol {4,6|4}
• 4 hexagons around each vertex, Coxeter symbol {6,4|4}
• 6 hexagons around each vertex, Coxeter symbol {6,6|3}

There are thirty regular apeirohedra in Euclidean space.[6] These include those listed above, as well as (in the plane) polytopes of type: {∞,3}, {∞,4}, {∞,6} and in 3-dimensional space, blends of these with either an apeirogon or a line segment, and the "pure" 3-dimensional apeirohedra (12 in number)

## References

### Bibliography

• Grünbaum, B. (1977). "Regular Polyhedra—Old and New". Aeqationes mathematicae. 16: 1–20.
• McMullen, Peter (1994), "Realizations of regular apeirotopes", Aequationes Mathematicae, 47 (2–3): 223–239, doi:10.1007/BF01832961, MR 1268033
• McMullen, Peter; Schulte, Egon (2002), Abstract Regular Polytopes, Encyclopedia of Mathematics and its Applications, vol. 92, Cambridge: Cambridge University Press, doi:10.1017/CBO9780511546686, ISBN 0-521-81496-0, MR 1965665