Boerdijk–Coxeter helix

Coxeter helixes from regular tetrahedra

CCW and CW turning

Edges can be colored into 6 groups, 3 main helixes (cyan), with the concave edges forming a slow forward helixes (magenta), and two backwards helixes (yellow and orange)

File:Boerdijk helical sphere packing.png

A Boerdijk helical sphere packing has each sphere centered at a vertex of the Coxeter helix. Each sphere is in contact with 6 neighboring spheres.

The Boerdijk–Coxeter helix, named after H. S. M. Coxeter and A. H. Boerdijk, is a linear stacking of regular tetrahedra, arranged so that the edges of the complex that belong to a single tetrahedron form three intertwined helices. There are two chiral forms, with either clockwise or counterclockwise windings. Contrary to any other stacking of Platonic solids, the Boerdijk–Coxeter helix is not rotationally repetitive. Even in an infinite string of stacked tetrahedra, no two tetrahedra will have the same orientation. This is because the helical pitch per cell is not a rational fraction of the circle.

Buckminster Fuller named it a tetrahelix and considered them with regular and irregular tetrahedral elements.

Architecture

See the Art Tower Mito.

Higher-dimensional geometry

30 tetrahedral ring from 600-cell projection

The 600-cell partitions into 20 rings of 30 tetrahedra, each a Boerdijk–Coxeter helix. When superimposed onto the 3-sphere curvature it becomes periodic, with a period of ten vertices, encompassing all 30 cells. The collective of such helices in the 600-cell represent a discrete Hopf fibration. While in 3 dimensions the edges are helices, in the imposed 3-sphere topology they are geodesics and have no torsion. They spiral around each other naturally due to the Hopf fibration.

In addition, the 16-cell partitions into two 8-tetrahedron rings, four edges long, and the 5-cell partitions into a single degenerate 5-tetrahedron ring.

4-polytope	Rings	Tetrahedra/ring	Cycle lengths	Net	Projection
600-cell	20	30	30, 103, 152	200px
16-cell	2	8	8, 8, 42
5-cell	1	5	(5, 5), 5

Related polyhedral helixes

Equilateral square pyramids can also be chained together as a helix, with two vertex configurations, 3.4.3.4 and 3.3.4.3.3.4. This helix exists as finite ring of 30 pyramids in a 4-dimensional polytope.

800px

And equilateral pentagonal pyramids can be chained with 3 vertex configurations, 3.3.5, 3.5.3.5, and 3.3.3.5.3.3.5:

800px

See also

• Toroidal polyhedron
• Line group#Helical symmetry
• Skew apeirogon#Helical apeirogons in 3-dimensions

References

• H.S.M. Coxeter, Regular Complex Polytopes, Cambridge University, 1974.
• A.H. Boerdijk, Philips Res. Rep. 7 (1952) 30
• The c-brass structure and the Boerdijk–Coxeter helix, E.A. Lord, S. Ranganathan, 2004, pp. 123–125 [1]
• Chiral Gold Nanowires with Boerdijk–Coxeter–Bernal Structure, Yihan Zhu, Jiating He, Cheng Shang, Xiaohe Miao, Jianfeng Huang, Zhipan Liu, Hongyu Chen and Yu Han, J. Am. Chem. Soc., 2014, 136 (36), pp 12746–12752 [2]
• Eric A. Lord, Alan Lindsay Mackay, Srinivasa Ranganathan, New geometries for new materials, p 64, sec 4.5 The Boerdijk–Coxeter helix
• J.F. Sadoc and N. Rivier, Boerdijk-Coxeter helix and biological helices The European Physical Journal B - Condensed Matter and Complex Systems, Volume 12, Number 2, 309-318, doi:10.1007/s100510051009 [3]
• Chapter 5: Joining polyhedra, 5.36 Tetrahelix p. 53

External links

• Boerdijk-Coxeter helix animation
• http://www.rwgrayprojects.com/rbfnotes/helix/helix01.html