View source for Kinematics of the cuboctahedron

{{Short description|Symmetrical transformations of the cuboctahedron into related uniform polyhedra}}
{{more sources needed|date=October 2024}}
[[File:A3-P5-P3.gif|thumb|Progressions between a cuboctahedron, [[pseudoicosahedron]] and [[octahedron]]. The cuboctahedron can flex this way even if its edges (but not its faces) are rigid.]]
The [[skeleton (topology)|skeleton]] of a [[cuboctahedron]], considering its edges as rigid beams connected at flexible joints at its vertices but omitting its faces, does not have [[structural rigidity]]. Consequently, its vertices can be repositioned by folding (changing the dihedral angle) at the edges and face diagonals. The cuboctahedron's [[kinematics]] is noteworthy in that its vertices can be repositioned to the vertex positions of the [[regular icosahedron]], the [[Jessen's icosahedron]], and the regular [[octahedron]], in accordance with the [[Icosahedron#Pyritohedral symmetry|pyritohedral symmetry of the icosahedron]].{{Sfn|Gunn|Sullivan|2008|loc=§3. Pyritohedral Symmetry|ps=; "The pyritohedral 3D symmetry group is the unique polyhedral point group that is neither a rotation group nor a reflection group."}}{{Sfn|Koca|Al-Mukhaini|Koca|Al Qanobi|2016|loc=4. Pyritohedral Group and Related Polyhedra|p=145|ps=; see Table 1.}}

== Rigid and kinematic cuboctahedra ==
When interpreted as a framework of rigid flat faces, connected along the edges by hinges, the cuboctahedron is a rigid structure, as are all convex polyhedra, by [[Cauchy's theorem (geometry)|Cauchy's theorem]]. However, when the faces are removed, leaving only rigid edges connected by flexible joints at the vertices, the result is not a [[rigid system]] (unlike polyhedra whose faces are all triangles, to which Cauchy's theorem applies despite the missing faces).

Adding a central vertex, connected by rigid edges to all the other vertices, subdivides the cuboctahedron into square pyramids and regular tetrahedra, meeting at the central vertex. Unlike the cuboctahedron itself, the resulting system of edges and joints is rigid, and forms the [[vertex figure]] of the infinite [[tetrahedral-octahedral honeycomb]].

== Cyclical cuboctahedron transformations ==
The cuboctahedron can be transformed cyclically through four polyhedra, repeating the cycle endlessly. Topologically, the transformation follows a [[Möbius loop]]: it is an [[orientable double cover]] of the octahedron. Physically, it is a [[spinor]].

In their spatial relationships the cuboctahedron, icosahedron, Jessen's icosahedron, and octahedron nest like [[Russian dolls]] and are related by a [[Helix|helical]] contraction. The contraction begins with the square faces of the cuboctahedron folding inward along their diagonals to form pairs of triangles. The 12 vertices of the cuboctahedron spiral inward (toward the center) and move closer together until they reach the points where they form a regular icosahedron; they move slightly closer together until they form a [[Jessen's icosahedron]]; and they continue to spiral toward each other until they coincide in pairs as the 6 vertices of the octahedron.{{sfn|Coxeter|1973|pp=50-52}}

The general cuboctahedron transformation can be parameterized along a continuum of special-case transformations with two limit cases: one in which the edges of the cuboctahedron are rigid, and one in which they are elastic.

== Rigid-edge transformation ==
[[File:Jitterbug pausing at jessen form.gif|alt=|thumb|Continuous transformation between the cuboctahedron and the octahedron pausing at the vertex position of the regular icosahedron.]]
The ''rigid-edge cuboctahedron transformation'' symmetrically transforms the cuboctahedron into a [[regular icosahedron]], a [[Jessen's icosahedron]], and a regular [[octahedron]], in the sense that the polyhedron's vertices take on the vertex positions of those polyhedra successively.

The cuboctahedron does not actually ''become'' those other polyhedra, and they cannot transform into each other (if they have rigid edges), because unlike the cuboctahedron they ''do'' have [[structural rigidity]] as a consequence of having only triangular faces.

What the cuboctahedron with rigid edges actually can transform into (and through) is a regular icosahedron from which 6 edges are missing (a [[pseudoicosahedron]]),{{Sfn|Koca|Al-Mukhaini|Koca|Al Qanobi|2016|loc=4.1 Construction of the vertices of the pseudoicosahedron}} a Jessen's icosahedron in which the 6 reflex edges are missing or elastic, and a [[Orientability#Orientable double cover|double cover]] of the octahedron that has ''two'' coincident rigid edges connecting each pair of vertices (formed by making pairs of cuboctahedron vertices coincide).

== Elastic-edge transformation ==
There is a [[tensegrity]] polyhedron that embodies and enforces the closely related ''elastic-edge cuboctahedron transformation''. The [[Tensegrity#Tensegrity icosahedra|tensegrity icosahedron]] has a dynamic structural rigidity called ''infinitesimal mobility'' and can ''only'' be deformed into symmetrical polyhedra along that spectrum from cuboctahedron to octahedron.{{Sfn|Kenner|1976|loc=§2. Spherical tensegrities|pp=11-19}} It is called the tensegrity icosahedron because its median stable form is Jessen's icosahedron.

[[File:Jessen's icosahedron.svg|thumb|Jessen's icosahedron]]
Although the transformation is described above as a contraction of the cuboctahedron, the stable equilibrium point of the tensegrity is Jessen's icosahedron; the tensegrity icosahedron resists being deformed from that shape and can only be forced to expand or contract from it to the extent that its edges are elastic (able to lengthen under tension). Forcing the polyhedron away from its stable resting shape (in either direction) involves stretching its 24 short edges slightly and equally. Force applied to any pair of parallel long edges, to move them closer together or farther apart, is transferred automatically to stretch ''all'' the short edges uniformly, shrinking the polyhedron from its medium-sized Jessen's icosahedron toward the smaller octahedron, or expanding it toward the larger regular icosahedron and still larger cuboctahedron, respectively. Releasing the force causes the polyhedron to spring back to its Jessen's icosahedron resting shape.

In the elastic-edge transformation the cuboctahedron edges are not rigid (though Jessen's icosahedron's 6 long edges are).{{Efn|The vertices of the regular icosahedron form five sets of three concentric, mutually [[orthogonal]] [[golden rectangle]]s, whose edges form [[Borromean rings]]. In a Jessen's icosahedron of unit short radius one set of these three rectangles (the set in which the Jessen's icosahedron's long edges are the rectangles' long edges) measures <math>2\times 4</math>. These three rectangles are the shortest possible representation of the Borromean rings using only edges of the [[integer lattice]].<ref name=lattice>{{citation
 | last1 = Uberti | first1 = R.
 | last2 = Janse van Rensburg | first2 = E. J.
 | last3 = Orlandini | first3 = E.
 | last4 = Tesi | first4 = M. C.
 | last5 = Whittington | first5 = S. G.
 | editor1-last = Whittington | editor1-first = Stuart G.
 | editor2-last = Sumners | editor2-first = Witt De
 | editor3-last = Lodge | editor3-first = Timothy
 | contribution = Minimal links in the cubic lattice
 | doi = 10.1007/978-1-4612-1712-1_9
 | location = New York
 | mr = 1655039
 | pages = 89–100
 | publisher = Springer
 | series = IMA Volumes in Mathematics and its Applications
 | title = Topology and Geometry in Polymer Science
 | volume = 103
 | year = 1998| isbn = 978-0-387-98580-0
 }}; see Table 2, p. 97</ref>}} What the cuboctahedron transforms into is a regular icosahedron of shorter radius and shorter edge length, a Jessen's icosahedron of still shorter radius and (minimum) edge length, and finally an octahedron of still shorter radius but the same (maximum) edge length as the cuboctahedron (but only after the edges have shortened and lengthened again, and come together in coincident pairs).

== Duality of the rigid-edge and elastic-edge transformations ==
The rigid-edge and elastic-edge cuboctahedron transformations differ only in having reciprocal parameters: in the elastic-edge transformation the Jessen's icosahedron's short edges stretch and its long edges are rigid, and in the rigid-edge transformation its long edges compress and its short edges are rigid. Everything in the descriptions above except the metrics applies to ''all'' cuboctahedron transformations. In particular, the vertices always move in helices toward the center as the cuboctahedron transforms into the octahedron,{{Sfn|Verheyen|1989|p=203|ps=; "As Clinton observed in his paper on expanding rigid structures,<ref>{{Cite book|last=Clinton|first=J.D.|year=1971|title=NASA Report: Advanced structural geometry studies, Part 2|chapter=A geometric transformation concept for expanding rigid structures|volume=CR-1735|publisher=Southern Ill. Univ.|place=Washington, D.C.}}</ref> each triangle is subject to a translation-rotation along its symmetry axis. When starting from the position in the octahedron, these axes are the four triangular symmetry axes of the octahedron. When describing cylinders about the triangles along the axes, each vertex common to two triangles moves along the intersecting [[Helix|[helical]]] curve of the two cylinders."}}{{Sfn|Itoh|Nara|2021|p=13|loc=§4. From the 24-cell onto an octahedron; "Lemma 4.2|ps=. ''There is a continuous motion of Q (the cuboctahedron without square faces) shown in Fig. 5a onto the octahedron W<sup>0</sup> satisfying the following conditions for each face F of Q, e.g. F = 𝚫a<sub>1</sub>a<sub>2</sub>a<sub>3</sub>.'' (1) ''F is rotated and moved toward along the line l joining the centroids of F and 𝚫v<sub>1</sub>v<sub>2</sub>v<sub>3</sub>.'' (2) ''F always touches the cylinder T(F), that is, F is always orthogonal to l.''"}} and Jessen's icosahedron (with 90° dihedral angles and three invariant orthogonal planes) is always the median point, stable to the extent that there is resistance to stretching or compressing.{{sfn|Kenner|1976|p=14|loc=Equilibrium}}

The elastic-edge cuboctahedron transformation is usually given as the mathematics of the [[Tensegrity#Tensegrity icosahedra|tensegrity icosahedron]]{{Sfn|Kenner|1976|pp=16-17|loc=Elasticity Multiplication}} because it comes closest to modeling how most actual tensegrity icosahedron structures behave. However, one could certainly construct a tensegrity icosahedron in which the short edges (cables) were perfectly inelastic, and the long edges (struts) were compressible springs. Such a tensegrity would perform the rigid-edge cuboctahedron transformation.

Finally, both transformations are pure abstractions, the two limit cases of an infinite family of cuboctahedron transformations in which there are two elasticity parameters and no requirement that one of them be 0. Neither limit case is apt to apply perfectly to most real tensegrity structures, which usually have some elasticity in ''both'' the cables and the struts, giving their actual behavior metrics that are non-trivial to calculate.{{sfn|Kenner|1976|p=12|loc=Equilibrium}} In engineering practice, only a tiny amount of elasticity is required to allow a significant degree of motion, so most tensegrity structures are constructed to be "drum-tight" using nearly inelastic struts ''and'' cables. A ''tensegrity icosahedron transformation'' is a kinematic cuboctahedron transformation with reciprocal small elasticity parameters.

== Jitterbug transformations ==
The twisting, expansive-contractive transformations between these polyhedra were named [[Synergetics (Fuller)#Tetrahedral accounting|Jitterbug transformations]] by [[Buckminster Fuller]]. Fuller did not give any mathematics;{{Sfn|Verheyen|1989}}{{Sfn|Itoh|Nara|2021|loc=Abstract|ps=; "This article addresses the [[24-cell]] and gives a continuous flattening motion for its 2-skeleton [the cuboctahedron], which is related to the Jitterbug by Buckminster Fuller."}} like many great geometers before him ([[Alicia Boole Stott]] for example) he did not have any mathematics to give. But he was the first to stress the importance of the cuboctahedron's [[Cuboctahedron#Radial equilateral symmetry|radial equilateral symmetry]] which he applied structurally (and patented) as the [[octet truss]], intuiting that it plays a fundamental role not only in [[structural integrity]] but in the dimensional relationships between [[polytopes]]. He discovered the symmetry transformations of the cuboctahedron, understood their relationship to the tensegrity icosahedron, and even gave demonstrations of the rigid-edge cuboctahedron transformation before audiences (in the days before computer-rendered animations). His demonstration with commentary of the "vector equilibrium",{{Sfn|Fuller|1975|loc=Fuller carefully folds a model of the cuboctahedron made of rigid struts with flexible joints through the entire ''rigid-edge'' transformation cycle|ps=; in this film, he does not demonstrate the ''elastic-edge'' transformation (which he observed in the tensegrity icosahedron), but he does show how a rigid regular icosahedron can be rotated inside an inscribing "vector edge cube" (a cube with an octahedron inscribed in it), keeping the 12 vertices on the surface of the cube (and on the edges of the octahedron inscribed in the cube) at all times; actually, Fuller could have rotated any of the kinematic polyhedra in an inscribing cube in this way: the entire cuboctahedron transformation cycle takes place inside an inscribing cube of varying edge length, with the 12 vertices always on the surface of the cube.}} as he called the cuboctahedron, is still far more illuminating than the animations in this article.

==Notes==
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==References==
{{reflist}}

==Bibliography==
{{refbegin}}
* {{Cite book | last=Coxeter | first=H.S.M. | author-link=Harold Scott MacDonald Coxeter | year=1973 | orig-year=1948 | title=Regular Polytopes | publisher=Dover | place=New York | edition=3rd | title-link=Regular Polytopes (book) }}
* {{Cite web| url = https://www.youtube.com/watch?v=9sM44p385Ws| title = Vector Equilibrium | first = R. Buckminster | last = Fuller | author-link=Buckminster Fuller | year = 1975 | work= Everything I Know Sessions | place = Philadelphia}}
* {{Cite book |first=Hugh |last=Kenner |title=Geodesic Math and How to Use It |publisher=University of California Press |year=1976 |isbn=978-0520029248 |url=https://archive.org/details/geodesicmathhowt0000kenn |url-access=registration }}
* {{cite journal
 | last = Verheyen | first = H. F.
 | doi = 10.1016/0898-1221(89)90160-0 | doi-access = free
 | issue = 1–3
 | journal = [[Computers and Mathematics with Applications]]
 | mr = 0994201
 | pages = 203–250
 | title = The complete set of Jitterbug transformers and the analysis of their motion
 | volume = 17
 | year = 1989
 }}
* {{cite journal
 | last1 = Itoh | first1 = Jin-ichi
 | last2 = Nara | first2 = Chie
 | doi = 10.1007/s00022-021-00575-6 | doi-access = free
 | issue = 13
 | journal = [[Journal of Geometry]]
 | title = Continuous flattening of the 2-dimensional skeleton of a regular 24-cell
 | volume = 112
 | year = 2021
 }}
* {{citation
 | last1 = Gunn | first1 = Charles
 | last2 = Sullivan | first2 = John M. | author2-link = John M. Sullivan (mathematician)
 | editor1-last = Sarhangi | editor1-first = Reza
 | editor2-last = Séquin | editor2-first = Carlo H. | editor2-link = Carlo H. Séquin
 | contribution = The Borromean Rings: A video about the New IMU logo
 | contribution-url = https://archive.bridgesmathart.org/2008/bridges2008-63.html
 | isbn = 978-0-9665201-9-4
 | location = London
 | pages = 63–70
 | publisher = Tarquin Publications
 | title = Bridges Leeuwarden: Mathematics, Music, Art, Architecture, Culture
 | year = 2008}}; see the video itself at "[http://torus.math.uiuc.edu/jms/Videos/imu/ The Borromean Rings: A new logo for the IMU]", ''International Mathematical Union''
* {{cite journal|
last1=Koca|first1=Nazife|
last2=Al-Mukhaini|first2=Aida|
last3=Koca|first3=Mehmet|
last4=Al Qanobi|first4=Amal|
date=2016-12-01|
page=139|
title=Symmetry of the Pyritohedron and Lattices|
volume=21|
doi=10.24200/squjs.vol21iss2pp139-149|doi-access=free|
journal=Sultan Qaboos University Journal for Science [SQUJS]|issue=2 }}
{{refend}}

==External links==
* {{cite web|url=https://www.mathunion.org/outreach/imu-logo/borromean-rings|title=Borromean Rings|publisher=International Mathematical Union}}

[[Category:Archimedean solids]]
[[Category:Quasiregular polyhedra]]
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