Jessen's icosahedron

Jessen's icosahedron
(3D model) (OFF file)
Rank	3
Elements
Faces	8 equilateral triangles, 12 isosceles triangles
Edges	24+6
Vertices	12
Measures (edge lengths $1$, ${\frac {2{\sqrt {6}}}{3}}$)
Central density	1
Related polytopes
Army	Pyrike
Convex hull	Pyritohedral icosahedron
Abstract & topological properties
Flag count	120
Euler characteristic	2
Orientable	Yes
Genus	0
Skeleton	Icosahedral graph
Properties
Symmetry	B3/2, order 24
Convex	No
Nature	Tame

Jessen's icosahedron or Jessen's orthogonal icosahedron is a concave isogonal icosahedron with pyritohedral symmetry. It is abstractly equivalent to the regular icosahedron. All edges have a dihedral angle of 90 degrees, but the six longer edges are concave.

The vertex coordinates are given by the twelve points $(\pm 2,\pm 1,0)$, $(0,\pm 2,\pm 1)$, and $(\pm 1,0,\pm 2)$.

Jessen's icosahedron is a well-known example of a "shaky polyhedron." That is, if the faces are made of a rigid material and the edges are hinges, the solid is flexible, but only infinitesimally so.

If the longer edges are realized with rigid bars and the shorter edges strings, the figure supports its own weight, forming a tensegrity structure known as the six-bar spherical tensegrity. Some research has been done on applying this structure in robotics.

A shape similar to Jessen's icosahedron can be produced by symmetrically "indenting" six edges of a regular icosahedron, but the longer-to-shorter edge ratio of the resulting figure is different (equal to the golden ratio in this case). This shape is sometimes mistakenly labeled as Jessen's icosahedron, but it is no longer shaky and does not form a tensegrity structure.

External links

• Wikipedia contributors. "Jessen's icosahedron".
• Weisstein, Eric W. "Jessen's Orthogonal Icosahedron" at MathWorld.
• McCooey, David. "Jessen's Orthogonal Icosahedron"

• Starck, Maurice. "Jessen's orthogonal icosahedron" on The Polyhedra World.