Snub dodecahedron

Snub dodecahedron

(Click here for rotating model)
TypeArchimedean solid
Uniform polyhedron
ElementsF = 92, E = 150, V = 60 (χ = 2)
Faces by sides(20+60){3}+12{5}
Conway notationsD
Schläfli symbolssr{5,3} or
ht0,1,2{5,3}
Wythoff symbol| 2 3 5
Coxeter diagram
Symmetry groupI, 1/2H3, [5,3]+, (532), order 60
Rotation groupI, [5,3]+, (532), order 60
Dihedral angle3-3: 164°10′31″ (164.18°)
3-5: 152°55′53″ (152.93°)
ReferencesU29, C32, W18
PropertiesSemiregular convex chiral

Colored faces
3.3.3.3.5
(Vertex figure)

Pentagonal hexecontahedron
(dual polyhedron)
Net

In geometry, the snub dodecahedron, or snub icosidodecahedron, is an Archimedean solid, one of thirteen convex isogonal nonprismatic solids constructed by two or more types of regular polygon faces.

The snub dodecahedron has 92 faces (the most of the 13 Archimedean solids): 12 are pentagons and the other 80 are equilateral triangles. It also has 150 edges, and 60 vertices.

It has two distinct forms, which are mirror images (or "enantiomorphs") of each other. The union of both forms is a compound of two snub dodecahedra, and the convex hull of both forms is a truncated icosidodecahedron.

Kepler first named it in Latin as dodecahedron simum in 1619 in his Harmonices Mundi. H. S. M. Coxeter, noting it could be derived equally from either the dodecahedron or the icosahedron, called it snub icosidodecahedron, with a vertical extended Schläfli symbol and flat Schläfli symbol sr{5,3}.

Cartesian coordinates

Cartesian coordinates for the vertices of a snub dodecahedron are all the even permutations of

(±2α, ±2, ±2β),
(±(α + β/φ + φ), ±(−αφ + β + 1/φ), ±(α/φ + βφ − 1)),
(±(α + β/φ − φ), ±(αφ − β + 1/φ), ±(α/φ + βφ + 1)),
(±(−α/φ + βφ + 1), ±(−α + β/φ − φ), ±(αφ + β − 1/φ)) and
(±(−α/φ + βφ − 1), ±(α − β/φ − φ), ±(αφ + β + 1/φ)),

with an even number of plus signs, where

α = ξ − 1/ξ

and

β = ξφ + φ2 + φ/ξ,

where φ = 1 + √5/2 is the golden ratio and ξ is the real solution to ξ3 − 2ξ = φ, which is the number:

This snub dodecahedron has an edge length of 4√1+α2 approximately 6.0437380841.

Taking the odd permutations of the above coordinates with an even number of plus signs gives another form, the enantiomorph of the other one. Though it may not be immediately obvious, the figure obtained by taking the even permutations with an even number of plus signs is the same as that obtained by taking the odd permutations with an odd number of plus signs. Similarly, the mirror image has either an odd permutation with an even number of plus signs or an even permutation with an odd number of plus signs.

Surface area and volume

Transformation from rhombicosidodecahedron to snub dodecahedron

For a snub dodecahedron whose edge length is 1, the surface area is

and the volume is

where φ is the golden ratio.

The snub dodecahedron has the highest sphericity (about 0.982) of all Archimedean solids.

Orthogonal projections

Dodecahedron snub demonstration
five coloured polyhedra mounted on vertical axes
Right-handed twisted dodecahedron
five solids of rotation formed by rotating polyhedra
Left-handed twisted dodecahedron

The snub dodecahedron has two special orthogonal projections, centered on two types of faces: triangles and pentagons, corresponding to the A2 and H2 Coxeter planes.

Orthogonal projections
Centered by Face
Triangle		 Face
Pentagon		 Edge
Image
Projective
symmetry		 [3] [5]+ [2]
Dual
image

Geometric relations

The snub dodecahedron can be generated by taking the twelve pentagonal faces of the dodecahedron and pulling them outward so they no longer touch. At a proper distance this can create the rhombicosidodecahedron by filling in square faces between the divided edges and triangle faces between the divided vertices. But for the snub form, only add the triangle faces and leave the square gaps empty. Then apply an equal rotation to the centers of the pentagons and triangles, continuing the rotation until the gaps can be filled by two equilateral triangles.


Dodecahedron
Rhombicosidodecahedron
(Expanded dodecahedron)
Snub dodecahedron

The snub dodecahedron can also be derived from the truncated icosidodecahedron by the process of alternation. Sixty of the vertices of the truncated icosidodecahedron form a polyhedron topologically equivalent to one snub dodecahedron; the remaining sixty form its mirror-image. The resulting polyhedron is vertex-transitive but not uniform, because its edges are of unequal lengths; some deformation is required to transform it into a uniform polyhedron.

This semiregular polyhedron is a member of a sequence of snubbed polyhedra and tilings with vertex figure (3.3.3.3.n) and Coxeter–Dynkin diagram . These figures and their duals have (n32) rotational symmetry, being in the Euclidean plane for n = 6, and hyperbolic plane for any higher n. The series can be considered to begin with n = 2, with one set of faces degenerated into digons.

Snub dodecahedral graph

Snub dodecahedral graph

5-fold symmetry Schlegel diagram
Vertices 60
Edges 150
Automorphisms 60
Properties Hamiltonian, regular

In the mathematical field of graph theory, a snub dodecahedral graph is the graph of vertices and edges of the snub dodecahedron, one of the Archimedean solids. It has 60 vertices and 150 edges, and is an Archimedean graph.[1]

Orthogonal projections

See also


References

  1. ↑ Read, R. C.; Wilson, R. J. (1998), An Atlas of Graphs, Oxford University Press, p. 269

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