# Snub dodecahedron

Snub dodecahedron

Type Archimedean solid
Uniform polyhedron
Elements F = 92, E = 150, V = 60 (χ = 2)
Faces by sides (20+60){3}+12{5}
Conway notation sD
Schläfli symbols sr{5,3} or ${\displaystyle s{\begin{Bmatrix}5\\3\end{Bmatrix}}}$
ht0,1,2{5,3}
Wythoff symbol | 2 3 5
Coxeter diagram
Symmetry group I, 1/2H3, [5,3]+, (532), order 60
Rotation group I, [5,3]+, (532), order 60
Dihedral angle 3-3: 164°10′31″ (164.18°)
3-5: 152°55′53″ (152.93°)
References U29, C32, W18
Properties Semiregular 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.

3D model of a snub dodecahedron

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 ${\displaystyle s\scriptstyle {\begin{Bmatrix}5\\3\end{Bmatrix}}}$ and flat Schläfli symbol sr{5,3}.

## Cartesian coordinates

Let ${\displaystyle \xi \approx 0.943\,151\,259\,24}$  be the real zero of the polynomial ${\displaystyle x^{3}+2x^{2}-\phi ^{2}}$ , where ${\displaystyle \phi }$  is the golden ratio. Let the point ${\displaystyle p}$  be given by

${\displaystyle p={\begin{pmatrix}\phi ^{2}-\phi ^{2}\xi \\-\phi ^{3}+\phi \xi +2\phi \xi ^{2}\\\xi \end{pmatrix}}}$ .

Then ${\displaystyle p}$  is a vertex of a snub dodecahedron, and the other 59 vertices are given by the images of ${\displaystyle p}$  under the action of the icosahedral rotation group I. Using the three group generators described here, the remaining 59 points are obtained by repeated and recursive use of the following three actions:

• rotation by 72° about the ${\displaystyle (\phi ,1,0)}$  axis
• rotation by 120° about the ${\displaystyle (1,1,1)}$  axis
• rotation by 180° about the ${\displaystyle (1,0,0)}$  x-axis

The coordinates of the vertices are integral linear combinations of ${\displaystyle 1}$ , ${\displaystyle \phi }$ , ${\displaystyle \xi }$ , ${\displaystyle \phi \xi }$ , ${\displaystyle \xi ^{2}}$  and ${\displaystyle \phi \xi ^{2}}$ . The edge length equals ${\displaystyle 2\xi {\sqrt {1-\xi }}\approx 0.449\,750\,618\,41}$ . Negating all coordinates gives the mirror image of this snub dodecahedron.

As a volume, the snub dodecahedron consists of 80 triangular and 12 pentagonal pyramids. The volume ${\displaystyle V_{3}}$  of one triangular pyramid is given by:

${\displaystyle V_{3}={\frac {1}{3}}\phi (3\xi ^{2}-\phi ^{2})\approx 0.027\,274\,068\,85,}$

and the volume ${\displaystyle V_{5}}$  of one pentagonal pyramid by:

${\displaystyle V_{5}={\frac {1}{3}}(3\phi +1)(\phi +3-2\xi -3\xi ^{2})\xi ^{3}\approx 0.103\,349\,665\,04.}$

The total volume is ${\displaystyle 80V_{3}+12V_{5}\approx 3.422\,121\,488\,76}$ .

The circumradius equals ${\displaystyle {\sqrt {4\xi ^{2}-\phi ^{2}}}\approx 0.969\,589\,192\,65}$ . The midradius equals ${\displaystyle \xi }$ . This gives an interesting geometrical interpretation of the number ${\displaystyle \xi }$ . The 20 "icosahedral" triangles of the snub dodecahedron described above are coplanar with the faces of a regular icosahedron. The midradius of this "circumscribed" icosahedron equals ${\displaystyle 1}$ . This means that ${\displaystyle \xi }$  is the ratio between the midradii of a snub dodecahedron and the icosahedron in which it is inscribed.

The triangle-triangle dihedral angle is given by

${\displaystyle \phi _{33}=180^{\circ }-\arccos({\frac {2}{3}}\xi +{\frac {1}{3}})\approx 164.175\,366\,056\,03^{\circ }.}$

The triangle-pentagon dihedral angle is given by

${\displaystyle \phi _{35}=180^{\circ }-\arccos({\sqrt {\frac {-(4\phi +8)\xi ^{2}-(4\phi +8)\xi +12\phi +19}{15}}})\approx 152.929\,920\,275\,84^{\circ }.}$

## Metric properties

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

${\displaystyle A=20{\sqrt {3}}+3{\sqrt {25+10{\sqrt {5}}}}\approx 55.286\,744\,958\,445\,15}$ .

Its volume is

${\displaystyle V={\frac {(3\phi +1)\xi ^{2}+(3\phi +1)\xi -\phi /6-2}{\sqrt {3\xi ^{2}-\phi ^{2}}}}\approx 37.616\,649\,962\,733\,36}$ .

${\displaystyle R={\frac {1}{2}}{\sqrt {\frac {2-\xi }{1-\xi }}}\approx 2.155\,837\,375}$ .

${\displaystyle r={\frac {1}{2}}{\sqrt {\frac {1}{1-\xi }}}\approx 2.097\,053\,835\,25}$ .

There are two inscribed spheres, one touching the triangular faces, and one, slightly smaller, touching the pentagonal faces. Their radii are, respectively:

${\displaystyle r_{3}={\frac {\phi {\sqrt {3}}}{6\xi }}{\sqrt {\frac {1}{1-\xi }}}\approx 2.077\,089\,659\,74}$

and

${\displaystyle r_{5}={\frac {1}{2}}{\sqrt {\phi ^{2}\xi ^{2}+3\phi ^{2}\xi +11\phi /5+12/5}}\approx 1.980\,915\,947\,28}$ .

The four positive real roots of the sextic in ${\displaystyle R^{2}}$

${\displaystyle 4096R^{12}-27648R^{10}+47104R^{8}-35776R^{6}+13872R^{4}-2696R^{2}+209=0}$

are the circumradii of the snub dodecahedron (U29), great snub icosidodecahedron (U57), great inverted snub icosidodecahedron (U69), and great retrosnub icosidodecahedron (U74).

The snub dodecahedron has the highest sphericity of all Archimedean solids. If sphericity is defined as the ratio of volume squared over surface area cubed, multiplied by a constant of 36 times pi (where this constant makes the sphericity of a sphere equal to 1), the sphericity of the snub dodecahedron is about 0.947.[1]

## Orthogonal projections

The snub dodecahedron has no point symmetry, so the vertex in the front does not correspond to an opposite vertex in the back.

The snub dodecahedron has two especially symmetric orthogonal projections as shown below, 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
Solid
Wireframe
Projective
symmetry
[3] [5]+ [2]
Dual

## Geometric relations

Dodecahedron, rhombicosidodecahedron and snub dodecahedron (animated expansion and twisting)

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, pull the pentagonal faces out slightly less, only add the triangle faces and leave the other gaps empty (the other gaps are rectangles at this point). 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. (The fact that the proper amount to pull the faces out is less in the case of the snub dodecahedron can be seen in either of two ways: the circumradius of the snub dodecahedron is smaller than that of the icosidodecahedron; or, the edge length of the equilateral triangles formed by the divided vertices increases when the pentagonal faces are rotated.)

Uniform alternation of a truncated icosidodecahedron

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.

## Related polyhedra and tilings

Family of uniform icosahedral polyhedra
Symmetry: [5,3], (*532) [5,3]+, (532)

{5,3} t{5,3} r{5,3} t{3,5} {3,5} rr{5,3} tr{5,3} sr{5,3}
Duals to uniform polyhedra

V5.5.5 V3.10.10 V3.5.3.5 V5.6.6 V3.3.3.3.3 V3.4.5.4 V4.6.10 V3.3.3.3.5

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.

n32 symmetry mutations of snub tilings: 3.3.3.3.n
Symmetry
n32
Spherical Euclidean Compact hyperbolic Paracomp.
232 332 432 532 632 732 832 ∞32
Snub
figures

Config. 3.3.3.3.2 3.3.3.3.3 3.3.3.3.4 3.3.3.3.5 3.3.3.3.6 3.3.3.3.7 3.3.3.3.8 3.3.3.3.∞
Gyro
figures

Config. V3.3.3.3.2 V3.3.3.3.3 V3.3.3.3.4 V3.3.3.3.5 V3.3.3.3.6 V3.3.3.3.7 V3.3.3.3.8 V3.3.3.3.∞

## Snub dodecahedral graph

Snub dodecahedral graph

5-fold symmetry Schlegel diagram
Vertices60
Edges150
Automorphisms60
PropertiesHamiltonian, regular
Table of graphs and parameters

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.[2]