Regular icosahedron

Regular icosahedron
Regular icosahedron
Type	Gyroelongated bipyramid; Deltahedron
Faces	20
Edges	30
Vertices	12
Vertex configuration
Symmetry group	Icosahedral symmetry
Dihedral angle (degrees)	138.190 (approximately)
Dual polyhedron	Regular dodecahedron
Properties	convex,; composite
Net

In geometry, the regular icosahedron (or simply icosahedron) is a convex polyhedron that can be constructed from pentagonal antiprism by attaching two pentagonal pyramids with regular faces to each of its pentagonal faces, or by putting points onto the cube. The resulting polyhedron has 20 equilateral triangles as its faces, 30 edges, and 12 vertices. It is an example of a Platonic solid and of a deltahedron. The icosahedral graph represents the skeleton of a regular icosahedron.

Many polyhedra are constructed from the regular icosahedron. For example, most of the Kepler–Poinsot polyhedron is constructed by faceting. Some of the Johnson solids can be constructed by removing the pentagonal pyramids. The regular icosahedron has many relations with other Platonic solids, one of them is the regular dodecahedron as its dual polyhedron and has the historical background on the comparison mensuration. It also has many relations with other polytopes.

The appearance of regular icosahedron can be found in nature, such as the virus with icosahedral-shaped shells and radiolarians. Other applications of the regular icosahedron are the usage of its net in cartography, twenty-sided dice that may have been found in ancient times and role-playing games.

Construction

The regular icosahedron can be constructed like other gyroelongated bipyramids, started from a pentagonal antiprism by attaching two pentagonal pyramids with regular faces to each of its faces. These pyramids cover the pentagonal faces, replacing them with five equilateral triangles, such that the resulting polyhedron has 20 equilateral triangles as its faces.^[1] This process construction is known as the gyroelongation.^[2]

Three mutually perpendicular golden ratio rectangles, with edges connecting their corners, form a regular icosahedron.

Another way to construct it is by putting two points on each surface of a cube. In each face, draw a segment line between the midpoints of two opposite edges and locate two points with the golden ratio distance from each midpoint. These twelve vertices describe the three mutually perpendicular planes, with edges drawn between each of them.^[3] Because of the constructions above, the regular icosahedron is Platonic solid, a family of polyhedra with regular faces. A polyhedron with only equilateral triangles as faces is called a deltahedron. There are only eight different convex deltahedra, one of which is the regular icosahedron.^[4]

The regular icosahedron can also be constructed starting from a regular octahedron. All triangular faces of a regular octahedron are breaking, twisting at a certain angle, and filling up with other equilateral triangles. This process is known as snub, and the regular icosahedron is also known as snub octahedron.^[5]

One possible system of Cartesian coordinate for the vertices of a regular icosahedron, giving the edge length 2, is: $\left(0,\pm 1,\pm \varphi \right),\left(\pm 1,\pm \varphi ,0\right),\left(\pm \varphi ,0,\pm 1\right),$ where $\varphi =(1+{\sqrt {5}})/2$ denotes the golden ratio.^[6]

Properties

Mensuration

The insphere of a convex polyhedron is a sphere inside the polyhedron, touching every face. The circumsphere of a convex polyhedron is a sphere that contains the polyhedron and touches every vertex. The midsphere of a convex polyhedron is a sphere tangent to every edge. Therefore, given that the edge length $a$ of a regular icosahedron, the radius of insphere (inradius) $r_{I}$ , the radius of circumsphere (circumradius) $r_{C}$ , and the radius of midsphere (midradius) $r_{M}$ are, respectively:^[7] $r_{I}={\frac {\varphi ^{2}a}{2{\sqrt {3}}}}\approx 0.756a,\qquad r_{C}={\frac {\sqrt {\varphi ^{2}+1}}{2}}a\approx 0.951a,\qquad r_{M}={\frac {\varphi }{2}}a\approx 0.809a.$

The surface area of a polyhedron is the sum of the areas of its faces. Therefore, the surface area $(A)$ of a regular icosahedron is 20 times that of each of its equilateral triangle faces. The volume $(V)$ of a regular icosahedron can be obtained as 20 times that of a pyramid whose base is one of its faces and whose apex is the icosahedron's center; or as the sum of two uniform pentagonal pyramids and a pentagonal antiprism. The expressions of both are:^[8] $A=5{\sqrt {3}}a^{2}\approx 8.660a^{2},\qquad V={\frac {5\varphi ^{2}}{6}}a^{3}\approx 2.182a^{3}.$ A problem dating back to the ancient Greeks is determining which of two shapes has a larger volume, an icosahedron inscribed in a sphere, or a dodecahedron inscribed in the same sphere. The problem was solved by Hero, Pappus, and Fibonacci, among others.^[9] Apollonius of Perga discovered the curious result that the ratio of volumes of these two shapes is the same as the ratio of their surface areas.^[10] Both volumes have formulas involving the golden ratio, but taken to different powers.^[11] As it turns out, the icosahedron occupies less of the sphere's volume (60.54%) than the dodecahedron (66.49%).^[12]

The dihedral angle of a regular icosahedron can be calculated by adding the angle of pentagonal pyramids with regular faces and a pentagonal antiprism. The dihedral angle of a pentagonal antiprism and pentagonal pyramid between two adjacent triangular faces is approximately 38.2°. The dihedral angle of a pentagonal antiprism between pentagon-to-triangle is 100.8°, and the dihedral angle of a pentagonal pyramid between the same faces is 37.4°. Therefore, for the regular icosahedron, the dihedral angle between two adjacent triangles, on the edge where the pentagonal pyramid and pentagonal antiprism are attached is 37.4° + 100.8° = 138.2°.^[13]

Symmetry

The rotational symmetry group of the regular icosahedron is isomorphic to the alternating group on five letters. This non-abelian simple group is the only non-trivial normal subgroup of the symmetric group on five letters. Since the Galois group of the general quintic equation is isomorphic to the symmetric group on five letters, and this normal subgroup is simple and non-abelian, the general quintic equation does not have a solution in radicals. The proof of the Abel–Ruffini theorem uses this simple fact, and Felix Klein wrote a book that made use of the theory of icosahedral symmetries to derive an analytical solution to the general quintic equation.^[14]

The full symmetry group of the icosahedron (including reflections) is known as the full icosahedral group. It is isomorphic to the product of the rotational symmetry group and the group $C_{2}$ of size two, which is generated by the reflection through the center of the icosahedron.

Icosahedral graph

Every Platonic graph, including the icosahedral graph, is a polyhedral graph. This means that they are planar graphs, graphs that can be drawn in the plane without crossing its edges; and they are 3-vertex-connected, meaning that the removal of any two of its vertices leaves a connected subgraph. According to Steinitz theorem, the icosahedral graph endowed with these heretofore properties represents the skeleton of a regular icosahedron.^[15]

The icosahedral graph is Hamiltonian, meaning that it contains a Hamiltonian cycle, or a cycle that visits each vertex exactly once.^[16]

Related polyhedra

In other Platonic solids

Aside from comparing the mensuration between the regular icosahedron and regular dodecahedron, they are dual to each other. An icosahedron can be inscribed in a dodecahedron by placing its vertices at the face centers of the dodecahedron, and vice versa.^[17]

An icosahedron can be inscribed in an octahedron by placing its 12 vertices on the 12 edges of the octahedron such that they divide each edge into its two golden sections. Because the golden sections are unequal, there are five different ways to do this consistently, so five disjoint icosahedra can be inscribed in each octahedron.^[18]

An icosahedron of edge length ${\textstyle {\frac {1}{\varphi }}\approx 0.618}$ can be inscribed in a unit-edge-length cube by placing six of its edges (3 orthogonal opposite pairs) on the square faces of the cube, centered on the face centers and parallel or perpendicular to the square's edges.^[19] Because there are five times as many icosahedron edges as cube faces, there are five ways to do this consistently, so five disjoint icosahedra can be inscribed in each cube. The edge lengths of the cube and the inscribed icosahedron are in the golden ratio.^[20]

Stellation

The icosahedron has a large number of stellations. Coxeter et al. (1938) stated 59 stellations were identified for the regular icosahedron. The first form is the icosahedron itself. One is a regular Kepler–Poinsot polyhedron. Three are regular compound polyhedra.^[21]

21 of 59 stellations
The faces of the icosahedron extended outwards as planes intersect, defining regions in space as shown by this stellation diagram of the intersections in a single plane.

Facetings

The great dodecahedron, small stellated dodecahedron, and the great icosahedron.

The small stellated dodecahedron, great dodecahedron, and great icosahedron are three facetings of the regular icosahedron. They share the same vertex arrangement. They all have 30 edges. The regular icosahedron and great dodecahedron share the same edge arrangement but differ in faces (triangles vs pentagons), as do the small stellated dodecahedron and great icosahedron (pentagrams vs triangles).

Diminishment

A Johnson solid is a polyhedron whose faces are all regular, but which is not uniform. This means the Johnson solids do not include the Archimedean solids, the Catalan solids, the prisms, or the antiprisms. Some of them are constructed involving the removal of the part of a regular icosahedron, a process known as diminishment. They are gyroelongated pentagonal pyramid, metabidiminished icosahedron, and tridiminished icosahedron, which remove one, two, and three pentagonal pyramids from the icosahedron, respectively.^[2] The similar dissected regular icosahedron has 2 adjacent vertices diminished, leaving two trapezoidal faces, and a bifastigium has 2 opposite sets of vertices removed and 4 trapezoidal faces.

Relations to the 600-cell and other 4-polytopes

The icosahedron is the dimensional analogue of the 600-cell, a regular 4-dimensional polytope. The 600-cell has icosahedral cross sections of two sizes, and each of its 120 vertices is an icosahedral pyramid; the icosahedron is the vertex figure of the 600-cell.

The unit-radius 600-cell has tetrahedral cells of edge length ${\textstyle {\frac {1}{\varphi }}}$ , 20 of which meet at each vertex to form an icosahedral pyramid (a 4-pyramid with an icosahedron as its base). Thus the 600-cell contains 120 icosahedra of edge length ${\textstyle {\frac {1}{\varphi }}}$ . The 600-cell also contains unit-edge-length cubes and unit-edge-length octahedra as interior features formed by its unit-length chords. In the unit-radius 120-cell (another regular 4-polytope which is both the dual of the 600-cell and a compound of 5 600-cells) we find all three kinds of inscribed icosahedra (in a dodecahedron, in an octahedron, and in a cube).

A semiregular 4-polytope, the snub 24-cell, has icosahedral cells.

Relations to other uniform polytopes

As mentioned above, the regular icosahedron is unique among the Platonic solids in possessing a dihedral angle is approximately $138.19^{\circ }$ . Thus, just as hexagons have angles not less than 120° and cannot be used as the faces of a convex regular polyhedron because such a construction would not meet the requirement that at least three faces meet at a vertex and leave a positive defect for folding in three dimensions, icosahedra cannot be used as the cells of a convex regular polychoron because, similarly, at least three cells must meet at an edge and leave a positive defect for folding in four dimensions (in general for a convex polytope in n dimensions, at least three facets must meet at a peak and leave a positive defect for folding in n-space). However, when combined with suitable cells having smaller dihedral angles, icosahedra can be used as cells in semi-regular polychora (for example the snub 24-cell), just as hexagons can be used as faces in semi-regular polyhedra (for example the truncated icosahedron). Finally, non-convex polytopes do not carry the same strict requirements as convex polytopes, and icosahedra are indeed the cells of the icosahedral 120-cell, one of the ten non-convex regular polychora.

There are distortions of the icosahedron that, while no longer regular, are nevertheless vertex-uniform. These are invariant under the same rotations as the tetrahedron, and are somewhat analogous to the snub cube and snub dodecahedron, including some forms which are chiral and some with $T_{h}$ -symmetry, i.e. have different planes of symmetry from the tetrahedron.

Appearances

Twenty-sided dice from Ptolemaic of Egypt, inscribed with Greek letters at the faces.

The Scattergories twenty-sided die, excluding the six letters Q, U, V, X, Y, and Z.

Dice are the most common objects using different polyhedra, one of them being the regular icosahedron. The twenty-sided die was found in many ancient times. One example is the die from the Ptolemaic of Egypt, which later used Greek letters inscribed on the faces in the period of Greece and Rome.^[22] Another example was found in the treasure of Tipu Sultan, which was made out of gold and with numbers written on each face.^[23] In several roleplaying games, such as Dungeons & Dragons, the twenty-sided die (labeled as d20) is commonly used in determining success or failure of an action. It may be numbered from "0" to "9" twice, in which form it usually serves as a ten-sided die (d10); most modern versions are labeled from "1" to "20".^[24] Scattergories is another board game in which the player names the category entires on a card within a given set time. The naming of such categories is initially with the letters contained in every twenty-sided dice.^[25]

The radiolarian Circogonia icosahedra

Dymaxion map, created by the net of a regular icosahedron

The regular icosahedron may also appear in many fields of science as follows:

In virology, herpes virus have icosahedral shells. The outer protein shell of HIV is enclosed in a regular icosahedron, as is the head of a typical myovirus.^[26] Several species of radiolarians discovered by Ernst Haeckel, described its shells as the like-shaped various regular polyhedra; one of which is Circogonia icosahedra, whose skeleton is shaped like a regular icosahedron.^[27]
In chemistry, the closo-carboranes are compounds with a shape resembling the regular icosahedron.^[28] The crystal twinning with icosahedral shapes also occurs in crystals, especially nanoparticles.^[29] Many borides and allotropes of boron such as α- and β-rhombohedral contain boron B₁₂ icosahedron as a basic structure unit.^[30]
In cartography, R. Buckminster Fuller used the net of a regular icosahedron to create a map known as Dymaxion map, by subdividing the net into triangles, followed by calculating the grid on the Earth's surface, and transferring the results from the sphere to the polyhedron. This projection was created during the time that Fuller realized that the Greenland is smaller than South America.^[31]
In the Thomson problem, concerning the minimum-energy configuration of $n$ charged particles on a sphere, and for the Tammes problem of constructing a spherical code maximizing the smallest distance among the points, the minimum solution known for $n=12$ places the points at the vertices of a regular icosahedron, inscribed in a sphere. This configuration is proven optimal for the Tammes problem, but a rigorous solution to this instance of the Thomson problem is unknown.^[32]

Sketch of a regular icosahedron by Johannes Kepler

Kepler's Platonic solid model of the Solar System

As mentioned above, the regular icosahedron is one of the five Platonic solids. The regular polyhedra have been known since antiquity, but are named after Plato who, in his Timaeus dialogue, identified these with the five elements, whose elementary units were attributed these shapes: fire (tetrahedron), air (octahedron), water (icosahedron), earth (cube) and the shape of the universe as a whole (dodecahedron). Euclid's Elements defined the Platonic solids and solved the problem of finding the ratio of the circumscribed sphere's diameter to the edge length.^[33] Following their identification with the elements by Plato, Johannes Kepler in his Harmonices Mundi sketched each of them, in particular, the regular icosahedron.^[34] In his Mysterium Cosmographicum, he also proposed a model of the Solar System based on the placement of Platonic solids in a concentric sequence of increasing radius of the inscribed and circumscribed spheres whose radii gave the distance of the six known planets from the common center. The ordering of the solids, from innermost to outermost, consisted of: regular octahedron, regular icosahedron, regular dodecahedron, regular tetrahedron, and cube.^[35]

Notes

^ Silvester 2001, p. 140–141; Cundy 1952.
^ ^a ^b Berman 1971.
^ Cromwell 1997, p. 70.
^ Shavinina 2013, p. 333; Cundy 1952.
^ Kappraff 1991, p. 475.
^ Steeb, Hardy & Tanski 2012, p. 211.
^ MacLean 2007, p. 43–44; Coxeter 1973, Table I(i), pp. 292–293. See column " ${}_{1}\!\mathrm {R} /\ell$ ", where ${}_{1}\!\mathrm {R}$ is Coxeter's notation for the midradius, also noting that Coxeter uses $2\ell$ as the edge length (see p. 2).
^
- MacLean 2007, p. 43–44
- Berman 1971
^ Herz-Fischler 2013, p. 138–140.
^ Simmons 2007, p. 50.
^ Sutton 2002, p. 55.
^ Numerical values for the volumes of the inscribed Platonic solids may be found in Buker & Eggleton 1969.
^ Johnson 1966, See table II, line 4.; MacLean 2007, p. 43–44.
^ Klein 1884. See icosahedral symmetry: related geometries for further history, and related symmetries on seven and eleven letters.
^ Bickle 2020, p. 147.
^ Hopkins 2004.
^ Herrmann & Sally 2013, p. 257.
^ Coxeter et al. 1938, p. 4.
^ Borovik 2006, pp. 8–9, §5. How to draw an icosahedron on a blackboard.
^ Reciprocally, the edge length of a cube inscribed in a dodecahedron is in the golden ratio to the dodecahedron's edge length. The cube's edges lie in pentagonal face planes of the dodecahedron as regular pentagon diagonals, which are always in the golden ratio to the regular pentagon's edge. When a cube is inscribed in a dodecahedron and an icosahedron is inscribed in the cube, the dodecahedron and icosahedron that do not share any vertices have the same edge length.
^ Coxeter et al. 1938, p. 8–26.
^ Smith 1958, p. 295; Minas-Nerpel 2007.
^ Cromwell 1997, p. 4.
^ "Dungeons & Dragons Dice". gmdice.com. Retrieved August 20, 2019.
^ Flanagan & Gregory 2015, p. 85.
^ Strauss & Strauss 2008, p. 35–62.
^ Haeckel 1904; Cromwell 1997, p. 6.
^ Spokoyny 2013.
^ Hofmeister 2004.
^ Dronskowski, Kikkawa & Stein 2017, p. 435–436.
^ Cromwell 1997, p. 7.
^ Whyte 1952.
^ Heath 1908, p. 262, 478, 480.
^ Cromwell 1997, p. 55.
^ Livio 2003, p. 147.

References

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Bickle, Allan (2020). Fundamentals of Graph Theory. American Mathematical Society. ISBN 9781470455491.
Borovik, Alexandre (2006). "Coxeter Theory: The Cognitive Aspects". In Davis, Chandler; Ellers, Erich (eds.). The Coxeter Legacy. Providence, Rhode Island: American Mathematical Society. pp. 17–43. ISBN 978-0821837221.
Buker, W. E.; Eggleton, R. B. (1969). "The Platonic Solids (Solution to problem E2053)". American Mathematical Monthly. 76 (2): 192. doi:10.2307/2317282. JSTOR 2317282.
Coxeter, H. S. M. (1973). "2.1 Regular polyhedra; 2.2 Reciprocation". Regular Polytopes (3rd ed.). Dover Publications. pp. 16–17. ISBN 0-486-61480-8. MR 0370327.
Coxeter, H.S.M.; du Val, Patrick; Flather, H. T.; Petrie, J. F. (1938). The Fifty-Nine Icosahedra. Vol. 6. University of Toronto Studies (Mathematical Series).
Cromwell, Peter R. (1997). Polyhedra. Cambridge University Press. ISBN 978-0-521-55432-9.
Cundy, H. Martyn (1952). "Deltahedra". The Mathematical Gazette. 36 (318): 263–266. doi:10.2307/3608204. JSTOR 3608204. S2CID 250435684.
Dronskowski, Richard; Kikkawa, Shinichi; Stein, Andreas (2017). Handbook of Solid State Chemistry, 6 Volume Set. John Sons & Wiley. ISBN 978-3-527-69103-6.
Flanagan, Kieran; Gregory, Dan (2015). Selfish, Scared and Stupid: Stop Fighting Human Nature and Increase Your Performance, Engagement and Influence. John Wiley & Sons. ISBN 9780730312796.
Haeckel, E. (1904). Kunstformen der Natur (in German). See here for an online book.
Herrmann, Diane L.; Sally, Paul J. (2013). Number, Shape, & Symmetry: An Introduction to Number Theory, Geometry, and Group Theory. CRC Press. ISBN 978-1-4665-5464-1.
Heath, Thomas L. (1908). The Thirteen Books of Euclid's Elements (3rd ed.). Cambridge University Press.
Herz-Fischler, Roger (2013). A Mathematical History of the Golden Number. Courier Dover Publications. ISBN 9780486152325.
Hofmeister, H. (2004). "Fivefold Twinned Nanoparticles". Encyclopedia of Nanoscience and Nanotechnology. 3: 431–452.
Hopkins, Brian (2004). "Hamiltonian paths on Platonic graphs". International Journal of Mathematics and Mathematical Sciences. 2004 (30): 1613–1616. doi:10.1155/S0161171204307118.
Johnson, Norman W. (1966). "Convex polyhedra with regular faces". Canadian Journal of Mathematics. 18: 169–200. doi:10.4153/cjm-1966-021-8. MR 0185507. Zbl 0132.14603.
Jones, Daniel (2003) [1917]. Roach, Peter; Hartmann, James; Setter, Jane (eds.). English Pronouncing Dictionary. Cambridge: Cambridge University Press. ISBN 3-12-539683-2.
Kappraff, Jay (1991). Connections: The Geometric Bridge Between Art and Science (2nd ed.). World Scientific. ISBN 978-981-281-139-4.</ref>
Klein, Felix (1888). Lectures on the ikosahedron and the solution of equations of the fifth degree. Courier Corporation. ISBN 978-0-486-49528-6, Dover edition{{cite book}}: CS1 maint: postscript (link), translated from Klein, Felix (1884). Vorlesungen über das Ikosaeder und die Auflösung der Gleichungen vom fünften Grade. Teubner.
Livio, Mario (2003) [2002]. The Golden Ratio: The Story of Phi, the World's Most Astonishing Number (1st trade paperback ed.). New York City: Broadway Books. ISBN 978-0-7679-0816-0.
MacLean, Kenneth J. M. (2007). A Geometric Analysis of the Platonic Solids and Other Semi-Regular Polyhedra. Loving Healing Press. ISBN 978-1-932690-99-6.
Minas-Nerpel, Martina (2007). "A Demotic Inscribed Icosahedron from Dakhleh Oasis". The Journal of Egyptian Archaeology. 93 (1): 137–148. doi:10.1177/030751330709300107. JSTOR 40345834.
Shavinina, Larisa V. (2013). The Routledge International Handbook of Innovation Education. Routledge. ISBN 978-0-203-38714-6.
Silvester, John R. (2001). Geometry: Ancient and Modern. Oxford University Publisher.
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Smith, David E. (1958). History of Mathematics. Vol. 2. Dover Publications. ISBN 0-486-20430-8.
Spokoyny, A. M. (2013). "New Ligand Platforms Featuring Boron-Rich Clusters as Organomimetic Sbstituents". Pure and Applied Chemistry. 85 (5): 903–919. doi:10.1351/PAC-CON-13-01-13. PMC 3845684. PMID 24311823.
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Sutton, Daud (2002). Platonic & Archimedean Solids. Wooden Books. Bloomsbury Publishing USA. ISBN 9780802713865.
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Whyte, L. L. (1952). "Unique arrangements of points on a sphere". The American Mathematical Monthly. 59 (9): 606–611. doi:10.1080/00029890.1952.11988207. JSTOR 2306764. MR 0050303.

External links

Klitzing, Richard. "3D convex uniform polyhedra x3o5o – ike".
Hartley, Michael. "Dr Mike's Math Games for Kids".
K.J.M. MacLean, A Geometric Analysis of the Five Platonic Solids and Other Semi-Regular Polyhedra
Virtual Reality Polyhedra The Encyclopedia of Polyhedra
Tulane.edu A discussion of viral structure and the icosahedron
Origami Polyhedra – Models made with Modular Origami
Video of icosahedral mirror sculpture
[1] Principle of virus architecture

Fundamental convex regular and uniform polytopes in dimensions 2–10
Family	A_n	B_n	I₂(p) / D_n	E₆ / E₇ / E₈ / F₄ / G₂	H_n
Regular polygon	Triangle	Square	p-gon	Hexagon	Pentagon
Uniform polyhedron	Tetrahedron	Octahedron • Cube	Demicube		Dodecahedron • Icosahedron
Uniform polychoron	Pentachoron	16-cell • Tesseract	Demitesseract	24-cell	120-cell • 600-cell
Uniform 5-polytope	5-simplex	5-orthoplex • 5-cube	5-demicube
Uniform 6-polytope	6-simplex	6-orthoplex • 6-cube	6-demicube	1₂₂ • 2₂₁
Uniform 7-polytope	7-simplex	7-orthoplex • 7-cube	7-demicube	1₃₂ • 2₃₁ • 3₂₁
Uniform 8-polytope	8-simplex	8-orthoplex • 8-cube	8-demicube	1₄₂ • 2₄₁ • 4₂₁
Uniform 9-polytope	9-simplex	9-orthoplex • 9-cube	9-demicube
Uniform 10-polytope	10-simplex	10-orthoplex • 10-cube	10-demicube
Uniform n-polytope	n-simplex	n-orthoplex • n-cube	n-demicube	1_k2 • 2_k1 • k₂₁	n-pentagonal polytope
Topics: Polytope families • Regular polytope • List of regular polytopes and compounds

(Convex) icosahedron	Small triambic icosahedron	Compound of five octahedra	Great icosahedron	Excavated dodecahedron
Notable stellations of the icosahedron
Regular	Uniform duals	Regular compounds	Regular star	Others


The stellation process on the icosahedron creates a number of related polyhedra and compounds with icosahedral symmetry.

[FOOTNOTESilvester2001[httpsbooksgooglecombooksidVtH_QG6scSUCpgPA141_140&ndash;141]Cundy1952-1] Silvester 2001, p. 140–141; Cundy 1952.

[FOOTNOTEBerman1971-2] Berman 1971.

[FOOTNOTECromwell1997[httpsarchiveorgdetailspolyhedra0000crompage70mode1upviewtheater_70]-3] Cromwell 1997, p. 70.

[FOOTNOTEShavinina2013[httpsbooksgooglecombooksidJcPd_JRc4FgCpgPA333_333]Cundy1952-4] Shavinina 2013, p. 333; Cundy 1952.

[FOOTNOTEKappraff1991[httpsbooksgooglecombooksidtz76s0ZGFiQCpgPA475_475]-5] Kappraff 1991, p. 475.

[FOOTNOTESteebHardyTanski2012[httpsbooksgooglecombooksidUdI7DQAAQBAJpgPA211_211]-6] Steeb, Hardy & Tanski 2012, p. 211.

[7] MacLean 2007, p. 43–44; Coxeter 1973, Table I(i), pp. 292–293. See column " ${}_{1}\!\mathrm {R} /\ell$ ", where ${}_{1}\!\mathrm {R}$ is Coxeter's notation for the midradius, also noting that Coxeter uses $2\ell$ as the edge length (see p. 2).

[8] 
MacLean 2007, p. 43–44
Berman 1971

[9] MacLean 2007, p. 43–44

[10] Berman 1971

[FOOTNOTEHerz-Fischler2013[httpsbooksgooglecombooksidaYjXZJwLARQCpgPA138_138–140]-9] Herz-Fischler 2013, p. 138–140.

[FOOTNOTESimmons2007[httpsbooksgooglecombooksid3KOst4Mon90CpgPA50_50]-10] Simmons 2007, p. 50.

[FOOTNOTESutton2002[httpsbooksgooglecombooksidvgo7bTxDmIsCpgPA55_55]-11] Sutton 2002, p. 55.

[12] Numerical values for the volumes of the inscribed Platonic solids may be found in Buker & Eggleton 1969.

[FOOTNOTEJohnson1966See_table_II,_line_4.MacLean2007[httpsbooksgooglecombooksidvINuAwAAQBAJpgPA44_43&ndash;44]-13] Johnson 1966, See table II, line 4.; MacLean 2007, p. 43–44.

[14] Klein 1884. See icosahedral symmetry: related geometries for further history, and related symmetries on seven and eleven letters.

[FOOTNOTEBickle2020[httpsbooksgooglecombooksid2sbVDwAAQBAJpgPA147_147]-15] Bickle 2020, p. 147.

[FOOTNOTEHopkins2004-16] Hopkins 2004.

[FOOTNOTEHerrmannSally2013[httpsbooksgooglecombooksidb2fjR81h6yECpgPA257_257]-17] Herrmann & Sally 2013, p. 257.

[FOOTNOTECoxeterdu_ValFlatherPetrie19384-18] Coxeter et al. 1938, p. 4.

[FOOTNOTEBorovik20068–9§5._How_to_draw_an_icosahedron_on_a_blackboard-19] Borovik 2006, pp. 8–9, §5. How to draw an icosahedron on a blackboard.

[20] Reciprocally, the edge length of a cube inscribed in a dodecahedron is in the golden ratio to the dodecahedron's edge length. The cube's edges lie in pentagonal face planes of the dodecahedron as regular pentagon diagonals, which are always in the golden ratio to the regular pentagon's edge. When a cube is inscribed in a dodecahedron and an icosahedron is inscribed in the cube, the dodecahedron and icosahedron that do not share any vertices have the same edge length.

[FOOTNOTECoxeterdu_ValFlatherPetrie19388&ndash;26-21] Coxeter et al. 1938, p. 8–26.

[FOOTNOTESmith1958[httpsbooksgooglecombooksiduTytJGnTf1kCpgPA295_295]Minas-Nerpel2007-22] Smith 1958, p. 295; Minas-Nerpel 2007.

[FOOTNOTECromwell1997[httpsarchiveorgdetailspolyhedra0000crompage4mode1upviewtheater_4]-23] Cromwell 1997, p. 4.

[24] "Dungeons & Dragons Dice". gmdice.com. Retrieved August 20, 2019.

[FOOTNOTEFlanaganGregory2015[httpsbooksgooglecombooksidhViuEAAAQBAJpgPA85_85]-25] Flanagan & Gregory 2015, p. 85.

[FOOTNOTEStraussStrauss200835&ndash;62-26] Strauss & Strauss 2008, p. 35–62.

[FOOTNOTEHaeckel1904Cromwell1997[httpsarchiveorgdetailspolyhedra0000crompage6mode1upviewtheater_6]-27] Haeckel 1904; Cromwell 1997, p. 6.

[FOOTNOTESpokoyny2013-28] Spokoyny 2013.

[FOOTNOTEHofmeister2004-29] Hofmeister 2004.

[FOOTNOTEDronskowskiKikkawaStein2017[httpsbooksgooglecombookside0VBDwAAQBAJpgPA436_435&ndash;436]-30] Dronskowski, Kikkawa & Stein 2017, p. 435–436.

[FOOTNOTECromwell1997[httpsarchiveorgdetailspolyhedra0000crompage7mode1upviewtheater_7]-31] Cromwell 1997, p. 7.

[FOOTNOTEWhyte1952-32] Whyte 1952.

[FOOTNOTEHeath1908262,_478,_480-33] Heath 1908, p. 262, 478, 480.

[FOOTNOTECromwell1997[httpsarchiveorgdetailspolyhedra0000crompage55_55]-34] Cromwell 1997, p. 55.

[FOOTNOTELivio2003147-35] Livio 2003, p. 147.

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Polyhedra
Listed by number of faces and type
1–10 faces	Monohedron Dihedron Trihedron Tetrahedron Pentahedron Hexahedron Heptahedron Octahedron Enneahedron Decahedron
11–20 faces	Hendecahedron Dodecahedron Tridecahedron Tetradecahedron Pentadecahedron Hexadecahedron Heptadecahedron Octadecahedron Enneadecahedron Icosahedron
>20 faces	Icositetrahedron (24) Triacontahedron (30) Icosidodecahedron (32) Hexoctahedron (48) Hexecontahedron (60) Enneacontahedron (90) Hectotriadiohedron (132) Apeirohedron (∞)
elemental things	face edge vertex uniform polyhedron (two infinite groups and 75) regular polyhedron (9) quasiregular polyhedron (16) semiregular polyhedron (two infinite groups and 50)
convex polyhedron	Platonic solid (5) Archimedean solid (13) Catalan solid (13) Johnson solid (92)
non-convex polyhedron	Kepler–Poinsot polyhedron (4) Star polyhedron (infinite) Uniform star polyhedron (57)
prismatoid‌s	prism antiprism frustum cupola wedge pyramid parallelepiped

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