Goldberg polyhedron

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Icosahedral Goldberg polyhedra with pentagons in red
Conway polyhedron Dk5k6st.png
GP(1,4) = {5+,3}1,4
Conway polyhedron dadkt5daD.png
GP(4,4) = {5+,3}4,4
Goldberg polyhedron 7 0.png
GP(7,0) = {5+,3}7,0
Goldberg polyhedron 5 3.png
GP(3,5) = {5+,3}3,5
Goldberg 10 0 equilateral-spherical.png
GP(10,0) = {5+,3}10,0
Equilateral and spherical

A Goldberg polyhedron is a convex polyhedron made from hexagons and pentagons. They were first described by Michael Goldberg (1902–1990) in 1937. They are defined by three properties: each face is either a pentagon or hexagon, exactly three faces meet at each vertex, they have rotational icosahedral symmetry. They are not necessarily mirror-symmetric; e.g. GP(5,3) and GP(3,5) are enantiomorphs of each other. A consequence of Euler's polyhedron formula is that there will be exactly twelve pentagons.

Icosahedral symmetry ensures that the pentagons are always regular, although many of the hexagons may not be. Typically all of the vertices lie on a sphere, but they can also be computed as equilateral.

It is a dual polyhedron of a geodesic sphere, with all triangle faces and 6 triangles per vertex, except for 12 vertices with 5 triangles.

Simple examples of Goldberg polyhedra include the dodecahedron and truncated icosahedron. Other forms can be described by taking a chess knight move from one pentagon to the next: first take m steps in one direction, then turn 60° to the left and take n steps. Such a polyhedron is denoted GP(m,n). A dodecahedron is GP(1,0) and a truncated icosahedron is GP(1,1).

A similar technique can be applied to construct polyhedra with tetrahedral symmetry and octahedral symmetry. These polyhedra will have triangles or squares rather than pentagons. These variations are given Roman numeral subscripts denoting the number of sides on the non-hexagon faces: GPIII(n,m), GPIV(n,m), and GPV(n,m).

Elements[edit]

The number of vertices, edges, and faces of GP(m,n) can be computed from m and n, with T = m2 + mn + n2 = (m + n)2 − mn, depending on one of three symmetry systems:[1]

Symmetry Icosahedral (Goldberg) Octahedral Tetrahedral
Base Dodecahedron
GPV(1,0) = {5+,3}1,0
Cube
GPIV(1,0) = {4+,3}1,0
Tetrahedron
GPIII(1,0) = {3+,3}1,0
Image Icosahedron Cube Tetrahedron
Symbol GPV(m,n) = {5+,3}m,n GPIV(m,n) = {4+,3}m,n GPIII(m,n) = {3+,3}m,n
Vertices
Edges
Faces
Faces by type 12 {5} and 10(T − 1) {6} 6 {4} and 4(T − 1) {6} 4 {3} and 2(T − 1) {6}

Construction[edit]

Most Goldberg polyhedra can be constructed using Conway polyhedron notation starting with (T)etrahedron, (C)ube, and (D)odecahedron seeds. The chamfer operator, c, replaces all edges by hexagons, transforming GP(m,n) to GP(2m,2n), with a T multiplier of 4. The truncated kis operator, y=tk, generates GP(3,0), transforming GP(m,n) to GP(3m,3n), with a T multiplier of 9.

For class 2 forms, the dual kis operator, z=dk, transforms GP(a,0) into GP(a,a), with a T multiplier of 3. For class 3 forms, the whirl operator, w, generates GP(2,1), with a T multiplier of 7. A clockwise and counterclockwise whirl generator, ww=wrw generates GP(7,0) in class 1. In general, a whirl can transform a GP(a,b) into GP(a+3b,2a-b) for a>b and the same chiral direction. If chiral directions are reversed, GP(a,b) becomes GP(2a+3b,a-2b) if a>=2b, and GP(3a+b,2b-a) if a<2b.

Examples[edit]

Class I polyhedra
Frequency (1,0) (2,0) (3,0) (4,0) (5,0) (6,0) (7,0) (8,0) (m,0)
T 1 4 9 16 25 36 49 64 m2
Icosahedral (Goldberg) Dodecahedron.svg Truncated rhombic triacontahedron.png Conway polyhedron Dk6k5tI.png Conway polyhedron dk6k5at5daD.png Goldberg polyhedron 5 0.png Conway polyhedron tkt5daD.png Goldberg polyhedron 7 0.png Conway polyhedron dk6k5adk6k5at5daD.png more
Octahedral Hexahedron.svg Truncated rhombic dodecahedron2.png Conway polyhedron dktO.png Conway polyhedron dk6k4at4daC.png Goldberg polyhedron 4 05 00.svg Conway wrwC.png Conway polyhedron dk6k4adk6k4at4daC.png more
Tetrahedral Tetrahedron.svg Alternate truncated cube.png Conway polyhedron dktT.png Conway ccT.png Conway wrwT.png more
Class II polyhedra
Frequency (1,1) (2,2) (3,3) (4,4) (5,5) (6,6) (7,7) (8,8) (m,m)
T 3 12 27 48 75 108 147 192 3m2
Icosahedral (Goldberg) Truncated icosahedron.png Conway polyhedron dkt5daD.png Conway polyhedron dkdktI.png Conway polyhedron dadkt5daD.png Conway du5zI.png Conway cyzD.png Conway wrwdkD.png Conway cccdkD.png more
Octahedral Truncated octahedron.png Conway polyhedron dkt4daC.png Conway polyhedron tktO.png Conway polyhedron dk6k4adk6k4adkC.png more
Tetrahedral Uniform polyhedron-33-t12.png Conway polyhedron tktT.png more
Class III polyhedra
Frequency (1,2) (1,3) (2,3) (1,4) (2,4) (3,4) (1,5) (m,n)
T 7 13 19 21 28 37 31 m2+mn+n2
Icosahedral (Goldberg) Conway polyhedron Dk5sI.png Goldberg polyhedron 3 1.png Goldberg polyhedron 3 2.png Conway polyhedron Dk5k6st.png Conway polyhedron dk6k5adk5sD.png Goldberg polyhedron 4 3.png Goldberg polyhedron 5 1.png more
Octahedral Conway polyhedron wC.png more
Tetrahedral Conway polyhedron wT.png more

See also[edit]

Notes[edit]

  1. ^ Clinton’s Equal Central Angle Conjecture, JOSEPH D. CLINTON

References[edit]

External links[edit]