Deltahedron

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Short description: Polyhedron made of equilateral triangles
The largest strictly-convex deltahedron is the regular icosahedron
This is a truncated tetrahedron with hexagons subdivided into triangles. This figure is not a strictly-convex deltahedron since coplanar faces are not allowed within the definition.

In geometry, a deltahedron (plural deltahedra) is a polyhedron whose faces are all equilateral triangles. The name is taken from the Greek upper case delta (Δ), which has the shape of an equilateral triangle. There are infinitely many deltahedra, all having an even number of faces by the handshaking lemma. Of these only eight are convex, having 4, 6, 8, 10, 12, 14, 16 and 20 faces.[1] The number of faces, edges, and vertices is listed below for each of the eight convex deltahedra.

The eight convex deltahedra

There are eight strictly-convex deltahedra: three are regular polyhedra and Platonic solids, all eight are Johnson solids.

Regular deltahedra
Image Name Faces Edges Vertices Vertex configurations Symmetry group
Tetrahedron.svg tetrahedron 4 6 4 4 × 33 Td, [3,3]
Octahedron.svg octahedron 8 12 6 6 × 34 Oh, [4,3]
Icosahedron.svg icosahedron 20 30 12 12 × 35 Ih, [5,3]
Irregular deltahedra
Image Name Faces Edges Vertices Vertex configurations Symmetry group
Triangular dipyramid.png triangular bipyramid 6 9 5 2 × 33
3 × 34
D3h, [3,2]
Pentagonal dipyramid.png pentagonal bipyramid 10 15 7 5 × 34
2 × 35
D5h, [5,2]
Snub disphenoid.png snub disphenoid 12 18 8 4 × 34
4 × 35
D2d, [2,2]
Triaugmented triangular prism.png triaugmented triangular prism 14 21 9 3 × 34
6 × 35
D3h, [3,2]
Gyroelongated square dipyramid.png gyroelongated square bipyramid 16 24 10 2 × 34
8 × 35
D4d, [4,2]

In the 6-faced deltahedron, some vertices have degree 3 and some degree 4. In the 10-, 12-, 14-, and 16-faced deltahedra, some vertices have degree 4 and some degree 5. These five irregular deltahedra belong to the class of Johnson solids: convex polyhedra with regular polygons for faces.

Deltahedra retain their shape even if the edges are free to rotate around their vertices so that the angles between edges are fluid. Not all polyhedra have this property: for example, if some of the angles of a cube are relaxed, the cube can be deformed into a non-right square prism.

There is no 18-faced convex deltahedron.[2] However, the edge-contracted icosahedron gives an example of an octadecahedron that can either be made convex with 18 irregular triangular faces, or made with equilateral triangles that include two coplanar sets of three triangles.

Non-strictly convex cases

There are infinitely many cases with coplanar triangles, allowing for sections of the infinite triangular tilings. If the sets of coplanar triangles are considered a single face, a smaller set of faces, edges, and vertices can be counted. The coplanar triangular faces can be merged into rhombic, trapezoidal, hexagonal, or other equilateral polygon faces. Each face must be a convex polyiamond such as Polyiamond-1-1.svg, 30px, 30px, 40px, 30px, 50px, 60px and Polyiamond-6-11.svg, ...[3]

Some smaller examples include:

Coplanar deltahedra
Image Name Faces Edges Vertices Vertex configurations Symmetry group
Augmented octahedron.png Augmented octahedron
Augmentation
1 tet + 1 oct
10 Polyiamond-1-1.svg 15 7 1 × 33
3 × 34
3 × 35
0 × 36
C3v, [3]
4 Polyiamond-1-1.svg
3 Polyiamond-2-1.svg
12
Gyroelongated triangular bipyramid.png Trigonal trapezohedron
Augmentation
2 tets + 1 oct
12 Polyiamond-1-1.svg 18 8 2 × 33
0 × 34
6 × 35
0 × 36
C3v, [3]
6 Polyiamond-2-1.svg 12
Tet2Oct solid.png Augmentation
2 tets + 1 oct
12 Polyiamond-1-1.svg 18 8 2 × 33
1 × 34
4 × 35
1 × 36
C2v, [2]
2 Polyiamond-1-1.svg
2 30px
2 Polyiamond-3-1.svg
11 7
Triangulated monorectified tetrahedron.png Triangular frustum
Augmentation
3 tets + 1 oct
14 Polyiamond-1-1.svg 21 9 3 × 33
0 × 34
3 × 35
3 × 36
C3v, [3]
1 Polyiamond-1-1.svg
3 30px
1 Polyiamond-4-3.svg
9 6
TetOct2 solid2.png Elongated octahedron
Augmentation
2 tets + 2 octs
16 Polyiamond-1-1.svg 24 10 0 × 33
4 × 34
4 × 35
2 × 36
D2h, [2,2]
4 Polyiamond-1-1.svg
4 Polyiamond-3-1.svg
12 6
Triangulated tetrahedron.png Tetrahedron
Augmentation
4 tets + 1 oct
16 Polyiamond-1-1.svg 24 10 4 × 33
0 × 34
0 × 35
6 × 36
Td, [3,3]
4 Polyiamond-4-3.svg 6 4
Tet3Oct2 solid.png Augmentation
3 tets + 2 octs
18 Polyiamond-1-1.svg 27 11 1 × 33
2 × 34
5 × 35
3 × 36
D2h, [2,2]
2 Polyiamond-1-1.svg
1 30px
2 30px
2 Polyiamond-4-2.svg
14 9
Double diminished icosahedron.png Edge-contracted icosahedron 18 Polyiamond-1-1.svg 27 11 0 × 33
2 × 34
8 × 35
1 × 36
C2v, [2]
12 Polyiamond-1-1.svg
2 Polyiamond-3-1.svg
22 10
Triangulated truncated triangular bipyramid.png Triangular bifrustum
Augmentation
6 tets + 2 octs
20 Polyiamond-1-1.svg 30 12 0 × 33
3 × 34
6 × 35
3 × 36
D3h, [3,2]
2 Polyiamond-1-1.svg
6 Polyiamond-3-1.svg
15 9
Augmented triangular cupola.png triangular cupola
Augmentation
4 tets + 3 octs
22 Polyiamond-1-1.svg 33 13 0 × 33
3 × 34
6 × 35
4 × 36
C3v, [3]
3 Polyiamond-1-1.svg
3 30px
1 30px
1 Polyiamond-6-11.svg
15 9
Triangulated bipyramid.png Triangular bipyramid
Augmentation
8 tets + 2 octs
24 Polyiamond-1-1.svg 36 14 2 × 33
3 × 34
0 × 35
9 × 36
D3h, [3]
6 Polyiamond-4-3.svg 9 5
Augmented hexagonal antiprism flat.png Hexagonal antiprism 24 Polyiamond-1-1.svg 36 14 0 × 33
0 × 34
12 × 35
2 × 36
D6d, [12,2+]
12 Polyiamond-1-1.svg
2 Polyiamond-6-11.svg
24 12
Triangulated truncated tetrahedron.png Truncated tetrahedron
Augmentation
6 tets + 4 octs
28 Polyiamond-1-1.svg 42 16 0 × 33
0 × 34
12 × 35
4 × 36
Td, [3,3]
4 Polyiamond-1-1.svg
4 Polyiamond-6-11.svg
18 12
Triangulated octahedgon.png Tetrakis cuboctahedron
Octahedron
Augmentation
8 tets + 6 octs
32 Polyiamond-1-1.svg 48 18 0 × 33
12 × 34
0 × 35
6 × 36
Oh, [4,3]
8 Polyiamond-4-3.svg 12 6

Non-convex forms

There are an infinite number of nonconvex forms.

Some examples of face-intersecting deltahedra:

Other nonconvex deltahedra can be generated by adding equilateral pyramids to the faces of all 5 Platonic solids:

5-cell net.png Pyramid augmented cube.png Stella octangula.png Pyramid augmented dodecahedron.png Tetrahedra augmented icosahedron.png
triakis tetrahedron tetrakis hexahedron triakis octahedron
(stella octangula)
pentakis dodecahedron triakis icosahedron
12 triangles 24 triangles 60 triangles

Other augmentations of the tetrahedron include:

Augmented tetrahedra
Biaugmented tetrahedron.png Triaugmented tetrahedron.png Quadaugmented tetrahedron.png
8 triangles 10 triangles 12 triangles

Also by adding inverted pyramids to faces:

  • Excavated dodecahedron
Third stellation of icosahedron.svg
Excavated dodecahedron
Toroidal polyhedron.gif
A toroidal deltahedron
60 triangles 48 triangles

See also

References

  1. Freudenthal, H; van der Waerden, B. L. (1947), "Over een bewering van Euclides ("On an Assertion of Euclid")" (in Dutch), Simon Stevin 25: 115–128  (They showed that there are just 8 convex deltahedra. )
  2. "An Infinite Class of Deltahedra", Mathematics Magazine 51 (1): 55–57, 1978, doi:10.1080/0025570X.1978.11976675 .
  3. The Convex Deltahedra And the Allowance of Coplanar Faces

Further reading

  • Rausenberger, O. (1915), "Konvexe pseudoreguläre Polyeder", Zeitschrift für mathematischen und naturwissenschaftlichen Unterricht 46: 135–142 .
  • Cundy, H. Martyn (December 1952), "Deltahedra", Mathematical Gazette 36: 263–266, doi:10.2307/3608204 .
  • Cundy, H. Martyn; Rollett, A. (1989), "3.11. Deltahedra", Mathematical Models (3rd ed.), Stradbroke, England: Tarquin Pub., pp. 142–144 .
  • Gardner, Martin (1992), Fractal Music, Hypercards, and More: Mathematical Recreations from Scientific American, New York: W. H. Freeman, pp. 40, 53, and 58-60 .
  • Pugh, Anthony (1976), Polyhedra: A visual approach, California: University of California Press Berkeley, ISBN 0-520-03056-7  pp. 35–36

External links