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Deltahedron

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Triangulated truncated tetrahedron

The truncated tetrahedron with hexagons replaced by triangles is not a convex deltahedron because it is not strictly convex.

A deltahedron (plural deltahedra) is a polyhedron whose faces are all equilateral triangles. The name is taken from the Greek majuscule delta (Δ), which has the shape of an equilateral triangle. There are infinitely many deltahedra, but of these only eight are convex, having 4, 6, 8, 10, 12, 14, 16 and 20 faces. The number of faces, edges, and vertices is listed below for each of the eight convex deltahedra.

The deltahedra should not be confused with the deltohedra (spelled with an "o"), polyhedra whose faces are geometric kites.

The eight convex deltahedra Edit

Name Image Faces Edges Vertices Vertex configurations Symmetry group
regular tetrahedron Tetrahedron 4 6 4 4 × 33 T_d
triangular dipyramid Triangular dipyramid 6 9 5 2 × 33
3 × 34
D_{3h}
regular octahedron Octahedron 8 12 6 6 × 34 O_h
pentagonal dipyramid Pentagonal dipyramid 10 15 7 5 × 34
2 × 35
D_{5h}
snub disphenoid Snub disphenoid 12 18 8 4 × 34
4 × 35
D_{2d}
triaugmented triangular prism Triaugmented triangular prism 14 21 9 3 × 34
6 × 35
D_{3h}
gyroelongated square dipyramid Gyroelongated square dipyramid 16 24 10 2 × 34
8 × 35
D_{4d}
regular icosahedron Icosahedron 20 30 12 12 × 35 I_h

Three of the deltahedra are Platonic solids (polyhedra in which a constant number of identical regular faces meet at each vertex). These are:

  • the 4-faced deltahedron (or tetrahedron), in which three faces meet at each vertex
  • the 8-faced deltahedron (or octahedron), in which four faces meet at each vertex
  • the 20-faced deltahedron (or icosahedron), in which five faces meet at each vertex

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 you relax some of the angles of a cube, the cube can be deformed into a non-right square prism.

Non-convex forms Edit

There are an infinite number of nonconvex forms.

Some examples of non-convex deltahedra:

Others can be generated by adding equilateral pyramids to the faces of all 5 regular polyhedra:

  1. Equilateral triakis tetrahedron
  2. Equilateral tetrakis hexahedron
  3. Equilateral triakis octahedron (stella octangula)
  4. Equilateral pentakis dodecahedron
  5. Equilateral triakis icosahedron

Also by adding inverted pyramids to faces:

Great icosahedron
Great icosahedron
(20 intersecting triangles)
Stella octangula
stella octangula
(24 triangles)
Third stellation of icosahedron
Third stellation of icosahedron
(60 triangles)
Toroidal polyhedron
A toroidal deltahedron
(48 triangles)

External links Edit

References Edit

  • Freudenthal, H. and van der Waerden, B. L. "Over een bewering van Euclides". ("On an Assertion of Euclid") Simon Stevin 25, 115—128, 1947. (They showed that there are just 8 convex deltahedra. )
  • H. Martyn Cundy Deltahedra. Math. Gaz. 36, 263-266, Dec 1952. [2]
  • H. Martyn Cundy and A. Rollett Deltahedra. §3.11 in Mathematical Models, 3rd ed. Stradbroke, England: Tarquin Pub., pp. 142-144, 1989.
  • Charles W. Trigg An Infinite Class of Deltahedra, Mathematics Magazine, Vol. 51, No. 1 (Jan., 1978), pp. 55-57 [3]
  • M. Gardner Fractal Music, Hypercards, and More: Mathematical Recreations, Scientific American Magazine. New York: W. H. Freeman, pp. 40, 53, and 58-60, 1992.
  • A. Pugh Polyhedra: A Visual Approach. Berkeley, CA: University of California Press, pp. 35-36, 1976.eo:Egallatera trianguledra pluredroit:Deltaedro

nl:Deltaëderpt:Deltaedro sk:Deltaéder th:เดลตาฮีดรอน

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