In geometry, any polyhedron is associated with a second dual figure, where the vertices of one correspond to the faces of the other and the edges between pairs of vertices of one correspond to the edges between pairs of faces of the other.[1] Such dual figures remain combinatorial or abstract polyhedra, but not all are also geometric polyhedra.[2] Starting with any given polyhedron, the dual of its dual is the original polyhedron. Duality preserves the symmetries of a polyhedron. Therefore, for many classes of polyhedra defined by their symmetries, the duals also belong to a symmetric class. Thus, the regular polyhedra – the (convex) Platonic solids and (star) Kepler-Poinsot polyhedra – form dual pairs, where the regular tetrahedron is self-dual. The dual of an isogonal polyhedron, having equivalent vertices, is one which is isohedral, having equivalent faces. The dual of an isotoxal polyhedron (having equivalent edges) is also isotoxal. Duality is closely related to reciprocity or polarity, a geometric transformation that, when applied to a convex polyhedron, realizes the dual polyhedron as another convex polyhedron. Contents 1 Kinds of duality 1.1 Polar reciprocation 1.1.1 Canonical duals 1.2 Topological duality 2 Dorman Luke construction 3 Self-dual polyhedra 3.1 Self-dual compound polyhedra 4 Dual polytopes and tessellations 4.1 Self-dual polytopes and tessellations 5 See also 6 References 6.1 Notes 6.2 Bibliography 7 External links Kinds of duality[edit] The dual of a
There are many kinds of duality. The kinds most relevant to elementary polyhedra are polar reciprocity and topological or abstract duality. Polar reciprocation[edit] See also: Polar reciprocation The duality of polyhedra is often defined in terms of polar reciprocation about a concentric sphere. Here, each vertex (pole) is associated with a face plane (polar plane or just polar) so that the ray from the center to the vertex is perpendicular to the plane, and the product of the distances from the center to each is equal to the square of the radius.[3] In coordinates, for reciprocation about the sphere x 2 + y 2 + z 2 = r 2 , displaystyle x^ 2 +y^ 2 +z^ 2 =r^ 2 , the vertex ( x 0 , y 0 , z 0 ) displaystyle (x_ 0 ,y_ 0 ,z_ 0 ) is associated with the plane x 0 x + y 0 y + z 0 z = r 2 displaystyle x_ 0 x+y_ 0 y+z_ 0 z=r^ 2 . The vertices of the dual are the poles reciprocal to the face planes of the original, and the faces of the dual lie in the polars reciprocal to the vertices of the original. Also, any two adjacent vertices define an edge, and these will reciprocate to two adjacent faces which intersect to define an edge of the dual. This dual pair of edges are always orthogonal (at right angles) to each other. If r 0 displaystyle r_ 0 is the radius of the sphere, and r 1 displaystyle r_ 1 and r 2 displaystyle r_ 2 respectively the distances from its centre to the pole and its polar, then: r 1 . r 2 = r 0 2 displaystyle r_ 1 .r_ 2 =r_ 0 ^ 2 For the more symmetrical polyhedra having an obvious centroid, it is
common to make the polyhedron and sphere concentric, as in the Dorman
Luke construction described below.
However, it is possible to reciprocate a polyhedron about any sphere,
and the resulting form of the dual will depend on the size and
position of the sphere; as the sphere is varied, so too is the dual
form. The choice of center for the sphere is sufficient to define the
dual up to similarity. If multiple symmetry axes are present, they
will necessarily intersect at a single point, and this is usually
taken to be the centroid. Failing that, a circumscribed sphere,
inscribed sphere, or midsphere (one with all edges as tangents) is
commonly used.
If a polyhedron in
The snub cube and its dual pentagonal icositetrahedron. The image in the middle shows the edges of both touching the shared midsphere. Any convex polyhedron can be distorted into a canonical form, in which
a unit midsphere (or intersphere) exists tangent to every edge, and
such that the average position of the points of tangency is the center
of the sphere. This form is unique up to congruences.
If we reciprocate such a canonical polyhedron about its midsphere, the
dual polyhedron will share the same edge-tangency points and so must
also be canonical. It is the canonical dual, and the two together form
a canonical dual pair.[5]
Topological duality[edit]
Even when a pair of polyhedra cannot be obtained by reciprocation from
each other, they may be called duals of each other as long as the
vertices of one correspond to the faces of the other, and the edges of
one correspond to the edges of the other, in an incidence-preserving
way. Such pairs of polyhedra are still topologically or abstractly
dual.
The vertices and edges of a convex polyhedron form a graph (the
1-skeleton of the polyhedron), embedded on a topological sphere, the
surface of the polyhedron. The same graph can be projected to form a
Compound of the cuboctahedron and its dual For a uniform polyhedron, the face of the dual polyhedron may be found from the original polyhedron's vertex figure using the Dorman Luke construction.[6] As an example, the illustration below shows the vertex figure (red) of the cuboctahedron being used to derive a face (blue) of the rhombic dodecahedron. Before beginning the construction, the vertex figure ABCD is obtained by cutting each connected edge at (in this case) its midpoint. Dorman Luke's construction then proceeds: Draw the vertex figure ABCD Draw the circumcircle (tangent to every corner A, B, C and D). Draw lines tangent to the circumcircle at each corner A, B, C, D. Mark the points E, F, G, H, where each tangent line meets the adjacent tangent. The polygon EFGH is a face of the dual polyhedron. In this example the size of the vertex figure was chosen so that its circumcircle lies on the intersphere of the cuboctahedron, which also becomes the intersphere of the dual rhombic dodecahedron. Dorman Luke's construction can only be used where a polyhedron has such an intersphere and the vertex figure is cyclic. For instance, it can be applied to the uniform polyhedra. Self-dual polyhedra[edit] Topologically, a self-dual polyhedron is one whose dual has exactly the same connectivity between vertices, edges and faces. Abstractly, they have the same Hasse diagram. A geometrically self-dual polyhedron is not only topologically self-dual, but its polar reciprocal about a certain point, typically its centroid, is a similar figure. For example, the dual of a regular tetrahedron is another regular tetrahedron, reflected through the origin. Every polygon is topologically self-dual (it has the same number of vertices as edges, and these are switched by duality), but will not in general be geometrically self-dual (up to rigid motion, for instance). Every polygon has a regular form which is geometrically self-dual about its intersphere: all angles are congruent, as are all edges, so under duality these congruences swap. Similarly, every topologically self-dual convex polyhedron can be realized by an equivalent geometrically self-dual polyhedron, its canonical polyhedron, reciprocal about the center of the midsphere. There are infinitely many geometrically self-dual polyhedra. The simplest infinite family are the canonical pyramids of n sides. Another infinite family, elongated pyramids, consists of polyhedra that can be roughly described as a pyramid sitting on top of a prism (with the same number of sides). Adding a frustum (pyramid with the top cut off) below the prism generates another infinite family, and so on. There are many other convex, self-dual polyhedra. For example, there are 6 different ones with 7 vertices, and 16 with 8 vertices.[7] A self-dual[clarification needed] non-convex icosahedron with hexagonal faces was identified by Brückner in 1900.[8][9][10] Other non-convex self-dual polyhedra have been found, under certain definitions of non-convex polyhedra and their duals.[clarification needed] Family of pyramids 3 4 5 6 Family of elongated pyramids 3 4 5 Family of diminished trapezohedra 3 4 5 6 7 Self-dual compound polyhedra[edit]
Trivially, the compound of any polyhedron and its dual is a self-dual
figure.
If a polyhedron is self-dual, then the compound of the polyhedron with
its dual will comprise congruent polyhedra. The regular compound of
two tetrahedra, known as the Stella octangula, is the only regular
compound with this property.
Dual polytopes and tessellations[edit]
Duality can be generalized to n-dimensional space and dual polytopes;
in two dimension these are called dual polygons.
The vertices of one polytope correspond to the (n − 1)-dimensional
elements, or facets, of the other, and the j points that define a (j
− 1)-dimensional element will correspond to j hyperplanes that
intersect to give a (n − j)-dimensional element. The dual of an
n-dimensional tessellation or honeycomb can be defined similarly.
In general, the facets of a polytope's dual will be the topological
duals of the polytope's vertex figures. For the polar reciprocals of
the regular and uniform polytopes, the dual facets will be polar
reciprocals of the original's vertex figure. For example, in four
dimensions, the vertex figure of the
The square tiling, 4,4 , is self-dual, as shown by these red and blue tilings The Infinite-order apeirogonal tiling, ∞,∞ in red, and its dual position in blue The primary class of self-dual polytopes are regular polytopes with palindromic Schläfli symbols. All regular polygons, a are self-dual, polyhedra of the form a,a , 4-polytopes of the form a,b,a , 5-polytopes of the form a,b,b,a , etc. The self-dual regular polytopes are: All regular polygons, a .
Regular tetrahedron: 3,3
In general, all regular n-simplexes, 3,3,...,3
The regular
The self-dual (infinite) regular Euclidean honeycombs are: Apeirogon: ∞ Square tiling: 4,4 Cubic honeycomb: 4,3,4 In general, all regular n-dimensional Euclidean hypercubic honeycombs: 4,3,...,3,4 . The self-dual (infinite) regular hyperbolic honeycombs are: Compact hyperbolic tilings: 5,5 , 6,6 , ... p,p . Paracompact hyperbolic tiling: ∞,∞ Compact hyperbolic honeycombs: 3,5,3 , 5,3,5 , and 5,3,3,5 Paracompact hyperbolic honeycombs: 3,6,3 , 6,3,6 , 4,4,4 , and 3,3,4,3,3 See also[edit] Conway polyhedron notation Dual polygon Self-dual graph Self-dual polygon References[edit] Notes[edit] ^ Wenninger (1983), "Basic notions about stellation and duality",
p. 1.
^ Grünbaum (2003)
^ Cundy & Rollett (1961), 3.2 Duality, pp. 78–79; Wenninger
(1983), Pages 3-5. (Note, Wenninger's discussion includes nonconvex
polyhedra.)
^ See for example Grünbaum & Shephard (2013), and Gailiunas &
Sharp (2005). Wenninger (1983) also discusses some issues on the way
to deriving his infinite duals.
^ Grünbaum (2007), Theorem 3.1, p. 449.
^ Cundy & Rollett (1961), p. 117; Wenninger (1983),
p. 30.
^ 3D Java models at Symmetries of Canonical Self-Dual Polyhedra, based
on paper by Gunnar Brinkmann, Brendan D. McKay, Fast generation of
planar graphs
Bibliography[edit] Cundy, H. Martyn; Rollett, A. P. (1961), Mathematical Models (2nd ed.), Oxford: Clarendon Press, MR 0124167 . Gailiunas, P.; Sharp, J. (2005), "Duality of polyhedra", International Journal of Mathematical Education in Science and Technology, 36 (6): 617–642, doi:10.1080/00207390500064049 . Grünbaum, Branko (2003), "Are your polyhedra the same as my polyhedra?", in Aronov, Boris; Basu, Saugata; Pach, János; Sharir, Micha, Discrete and Computational Geometry: The Goodman–Pollack Festschrift, Algorithms and Combinatorics, 25, Berlin: Springer, pp. 461–488, CiteSeerX 10.1.1.102.755 , doi:10.1007/978-3-642-55566-4_21, ISBN 978-3-642-62442-1, MR 2038487 . Grünbaum, Branko (2007), "Graphs of polyhedra; polyhedra as graphs", Discrete Mathematics, 307 (3–5): 445–463, doi:10.1016/j.disc.2005.09.037, MR 2287486 . Grünbaum, Branko; Shephard, G. C. (2013), "Duality of polyhedra", in Senechal, Marjorie, Shaping Space: Exploring polyhedra in nature, art, and the geometrical imagination, New York: Springer, pp. 211–216, doi:10.1007/978-0-387-92714-5_15, ISBN 978-0-387-92713-8, MR 3077226 . Wenninger, Magnus (1983), Dual Models, Cambridge University Press, ISBN 0-521-54325-8, MR 0730208 . External links[edit] Wikimedia Commons has media related to Dual polyhedron. Weisstein, Eric W. "Dual polyhedron". MathWorld. Weisstein, Eric W. "Dual tessellation". MathWorld. Weisstein, Eric W. "Self-dual polyhedron" |

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