geometry Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is ...

, a vertex figure, broadly speaking, is the figure exposed when a corner of a

polyhedron In geometry, a polyhedron (plural polyhedra or polyhedrons; ) is a three-dimensional shape with flat polygonal faces, straight edges and sharp corners or vertices. A convex polyhedron is the convex hull of finitely many points, not all on ...

polytope In elementary geometry, a polytope is a geometric object with flat sides ('' faces''). Polytopes are the generalization of three-dimensional polyhedra to any number of dimensions. Polytopes may exist in any general number of dimensions as an ...

is sliced off.

Definitions

Take some corner or

vertex Vertex, vertices or vertexes may refer to: Science and technology Mathematics and computer science *Vertex (geometry), a point where two or more curves, lines, or edges meet * Vertex (computer graphics), a data structure that describes the positio ...

of a

. Mark a point somewhere along each connected edge. Draw lines across the connected faces, joining adjacent points around the face. When done, these lines form a complete circuit, i.e. a polygon, around the vertex. This polygon is the vertex figure. More precise formal definitions can vary quite widely, according to circumstance. For example

Coxeter Harold Scott MacDonald "Donald" Coxeter, (9 February 1907 – 31 March 2003) was a British and later also Canadian geometer. He is regarded as one of the greatest geometers of the 20th century. Biography Coxeter was born in Kensington to ...

(e.g. 1948, 1954) varies his definition as convenient for the current area of discussion. Most of the following definitions of a vertex figure apply equally well to infinite tilings or, by extension, to space-filling tessellation with

cells and other higher-dimensional

As a flat slice

Make a slice through the corner of the polyhedron, cutting through all the edges connected to the vertex. The cut surface is the vertex figure. This is perhaps the most common approach, and the most easily understood. Different authors make the slice in different places. Wenninger (2003) cuts each edge a unit distance from the vertex, as does Coxeter (1948). For uniform polyhedra the Dorman Luke construction cuts each connected edge at its midpoint. Other authors make the cut through the vertex at the other end of each edge. For an irregular polyhedron, cutting all edges incident to a given vertex at equal distances from the vertex may produce a figure that does not lie in a plane. A more general approach, valid for arbitrary convex polyhedra, is to make the cut along any plane which separates the given vertex from all the other vertices, but is otherwise arbitrary. This construction determines the combinatorial structure of the vertex figure, similar to a set of connected vertices (see below), but not its precise geometry; it may be generalized to convex polytopes in any dimension. However, for non-convex polyhedra, there may not exist a plane near the vertex that cuts all of the faces incident to the vertex.

As a spherical polygon

Cromwell (1999) forms the vertex figure by intersecting the polyhedron with a sphere centered at the vertex, small enough that it intersects only edges and faces incident to the vertex. This can be visualized as making a spherical cut or scoop, centered on the vertex. The cut surface or vertex figure is thus a spherical polygon marked on this sphere. One advantage of this method is that the shape of the vertex figure is fixed (up to the scale of the sphere), whereas the method of intersecting with a plane can produce different shapes depending on the angle of the plane. Additionally, this method works for non-convex polyhedra.

As the set of connected vertices

Many combinatorial and computational approaches (e.g. Skilling, 1975) treat a vertex figure as the ordered (or partially ordered) set of points of all the neighboring (connected via an edge) vertices to the given vertex.

Abstract definition

In the theory of

abstract polytope In mathematics, an abstract polytope is an algebraic partially ordered set which captures the dyadic property of a traditional polytope without specifying purely geometric properties such as points and lines. A geometric polytope is said to be ...

s, the vertex figure at a given vertex ''V'' comprises all the elements which are incident on the vertex; edges, faces, etc. More formally it is the (''n''−1)-section ''F_n''/''V'', where ''F_n'' is the greatest face. This set of elements is elsewhere known as a ''vertex star''. The geometrical vertex figure and the vertex star may be understood as distinct ''realizations'' of the same abstract section.

General properties

A vertex figure of an ''n''-polytope is an (''n''−1)-polytope. For example, a vertex figure of a

is a

polygon In geometry, a polygon () is a plane figure that is described by a finite number of straight line segments connected to form a closed ''polygonal chain'' (or ''polygonal circuit''). The bounded plane region, the bounding circuit, or the two to ...

, and the vertex figure for a

4-polytope In geometry, a 4-polytope (sometimes also called a polychoron, polycell, or polyhedroid) is a four-dimensional polytope. It is a connected and closed figure, composed of lower-dimensional polytopal elements: vertices, edges, faces (polygons), an ...

is a polyhedron. In general a vertex figure need not be planar. For nonconvex polyhedra, the vertex figure may also be nonconvex. Uniform polytopes, for instance, can have star polygons for faces and/or for vertex figures.

Isogonal figures

Vertex figures are especially significant for

uniform A uniform is a variety of clothing worn by members of an organization while participating in that organization's activity. Modern uniforms are most often worn by armed forces and paramilitary organizations such as police, emergency services, ...

s and other isogonal (vertex-transitive) polytopes because one vertex figure can define the entire polytope. For polyhedra with regular faces, a vertex figure can be represented in

vertex configuration In geometry, a vertex configurationCrystallography ...

notation, by listing the faces in sequence around the vertex. For example 3.4.4.4 is a vertex with one triangle and three squares, and it defines the uniform

rhombicuboctahedron In geometry, the rhombicuboctahedron, or small rhombicuboctahedron, is a polyhedron with eight triangular, six square, and twelve rectangular faces. There are 24 identical vertices, with one triangle, one square, and two rectangles meeting at ea ...

. If the polytope is isogonal, the vertex figure will exist in a hyperplane surface of the ''n''-space.

Constructions

From the adjacent vertices

By considering the connectivity of these neighboring vertices, a vertex figure can be constructed for each vertex of a polytope: *Each

of the ''vertex figure'' coincides with a vertex of the original polytope. *Each

edge Edge or EDGE may refer to: Technology Computing * Edge computing, a network load-balancing system * Edge device, an entry point to a computer network * Adobe Edge, a graphical development application * Microsoft Edge, a web browser developed ...

of the ''vertex figure'' exists on or inside of a face of the original polytope connecting two alternate vertices from an original face. *Each

face The face is the front of an animal's head that features the eyes, nose and mouth, and through which animals express many of their emotions. The face is crucial for human identity, and damage such as scarring or developmental deformities may aff ...

of the ''vertex figure'' exists on or inside a cell of the original ''n''-polytope (for ''n'' > 3). *... and so on to higher order elements in higher order polytopes.

Dorman Luke construction

For a uniform polyhedron, the face of the

dual polyhedron In geometry, every polyhedron is associated with a second dual structure, 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. ...

may be found from the original polyhedron's vertex figure using the " Dorman Luke" construction.

Regular polytopes

If a polytope is regular, it can be represented by a Schläfli symbol and both the

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

and the vertex figure can be trivially extracted from this notation. In general a regular polytope with Schläfli symbol has cells as , and ''vertex figures'' as . #For a

regular polyhedron A regular polyhedron is a polyhedron whose symmetry group acts transitively on its flags. A regular polyhedron is highly symmetrical, being all of edge-transitive, vertex-transitive and face-transitive. In classical contexts, many different equival ...

, the vertex figure is , a ''q''-gon. #*Example, the vertex figure for a cube , is the triangle . #For a

regular 4-polytope In mathematics, a regular 4-polytope is a regular four-dimensional polytope. They are the four-dimensional analogues of the regular polyhedra in three dimensions and the regular polygons in two dimensions. There are six convex and ten star reg ...

or space-filling tessellation , the vertex figure is . #*Example, the vertex figure for a hypercube , the vertex figure is a regular tetrahedron . #*Also the vertex figure for a

cubic honeycomb The cubic honeycomb or cubic cellulation is the only proper regular space-filling tessellation (or honeycomb) in Euclidean 3-space made up of cubic cells. It has 4 cubes around every edge, and 8 cubes around each vertex. Its vertex figure is a r ...

, the vertex figure is a regular octahedron . Since the dual polytope of a regular polytope is also regular and represented by the Schläfli symbol indices reversed, it is easy to see the dual of the vertex figure is the cell of the dual polytope. For regular polyhedra, this is a special case of the

Dorman Luke construction In geometry, every polyhedron is associated with a second dual structure, 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. ...

An example vertex figure of a honeycomb

The vertex figure of a truncated cubic honeycomb is a nonuniform square pyramid. One octahedron and four truncated cubes meet at each vertex form a space-filling

tessellation A tessellation or tiling is the covering of a surface, often a plane, using one or more geometric shapes, called ''tiles'', with no overlaps and no gaps. In mathematics, tessellation can be generalized to higher dimensions and a variety o ...

Edge figure

Related to the ''vertex figure'', an ''edge figure'' is the ''vertex figure'' of a ''vertex figure''.Klitzing: Vertex figures, etc.
/ref> Edge figures are useful for expressing relations between the elements within regular and uniform polytopes. An ''edge figure'' will be a (''n''−2)-polytope, representing the arrangement of

facets A facet is a flat surface of a geometric shape, e.g., of a cut gemstone. Facet may also refer to: Arts, entertainment, and media * ''Facets'' (album), an album by Jim Croce * ''Facets'', a 1980 album by jazz pianist Monty Alexander and his tri ...

around a given edge. Regular and single-ringed

coxeter diagram Harold Scott MacDonald "Donald" Coxeter, (9 February 1907 – 31 March 2003) was a British and later also Canadian geometer. He is regarded as one of the greatest geometers of the 20th century. Biography Coxeter was born in Kensington t ...

uniform polytopes will have a single edge type. In general, a uniform polytope can have as many edge types as active mirrors in the construction, since each active mirror produces one edge in the fundamental domain. Regular polytopes (and honeycombs) have a single ''edge figure'' which is also regular. For a regular polytope , the ''edge figure'' is . In four dimensions, the edge figure of a

or 3-honeycomb is a polygon representing the arrangement of a set of facets around an edge. For example, the ''edge figure'' for a regular

is a

square In Euclidean geometry, a square is a regular quadrilateral, which means that it has four equal sides and four equal angles (90- degree angles, π/2 radian angles, or right angles). It can also be defined as a rectangle with two equal-length a ...

, and for a regular 4-polytope is the polygon . Less trivially, the truncated cubic honeycomb t_0,1, has a square pyramid vertex figure, with

truncated cube In geometry, the truncated cube, or truncated hexahedron, is an Archimedean solid. It has 14 regular faces (6 octagonal and 8 triangular), 36 edges, and 24 vertices. If the truncated cube has unit edge length, its dual triakis octahedron has edg ...

and

octahedron In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...

cells. Here there are two types of ''edge figures''. One is a square edge figure at the apex of the pyramid. This represents the four ''truncated cubes'' around an edge. The other four edge figures are isosceles triangles on the base vertices of the pyramid. These represent the arrangement of two truncated cubes and one octahedron around the other edges.

References

Notes

Bibliography

* H. S. M. Coxeter, ''Regular Polytopes'', Hbk (1948), ppbk (1973). *H.S.M. Coxeter (et al.), Uniform Polyhedra, ''Phil. Trans''. 246 A (1954) pp. 401–450. *P. Cromwell, ''Polyhedra'', CUP pbk. (1999). *H.M. Cundy and A.P. Rollett, ''

Mathematical Models A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used in the natural sciences (such as physics, ...

'', Oxford Univ. Press (1961). *J. Skilling, The Complete Set of Uniform Polyhedra, ''Phil. Trans''. 278 A (1975) pp. 111–135. *M. Wenninger, ''Dual Models'', CUP hbk (1983) ppbk (2003). *''The Symmetries of Things'' 2008, John H. Conway, Heidi Burgiel, Chaim Goodman-Strass, (p289 Vertex figures)

External links

* *
Vertex Figures
{{Use dmy dates, date=February 2021 Polyhedra Polytopes