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# Peak (geometry)

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### Peak (geometry)

In solid geometry, a face is a flat (planar) surface that forms part of the boundary of a solid object;[1] a three-dimensional solid bounded exclusively by flat faces is a polyhedron.

In more technical treatments of the geometry of polyhedra and higher-dimensional polytopes, the term is also used to mean an element of any dimension of a more general polytope (in any number of dimensions).[2]

## Polygonal face

In elementary geometry, a face is a two-dimensional polygon on the boundary of a polyhedron.[2][3] Other names for a polygonal face include side of a polyhedron, and tile of a Euclidean plane tessellation.

For example, any of the six squares that bound a cube is a face of the cube. Sometimes "face" is also used to refer to the 2-dimensional features of a 4-polytope. With this meaning, the 4-dimensional tesseract has 24 square faces, each sharing two of 8 cubic cells.

Regular examples by Schläfli symbol
Polyhedron Star polyhedron Euclidean tiling Hyperbolic tiling 4-polytope
{4,3} {5/2,5} {4,4} {4,5} {4,3,3}
cube has 3 square faces per vertex. pentagrammic faces per vertex. square tiling in the Euclidean plane has 4 square faces per vertex. order-5 square tiling has 5 square faces per vertex. tesseract has 3 square faces per edge.

Some other polygons, which are not faces, are also important for polyhedra and tessellations. These include Petrie polygons, vertex figures and facets (flat polygons formed by coplanar vertices which do not lie in the same face of the polyhedron).

## k-face

In higher-dimensional geometry the faces of a polytope are features of all dimensions.[2][4][5] A face of dimension k is called a k-face. For example the polygonal faces of an ordinary polyhedron are 2-faces. In set theory, the set of faces of a polytope includes the polytope itself and the empty set where the empty set is for consistency given a "dimension" of −1. For any n-polytope (n-dimensional polytope), −1 ≤ kn.

For example, with this meaning, the faces of a cube include the empty set, its vertices (0-faces), edges (1-faces) and squares (2-faces), and the cube itself (3-face).

All of the following are the faces of a 4-dimensional polytope:

• 4-face – the 4-dimensional 4-polytope itself
• 3-faces – 3-dimensional cells (polyhedral faces)
• 2-faces – 2-dimensional faces (polygonal faces)
• 1-faces – 1-dimensional edges
• 0-faces – 0-dimensional vertices
• the empty set, which has dimension −1

In some areas of mathematics, such as polyhedral combinatorics, a polytope is by definition convex. Formally, a face of a polytope P is the intersection of P with any closed halfspace whose boundary is disjoint from the interior of P.[6] From this definition it follows that the set of faces of a polytope includes the polytope itself and the empty set.[4][5]

In other areas of mathematics, such as the theories of abstract polytopes and star polytopes, the requirement for convexity is relaxed. Abstract theory still requires that the set of faces include the polytope itself and the empty set.

## Cell or 3-face

A cell is a polyhedral element (3-face) of a 4 dimensional polytope or 3 dimensional tessellation, or higher. Cells are facets for 4-polytopes and 3-honeycombs.

Examples:

Regular examples by Schläfli symbol
4-polytopes 3-honeycombs
{4,3,3} {5,3,3} {4,3,4} {5,3,4}
tesseract has 3 cubic cells (3-faces) per edge. dodecahedron cells (3-faces) per edge. cubic honeycomb fills Euclidean 3-space with cubes, with 4 cells (3-faces) per edge. order-4 dodecahedral honeycomb fills 3-dimensional hyperbolic space with dodecahedra, 4 cells (3-faces) per edge.

## Facet or (n-1)-face

In higher-dimensional geometry, the facets of a n-polytope are the (n-1)-faces of dimension one less than the polytope itself.[7] For example:

## Ridge or (n-2)-face

In related terminology, a (n − 2)-face of an n-polytope is called a ridge.[8]

## Peak or (n-3)-face

A (n − 3)-face of an n-polytope is called a peak.