وجه (هندسة رياضية)

مكعب له جميع ثلاث أوجه مربعة تشهجر بنقطة هندسية

في الهندسة الرياضية، يطلق اسم وجه لجسم متعدد الأوجه على أي مضلع يشكل جزءاً من سطح الجسم.

أمثلة

أي مربع على سطح المكعب هووجه للمكعب.
أي مستطيل على سطح متوازي المستطيلات هووجه لمتوازي المستطيلات.
أي مثلث من سطح الهرم هووجه لهذا الهرم.

وجه مضلعي

أمثلة منتظمة حسب رمز شلفلي
Polyhedron	Star polyhedron	Euclidean tiling	Hyperbolic tiling	4-polytope
{4,3	{5/2,5	{4,4	{4,5	{4,3,3
The cube has ثلاثة square faces per vertex.	The small stellated dodecahedron hasخمسة pentagrammic faces per vertex.	The square tiling in the Euclidean plane has أربعة square faces per vertex.	The order-5 square tiling hasخمسة square faces per vertex.	The tesseract has ثلاثة 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).

Number of polygonal faces of a polyhedron

Any convex polyhedron's surface has Euler characteristic

{\displaystyle V-E+F=2,

where V is the number of vertices, E is the number of edges, and F is the number of faces. This equation is known as Euler's polyhedron formula. Thus the number of faces is 2 more than the excess of the number of edges over the number of vertices. For example, a cube has 12 edges andثمانية vertices, and henceستة faces.

k-face

In higher-dimensional geometry the faces of a polytope are features of all dimensions. 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 ≤ k ≤ n.

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. From this definition it follows that the set of faces of a polytope includes the polytope itself and the empty set.

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:

أمثلة منتظمة حسب رمز شلفلي
4-polytopes		3-honeycombs
{4,3,3	{5,3,3	{4,3,4	{5,3,4
The tesseract has ثلاثة cubic cells (3-faces) per edge.	The 120-cell has ثلاثة dodecahedral cells (3-faces) per edge.	The cubic honeycomb fills Euclidean 3-space with cubes, with أربعة cells (3-faces) per edge.	The order-4 dodecahedral honeycomb fills 3-dimensional hyperbolic space with dodecahedra, أربعة 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. A polytope is bounded by its facets.

For example:

The facets of a line segment are its 0-faces or vertices.
The facets of a polygon are its 1-faces or edges.
The facets of a polyhedron or plane tiling are its 2-faces.
The facets of a 4D polytope or 3-honeycomb are its 3-faces.
The facets of a 5D polytope or 4-honeycomb are its 4-faces.

Ridge or (n-2)-face

In related terminology, a (n − 2)-face of an n-polytope is called a ridge (also subfacet). A ridge is seen as the boundary between exactly two facets of a polytope or honeycomb.

For example:

The ridges of a 2D polygon or 1D tiling are its 0-faces or vertices.
The ridges of a 3D polyhedron or plane tiling are its 1-faces or edges.
The ridges of a 4D polytope or 3-honeycomb are its 2-faces or simply faces.
The ridges of a 5D polytope or 4-honeycomb are its 3-faces or cells.

Peak or (n-3)-face

A (n − 3)-face of an n-polytope is called a peak. A peak contain a rotational axis of facets and ridges in a regular polytope or honeycomb.

For example:

The peaks of a 3D polyhedron or plane tiling are its 0-faces or vertices.
The peaks of a 4D polytope or 3-honeycomb are its 1-faces or edges.
The peaks of a 5D polytope or 4-honeycomb are its 2-faces or simply faces.

الهامش

^ خطأ استشهاد: وسم <ref> غير سليم؛ لا نص تم توفيره للمراجع المسماة m
^ Grünbaum, Branko (2003), Convex Polytopes, Graduate Texts in Mathematics, 221 (2nd ed.), Springer, p. 17, https://books.google.com/books?id=ISHO86XJ1CsC&pg=PA17 .
^ Ziegler, Günter M. (1995), Lectures on Polytopes, Graduate Texts in Mathematics, 152, Springer, Definition 2.1, p. 51, https://books.google.com/books?id=xd25TXSSUcgC&pg=PA51 .
^ Matoušek (2002) and Ziegler (1995) use a slightly different but equivalent definition, which amounts to intersecting P with either a hyperplane disjoint from the interior of P or the whole space.
^ Matoušek (2002), p. 87; Grünbaum (2003), p. 27; Ziegler (1995), p. 17.
^ Matoušek (2002), p. 87; Ziegler (1995), p. 71.