# Polytope

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## polytope

[′päl·i‚tōp]
(mathematics)
A finite region in n-dimensional space (n = 2, 3, 4, …), enclosed by a finite number of hyperplanes; it is the n-dimensional analog of a polygon (n = 2) and a polyhedron (n = 3).

## Polytope

(1) A polyhedron.

(2) A geometric figure that is the union of a finite number of convex polyhedrons of an arbitrary number of dimensions arbitrarily arranged in n-dimensional space. This concept is often made use of in topology and can easily be extended to the case of n-dimensional space.

Let us consider a half space in the n-dimensional space Rn that is, the set of all points located on one side of some in — 1)-dimensional hyperplane of the space along with the points of the hyperplane itself. Analytically, the half space is the set of all points of Rn whose coordinates satisfy a linear inequality of the form a1x1 + a2x2 + … + anxn + b ≥0. The intersection of a finite number of half spaces—if it is bounded— is the most general convex polyhedron of arbitrary dimension ≤ n located in Rn. A poly tope in the general sense of the word is the union of a finite number of such polyhedrons. When n = 2, we obtain two-dimensional polytopes, or polygons, which are not necessarily convex. One-dimensional polytopes are broken lines that need not be connected and may be branched—at any vertex any number of segments may meet. A zero-dimensional polytope is a finite set of points. A three-dimensional polytope can always be partitioned into polyhedrons of the simplest type —that is, into simplexes. Simplexes of dimension 0, 1, 2, and 3 correspond, respectively, to a point, a line segment, a triangle, and a tetrahedron, which is in general irregular. This partitioning, moreover, can be performed in such a way that either two of the resulting simplexes have no points in common or they share a face. Such partitions of a polytope into simplexes are called triangulations and constitute a fundamental research technique in combinatorial topology.

The concept of polytope permits of various generalizations. For example, curved polytopes are the images of polytopes under topological mappings; thus an arbitrary curved surface may be regarded as the topological image of a polyhedral surface. Another example is infinite polytope, which consists of an infinite set of convex polyhedrons (simplexes).

### REFERENCES

Aleksandrov, P. S. Lektsii po analiticheskoi geometrii…. Moscow, 1968.
Aleksandrov, P. S. Kombinatornaia topologiia. Moscow-Leningrad, 1947.
Pontriagin, L. S. Osnovy kombinatornoi topologii. Moscow-Leningrad, 1947.
Aleksandrov, P. S., and B. A. Pasynkov. Vvedenie v teoriiu razmernosti. Moscow, 1973.

P. S. ALEKSANDROV

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