Polygon

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Some polygons of different kinds: open (excluding its boundary), boundary only (excluding interior), closed (including both boundary and interior), and self-intersecting. Assorted polygons.svg
Some polygons of different kinds: open (excluding its boundary), boundary only (excluding interior), closed (including both boundary and interior), and self-intersecting.

In geometry, a polygon ( /ˈpɒlɪɡɒn/ ) 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 together, may be called a polygon.

Contents

The segments of a polygonal circuit are called its edges or sides. The points where two edges meet are the polygon's vertices (singular: vertex) or corners. The interior of a solid polygon is sometimes called its body. An n-gon is a polygon with n sides; for example, a triangle is a 3-gon.

A simple polygon is one which does not intersect itself. Mathematicians are often concerned only with the bounding polygonal chains of simple polygons and they often define a polygon accordingly. A polygonal boundary may be allowed to cross over itself, creating star polygons and other self-intersecting polygons.

A polygon is a 2-dimensional example of the more general polytope in any number of dimensions. There are many more generalizations of polygons defined for different purposes.

Etymology

The word polygon derives from the Greek adjective πολύς (polús) 'much', 'many' and γωνία (gōnía) 'corner' or 'angle'. It has been suggested that γόνυ (gónu) 'knee' may be the origin of gon. [1]

Classification

Some different types of polygon Polygon types.svg
Some different types of polygon

Number of sides

Polygons are primarily classified by the number of sides. See the table below.

Convexity and intersection

Polygons may be characterized by their convexity or type of non-convexity:

Equality and symmetry

The property of regularity may be defined in other ways: a polygon is regular if and only if it is both isogonal and isotoxal, or equivalently it is both cyclic and equilateral. A non-convex regular polygon is called a regular star polygon .

Miscellaneous

Properties and formulas

Partitioning an n-gon into n - 2 triangles Winkelsumme-polygon.svg
Partitioning an n-gon into n − 2 triangles

Euclidean geometry is assumed throughout.

Angles

Any polygon has as many corners as it has sides. Each corner has several angles. The two most important ones are:

Area

Coordinates of a non-convex pentagon. Polygon vertex labels.svg
Coordinates of a non-convex pentagon.

In this section, the vertices of the polygon under consideration are taken to be in order. For convenience in some formulas, the notation (xn, yn) = (x0, y0) will also be used.

Simple polygons

If the polygon is non-self-intersecting (that is, simple), the signed area is

or, using determinants

where is the squared distance between and [4] [5]

The signed area depends on the ordering of the vertices and of the orientation of the plane. Commonly, the positive orientation is defined by the (counterclockwise) rotation that maps the positive x-axis to the positive y-axis. If the vertices are ordered counterclockwise (that is, according to positive orientation), the signed area is positive; otherwise, it is negative. In either case, the area formula is correct in absolute value. This is commonly called the shoelace formula or surveyor's formula. [6]

The area A of a simple polygon can also be computed if the lengths of the sides, a1, a2, ..., an and the exterior angles, θ1, θ2, ..., θn are known, from:

The formula was described by Lopshits in 1963. [7]

If the polygon can be drawn on an equally spaced grid such that all its vertices are grid points, Pick's theorem gives a simple formula for the polygon's area based on the numbers of interior and boundary grid points: the former number plus one-half the latter number, minus 1.

In every polygon with perimeter p and area A , the isoperimetric inequality holds. [8]

For any two simple polygons of equal area, the Bolyai–Gerwien theorem asserts that the first can be cut into polygonal pieces which can be reassembled to form the second polygon.

The lengths of the sides of a polygon do not in general determine its area. [9] However, if the polygon is simple and cyclic then the sides do determine the area. [10] Of all n-gons with given side lengths, the one with the largest area is cyclic. Of all n-gons with a given perimeter, the one with the largest area is regular (and therefore cyclic). [11]

Regular polygons

Many specialized formulas apply to the areas of regular polygons.

The area of a regular polygon is given in terms of the radius r of its inscribed circle and its perimeter p by

This radius is also termed its apothem and is often represented as a.

The area of a regular n-gon in terms of the radius R of its circumscribed circle can be expressed trigonometrically as: [12] [13]

The area of a regular n-gon inscribed in a unit-radius circle, with side s and interior angle can also be expressed trigonometrically as:

Self-intersecting

The area of a self-intersecting polygon can be defined in two different ways, giving different answers:

  • Using the formulas for simple polygons, we allow that particular regions within the polygon may have their area multiplied by a factor which we call the density of the region. For example, the central convex pentagon in the center of a pentagram has density 2. The two triangular regions of a cross-quadrilateral (like a figure 8) have opposite-signed densities, and adding their areas together can give a total area of zero for the whole figure. [14]
  • Considering the enclosed regions as point sets, we can find the area of the enclosed point set. This corresponds to the area of the plane covered by the polygon or to the area of one or more simple polygons having the same outline as the self-intersecting one. In the case of the cross-quadrilateral, it is treated as two simple triangles.[ citation needed ]

Centroid

Using the same convention for vertex coordinates as in the previous section, the coordinates of the centroid of a solid simple polygon are

In these formulas, the signed value of area must be used.

For triangles (n = 3), the centroids of the vertices and of the solid shape are the same, but, in general, this is not true for n > 3. The centroid of the vertex set of a polygon with n vertices has the coordinates

Generalizations

The idea of a polygon has been generalized in various ways. Some of the more important include:

Naming

The word polygon comes from Late Latin polygōnum (a noun), from Greek πολύγωνον (polygōnon/polugōnon), noun use of neuter of πολύγωνος (polygōnos/polugōnos, the masculine adjective), meaning "many-angled". Individual polygons are named (and sometimes classified) according to the number of sides, combining a Greek-derived numerical prefix with the suffix -gon, e.g. pentagon , dodecagon . The triangle, quadrilateral and nonagon are exceptions.

Beyond decagons (10-sided) and dodecagons (12-sided), mathematicians generally use numerical notation, for example 17-gon and 257-gon. [17]

Exceptions exist for side counts that are easily expressed in verbal form (e.g. 20 and 30), or are used by non-mathematicians. Some special polygons also have their own names; for example the regular star pentagon is also known as the pentagram.

Polygon names and miscellaneous properties
NameSidesProperties
monogon 1Not generally recognised as a polygon, [18] although some disciplines such as graph theory sometimes use the term. [19]
digon 2Not generally recognised as a polygon in the Euclidean plane, although it can exist as a spherical polygon. [20]
triangle (or trigon)3The simplest polygon which can exist in the Euclidean plane. Can tile the plane.
quadrilateral (or tetragon)4The simplest polygon which can cross itself; the simplest polygon which can be concave; the simplest polygon which can be non-cyclic. Can tile the plane.
pentagon 5 [21] The simplest polygon which can exist as a regular star. A star pentagon is known as a pentagram or pentacle.
hexagon 6 [21] Can tile the plane.
heptagon (or septagon)7 [21] The simplest polygon such that the regular form is not constructible with compass and straightedge. However, it can be constructed using a neusis construction.
octagon 8 [21]
nonagon (or enneagon)9 [21] "Nonagon" mixes Latin [novem = 9] with Greek; "enneagon" is pure Greek.
decagon 10 [21]
hendecagon (or undecagon)11 [21] The simplest polygon such that the regular form cannot be constructed with compass, straightedge, and angle trisector. However, it can be constructed with neusis. [22]
dodecagon (or duodecagon)12 [21]
tridecagon (or triskaidecagon)13 [21]
tetradecagon (or tetrakaidecagon)14 [21]
pentadecagon (or pentakaidecagon)15 [21]
hexadecagon (or hexakaidecagon)16 [21]
heptadecagon (or heptakaidecagon)17 Constructible polygon [17]
octadecagon (or octakaidecagon)18 [21]
enneadecagon (or enneakaidecagon)19 [21]
icosagon 20 [21]
icositrigon (or icosikaitrigon)23The simplest polygon such that the regular form cannot be constructed with neusis. [23] [22]
icositetragon (or icosikaitetragon)24 [21]
icosipentagon (or icosikaipentagon)25The simplest polygon such that it is not known if the regular form can be constructed with neusis or not. [23] [22]
triacontagon 30 [21]
tetracontagon (or tessaracontagon)40 [21] [24]
pentacontagon (or pentecontagon)50 [21] [24]
hexacontagon (or hexecontagon)60 [21] [24]
heptacontagon (or hebdomecontagon)70 [21] [24]
octacontagon (or ogdoëcontagon)80 [21] [24]
enneacontagon (or enenecontagon)90 [21] [24]
hectogon (or hecatontagon) [25] 100 [21]
257-gon 257 Constructible polygon [17]
chiliagon 1000Philosophers including René Descartes, [26] Immanuel Kant, [27] David Hume, [28] have used the chiliagon as an example in discussions.
myriagon 10,000Used as an example in some philosophical discussions, for example in Descartes's Meditations on First Philosophy
65537-gon 65,537 Constructible polygon [17]
megagon [29] [30] [31] 1,000,000As with René Descartes's example of the chiliagon, the million-sided polygon has been used as an illustration of a well-defined concept that cannot be visualised. [32] [33] [34] [35] [36] [37] [38] The megagon is also used as an illustration of the convergence of regular polygons to a circle. [39]
apeirogon A degenerate polygon of infinitely many sides.

To construct the name of a polygon with more than 20 and less than 100 edges, combine the prefixes as follows. [21] The "kai" term applies to 13-gons and higher and was used by Kepler, and advocated by John H. Conway for clarity of concatenated prefix numbers in the naming of quasiregular polyhedra, [25] though not all sources use it.

TensandOnesfinal suffix
-kai-1-hena--gon
20icosi- (icosa- when alone)2-di-
30triaconta- (or triconta-)3-tri-
40tetraconta- (or tessaraconta-)4-tetra-
50pentaconta- (or penteconta-)5-penta-
60hexaconta- (or hexeconta-)6-hexa-
70heptaconta- (or hebdomeconta-)7-hepta-
80octaconta- (or ogdoëconta-)8-octa-
90enneaconta- (or eneneconta-)9-ennea-

History

Historical image of polygons (1699) Fotothek df tg 0003352 Geometrie ^ Dreieck ^ Viereck ^ Vieleck ^ Winkel.jpg
Historical image of polygons (1699)

Polygons have been known since ancient times. The regular polygons were known to the ancient Greeks, with the pentagram, a non-convex regular polygon (star polygon), appearing as early as the 7th century B.C. on a krater by Aristophanes, found at Caere and now in the Capitoline Museum. [40] [41]

The first known systematic study of non-convex polygons in general was made by Thomas Bradwardine in the 14th century. [42]

In 1952, Geoffrey Colin Shephard generalized the idea of polygons to the complex plane, where each real dimension is accompanied by an imaginary one, to create complex polygons. [43]

In nature

The Giant's Causeway, in Northern Ireland Giants causeway closeup.jpg
The Giant's Causeway, in Northern Ireland

Polygons appear in rock formations, most commonly as the flat facets of crystals, where the angles between the sides depend on the type of mineral from which the crystal is made.

Regular hexagons can occur when the cooling of lava forms areas of tightly packed columns of basalt, which may be seen at the Giant's Causeway in Northern Ireland, or at the Devil's Postpile in California.

In biology, the surface of the wax honeycomb made by bees is an array of hexagons, and the sides and base of each cell are also polygons.

Computer graphics

In computer graphics, a polygon is a primitive used in modelling and rendering. They are defined in a database, containing arrays of vertices (the coordinates of the geometrical vertices, as well as other attributes of the polygon, such as color, shading and texture), connectivity information, and materials. [44] [45]

Any surface is modelled as a tessellation called polygon mesh. If a square mesh has n + 1 points (vertices) per side, there are n squared squares in the mesh, or 2n squared triangles since there are two triangles in a square. There are (n + 1)2 / 2(n2) vertices per triangle. Where n is large, this approaches one half. Or, each vertex inside the square mesh connects four edges (lines).

The imaging system calls up the structure of polygons needed for the scene to be created from the database. This is transferred to active memory and finally, to the display system (screen, TV monitors etc.) so that the scene can be viewed. During this process, the imaging system renders polygons in correct perspective ready for transmission of the processed data to the display system. Although polygons are two-dimensional, through the system computer they are placed in a visual scene in the correct three-dimensional orientation.

In computer graphics and computational geometry, it is often necessary to determine whether a given point lies inside a simple polygon given by a sequence of line segments. This is called the point in polygon test. [46]

See also

Related Research Articles

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<span class="mw-page-title-main">Octahedron</span> Polyhedron with 8 triangular faces

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In geometry a quadrilateral is a four-sided polygon, having four edges (sides) and four corners (vertices). The word is derived from the Latin words quadri, a variant of four, and latus, meaning "side". It is also called a tetragon, derived from greek "tetra" meaning "four" and "gon" meaning "corner" or "angle", in analogy to other polygons. Since "gon" means "angle", it is analogously called a quadrangle, or 4-angle. A quadrilateral with vertices , , and is sometimes denoted as .

<span class="mw-page-title-main">Tetrahedron</span> Polyhedron with 4 faces

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<span class="mw-page-title-main">Diagonal</span> In geometry a line segment joining two nonconsecutive vertices of a polygon or polyhedron

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References

Bibliography

Notes

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  41. Cratere with the blinding of Polyphemus and a naval battle Archived 2013-11-12 at the Wayback Machine , Castellani Halls, Capitoline Museum, accessed 2013-11-11. Two pentagrams are visible near the center of the image,
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Family An Bn I2(p) / Dn E6 / E7 / E8 / F4 / G2 Hn
Regular polygon Triangle Square p-gon Hexagon Pentagon
Uniform polyhedron Tetrahedron OctahedronCube Demicube DodecahedronIcosahedron
Uniform polychoron Pentachoron 16-cellTesseract Demitesseract 24-cell 120-cell600-cell
Uniform 5-polytope 5-simplex 5-orthoplex5-cube 5-demicube
Uniform 6-polytope 6-simplex 6-orthoplex6-cube 6-demicube 122221
Uniform 7-polytope 7-simplex 7-orthoplex7-cube 7-demicube 132231321
Uniform 8-polytope 8-simplex 8-orthoplex8-cube 8-demicube 142241421
Uniform 9-polytope 9-simplex 9-orthoplex9-cube 9-demicube
Uniform 10-polytope 10-simplex 10-orthoplex10-cube 10-demicube
Uniform n-polytope n-simplex n-orthoplexn-cube n-demicube 1k22k1k21 n-pentagonal polytope
Topics: Polytope familiesRegular polytopeList of regular polytopes and compounds