Misplaced Pages

This is an old revision of this page, as edited by 173.76.182.113 (talk) at 23:04, 13 April 2009 (→Classification). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

I|thumb|400px|right|An assortment of polygons]]

Generalizations of polygons

In a broad sense, a polygon is an unbounded (without ends) sequence or circuit of alternating segments (sides) and angles (corners). An ordinary polygon is unbounded because the sequence closes back in itself in a loop or circuit, while an apeirogon (infinite polygon) is unbounded because it goes on for ever so you can never reach any bounding end point. The modern mathematical understanding is to describe such a structural sequence in terms of an 'abstract' polygon which is a partially ordered set (poset) of elements. The interior (body) of the polygon is another element, and (for technical reasons) so is the null polytope or nullitope.

A geometric polygon is understood to be a 'realization' of the associated abstract polygon; this involves some 'mapping' of elements from the abstract to the geometric. Such a polygon does not have to lie in a plane, or have straight sides, or enclose an area, and individual elements can overlap or even coincide. For example a spherical polygon is drawn on the surface of a sphere, and its sides are arcs of great circles. So when we talk about "polygons" we must be careful to explain what kind we are talking about.

A digon is a closed polygon having two sides and two corners. On the sphere, we can mark two opposing points (like the North and South poles) and join them by half a great circle. Add another arc of a different great circle and you have a digon. Tile the sphere with digons and you have a polyhedron called a hosohedron. Take just one great circle instead, run it all the way round, and add just one "corner" point, and you have a monogon or henagon - although many authorities do not regard this as a proper polygon.

Other realizations of these polygons are possible on other surfaces - but in the Euclidean (flat) plane, their bodies cannot be sensibly realized and we think of them as degenerate.

The idea of a polygon has been generalized in various ways. Here is a short list of some degenerate cases (or special cases, depending on your point of view):

• Digon. Interior angle of 0° in the Euclidean plane. See remarks above re. on the sphere.
• Interior angle of 180°: In the plane this gives an apeirogon (see below), on the sphere a dihedron
• A skew polygon does not lie in a flat plane, but zigzags in three (or more) dimensions. The Petrie polygons of the regular polyhedra are classic examples.
• A spherical polygon is a circuit of sides and corners on the surface of a sphere.
• An apeirogon is an infinite sequence of sides and angles, which is not closed but it has no ends because it extends infinitely.
• A complex polygon is a figure analogous to an ordinary polygon, which exists in the complex Hilbert plane.

Naming polygons

The word 'polygon' comes from Late Latin polygōnum (a noun), from Greek polygōnon/polugōnon πολύγωνον, noun use of neuter of polygōnos/polugōnos πολύγωνος (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 or quadrangle, and nonagon are exceptions. For large numbers, mathematicians usually write the numeral itself, e.g. 17-gon. A variable can even be used, usually n-gon. This is useful if the number of sides is used in a formula.

Some special polygons also have their own names; for example the regular star pentagon is also known as the pentagram.

To construct the name of a polygon with more than 20 and less than 100 edges, combine the prefixes as follows

The 'kai' is not always used. Opinions differ on exactly when it should, or need not, be used (see also examples above).

That is, a 42-sided figure would be named as follows:

and a 50-sided figure

But beyond enneagons and decagons, professional mathematicians generally prefer the aforementioned numeral notation (for example, MathWorld has articles on 17-gons and 257-gons). Exceptions exist for side numbers that are difficult to express in numerical form.

History

This article may require cleanup to meet Misplaced Pages's quality standards. No cleanup reason has been specified. Please help improve this article if you can. (February 2009) (Learn how and when to remove this message)

Polygons have been known since ancient times. The regular polygons were known to the ancient Greeks, and the pentagram, a non-convex regular polygon (star polygon), appears on the vase of Aristophonus, Caere, dated to the 7th century B.C.. Non-convex polygons in general were not systematically studied until the 14th century by Thomas Bredwardine.

In 1952, Shephard generalised the idea of polygons to the complex plane, where each real dimension is accompanied by an imaginary one, to create complex polygons.

Polygons in nature

Numerous regular polygons may be seen in nature. In the world of geology, crystals have flat faces, or facets, which are polygons. Quasicrystals can even have regular pentagons as faces. Another fascinating example of regular polygons occurs when the cooling of lava forms areas of tightly packed hexagonal columns of basalt, which may be seen at the Giant's Causeway in Ireland, or at the Devil's Postpile in California.

The most famous hexagons in nature are found in the animal kingdom. The wax honeycomb made by bees is an array of hexagons used to store honey and pollen, and as a secure place for the larvae to grow. There also exist animals who themselves take the approximate form of regular polygons, or at least have the same symmetry. For example, sea stars display the symmetry of a pentagon or, less frequently, the heptagon or other polygons. Other echinoderms, such as sea urchins, sometimes display similar symmetries. Though echinoderms do not exhibit exact radial symmetry, jellyfish and comb jellies do, usually fourfold or eightfold.

Radial symmetry (and other symmetry) is also widely observed in the plant kingdom, particularly amongst flowers, and (to a lesser extent) seeds and fruit, the most common form of such symmetry being pentagonal. A particularly striking example is the Starfruit, a slightly tangy fruit popular in Southeast Asia, whose cross-section is shaped like a pentagonal star.

Moving off the earth into space, early mathematicians doing calculations using Newton's law of gravitation discovered that if two bodies (such as the sun and the earth) are orbiting one another, there exist certain points in space, called Lagrangian points, where a smaller body (such as an asteroid or a space station) will remain in a stable orbit. The sun-earth system has five Lagrangian points. The two most stable are exactly 60 degrees ahead and behind the earth in its orbit; that is, joining the centre of the sun and the earth and one of these stable Lagrangian points forms an equilateral triangle. Astronomers have already found asteroids at these points. It is still debated whether it is practical to keep a space station at the Lagrangian point — although it would never need course corrections, it would have to frequently dodge the asteroids that are already present there. There are already satellites and space observatories at the less stable Lagrangian points.

Uses for polygons

• Cut up a piece of paper into polygons, and put them back together as a tangram.
• Join many edge-to-edge as a tiling or tessellation.
• Join several edge-to-edge and fold them all up so there are no gaps, to make a three-dimensional polyhedron.
• Join many edge-to-edge, folding them into a crinkly thing called an infinite polyhedron.
• Use computer-generated polygons to build up a three-dimensional world full of monsters, theme parks, aeroplanes or anything - see Polygons in computer graphics below.

Polygons in computer graphics

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (April 2007) (Learn how and when to remove this message)

A polygon in a computer graphics (image generation) system is a two-dimensional shape that is modelled and stored within its database. A polygon can be coloured, shaded and textured, and its position in the database is defined by the co-ordinates of its vertices (corners).

Naming conventions differ from those of mathematicians:

• A simple polygon does not cross itself.
• a concave polygon is a simple polygon having at least one interior angle greater than 180 deg.
• A complex polygon does cross itself.

Use of Polygons in Real-time imagery. 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 so that as the viewing point moves through the scene, it is perceived in 3D.

Morphing. To avoid artificial effects at polygon boundaries where the planes of contiguous polygons are at different angle, so called 'Morphing Algorithms' are used. These blend, soften or smooth the polygon edges so that the scene looks less artificial and more like the real world.

Polygon Count. Since a polygon can have many sides and need many points to define it, in order to compare one imaging system with another, "polygon count" is generally taken as a triangle. A triangle is processed as three points in the x,y, and z axes, needing nine geometrical descriptors. In addition, coding is applied to each polygon for colour, brightness, shading, texture, NVG (intensifier or night vision), Infra-Red characteristics and so on. When analysing the characteristics of a particular imaging system, the exact definition of polygon count should be obtained as it applies to that system.

Meshed Polygons. The number of meshed polygons (`meshed' is like a fish net) can be up to twice that of free-standing unmeshed polygons, particularly if the polygons are contiguous. 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) 2/2n2 vertices per triangle. Where n is large, this approaches one half. Or, each vertex inside the square mesh connects four edges (lines).

Vertex Count. Because of effects such as the above, a count of Vertices may be more reliable than Polygon count as an indicator of the capability of an imaging system.

Point in polygon test. In computer graphics and computational geometry, it is often necessary to determine whether a given point P = (x₀,y₀) lies inside a simple polygon given by a sequence of line segments. It is known as the Point in polygon test.

Pop culture references

They Might Be Giants have a song entitled "Nonagon" on their children's album "Here Come the 123s." The song anthropomorphizes each of the regular polygons with three through eight sides (except the heptagon), placing them at a party hosted by the Nonagon. A video on the DVD featuring this song shows each of the polygons as equiangular shapes with simply-drawn human characteristics.

External links

• Polygon name generator, enter the number of sides to see the polygon's name
• Weisstein, Eric W. "Polygon". MathWorld.
• What Are Polyhedra?, with Greek Numerical Prefixes
• Polygons, types of polygons, and polygon properties, with interactive animation
• How to draw monochrome orthogonal polygons on screens, by Herbert Glarner
• comp.graphics.algorithms Frequently Asked Questions, solutions to mathematical problems computing 2D and 3D polygons
• Comparison of the different algorithms for Polygon Boolean operations, compares capabilities, speed and numerical robustness

See also

• Constructible polygon
• Cyclic polygon
• Geometric shape
• Polygon triangulation
• Polyform
• Polyhedron
• Polytope
• Regular polygon
• Simple polygon
• Star polygon
• Synthetic geometry
• Tiling
• Tiling puzzle
• Golygon

• Boolean operations on polygons, boolean operations (AND, OR, NOT, XOR, etc.) operating on one or more sets of polygons.

References

1. Meditation VI by Descartes (English translation).
2. Geometry Demystified: A Self-teaching Guide By Stan Gibilisco Published by McGraw-Hill Professional, 2003 ISBN 0071416501, 9780071416504

• Coxeter, H.S.M.; Regular Polytopes, (Methuen and Co., 1948).
• Cromwell, P.;Polyhedra, CUP hbk (1997), pbk. (1999).
• Grünbaum, B.; Are your polyhedra the same as my polyhedra? Discrete and comput. geom: the Goodman-Pollack festschrift, ed. Aronov et al. Springer (2003) pp. 461-488. (pdf)

• Articles needing cleanup from February 2009
• Cleanup tagged articles without a reason field from February 2009
• Misplaced Pages pages needing cleanup from February 2009
• Polygons
• Polytopes
• Geometry
• Euclidean plane geometry