Star polygon
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Set of regular star polygons | |
---|---|
Example: A regular pentagram (Pentagonal star polygon: {5/2}) | |
Schläfli symbol 2<2q<p gcd(p,q)=1 | {p/q} |
Vertices and Edges | p |
Coxeter–Dynkin diagram | |
Symmetry group | Dihedral (D_{p}) |
Dual polygon | Self-dual |
Internal angle (degrees) |
A star polygon is a non-convex polygon which looks in some way like a star. Only the regular ones have been studied in any depth; star polygons in general have never been formally defined. They are not the same thing as polygons which are star domains.
Regular star polygons
In geometry, a regular star polygon is a self-intersecting, equilateral equiangular polygon, created by connecting one vertex of a simple, regular, n-sided polygon to another, non-adjacent vertex and continuing the process until the original vertex is reached again.^{[citation needed]} For instance, in a regular pentagon, a five-pointed star can be obtained by drawing a line from the first to the third vertex, from the third vertex to the fifth vertex, from the fifth vertex to the second vertex, from the second vertex to the fourth vertex, and from the fourth vertex to the first vertex. This involves repeated addition with a modulus of n, where n is the number of sides of the polygon and the number x to be repeatedly added is greater than 1 and less than n-1, or: 1 < x < n-1. The notation for such a polygon is {n/x} (see Schläfli symbol), which is equal to {n/n-x}. The polygon at right is {5/2}.
A regular star polygon can also be represented as a sequence of stellations of a convex regular core polygon.
Regular star polygons were first studied systematically by Thomas Bradwardine.
Examples
{5/2} | {7/2} | {7/3} | {8/3} | {9/2} | {9/4} | {10/3} |
Star figures
If the number of sides n is evenly divisible by m, the star polygon obtained will be a regular polygon with n/m sides. A new figure is obtained by rotating these regular n/m-gons one vertex to the left on the original polygon until the number of vertices rotated equals n/m minus one, and combining these figures. An extreme case of this is where n/m is 2, producing a figure consisting of n/2 straight line segments; this is called a degenerate star polygon.
In other cases where n and m have a common factor, a star polygon for a lower n is obtained, and rotated versions can be combined. These figures are called star figures or improper star polygons or compound polygons. The same notation {n/m} is often used for them, although authorities such as Grünbaum (1994) regard (with some justification) the form k{n} as being more correct, where usually k=m.
A further complication comes when we compound two or more star polygons, as for example two pentagrams, differing by a rotation of 36°, inscribed in a decagon. This is correctly written in the form k{n/m}, as 2{5/2}, rather than the commonly-used {10/4}.
A six-pointed star, like a hexagon, can be created using a compass and a straight edge:
- Make a circle of any size with the compass.
- Without changing the radius of the compass, set its pivot on the circle's circumference, and find one of the two points where a new circle would intersect the first circle.
- With the pivot on the last point found, similarly find a third point on the circumference, and repeat until six such points have been marked.
- With a straight edge, join alternate points on the circumference to form two overlapping equilateral triangles.
Symmetry
Regular star polygons and star figures can be thought of as diagramming cosets of the subgroups of the finite group .
The symmetry group of {n/k} is dihedral group D_{n} of order 2n, independent of k.
Irregular star polygons
A star polygon need not be regular.^{[citation needed]}
Irregular cyclic star polygons occur as vertex figures for the uniform polyhedra, defined by the sequence of regular polygon faces around each vertex, allowing for both multiple turns, and retrograde directions. (See vertex figures at List of uniform polyhedra) ^{[1]}
The unicursal hexagram is another example of a cyclic irregular star polygon, containing only D_{2h} Dihedral symmetry.
Interiors of star polygons
Star polygons leave an ambiguity of interpretation for interiors. This diagram demonstrates three interpretations of a pentagram.
- 1. The left hand interpretation has the 5 vertices of a regular pentagon connected alternately on a cyclic path, skipping alternate vertices. The interior is everything immediately left (or right) from each edge (until the next intersection). This makes the core convex pentagonal region actually "outside", and in general you can determine inside by a binary even-odd rule of counting how many edges are intersected from a point along a ray to infinity.
- 2. The middle interpretation also has the 5 vertices of a regular pentagon connected alternately on a cyclic path. The interior may be treated as either of:
- a) the inside a simple 10-sided polygon perimeter boundary, as for 3.
- b) having the central convex pentagonal region surrounded twice, because the starry perimeter winds round it twice.
- 3. The right hand interpretation creates new vertices at the intersections of the edges (5 in this case) and defines a new concave decagon (10-pointed polygon) formed by perimeter path of the middle interpretation - it is in fact no longer a pentagram, though it may still be considered a star polygon.
What is the area inside the pentagram? Each interpretation leads to a different answer.
Example interpretations of a star prism
Heptagrams with 2-sided interior | Heptagrams with a simple perimeter interior |
The heptagrammic prism above shows different interpretations can create very different appearances.
Builders of polyhedron models, like Magnus Wenninger, usually represent star polygon faces in the concave form, without internal edges shown.
Star polygons in art and culture
Star polygons feature prominently in art and culture. Such polygons are may or may not be regular but they are always highly symmetrical. Examples include:
- The {5/2} star pentagon is also known as a pentagram, pentalpha or pentangle, and historically has been considered by many magical and religious cults to have occult significance.
- The simplest non-degenerate complex star polygon which is two {6/2} polygons (i.e., triangles), the hexagram (Star of David, Seal of Solomon).
- The {7/3} and {7/2} star polygons which are known as heptagrams and also have occult significance, particularly in the Kabbalah and in Wicca.
- The complex {8/2} star polygon (i.e. two squares), which is known as the Star of Lakshmi and figures in Hinduism;
- The {8/3} star polygon (octagram), and the complex star polygon of two {16/6} polygons, which are frequent geometrical motifs in Mughal Islamic art and architecture; the first is on the coat of arms of Azerbaijan.
- An eleven pointed star called the hendecagram, which apparently was used on the tomb of Shah Nemat Ollah Vali.
Some symbols based on a star polygon have interlacing, by small gaps, and/or, in the case of a star figure, using different colors.
See also
- Complex polygon
- List of regular polytopes - Nonconvex forms (2D)
- Magic star
- Star polyhedron
- Star polychoron (4-polytopes)
- Star-shaped polygon
- Stellation#Stellated polygons
References
- ↑ H. S. M. Coxeter, M. S. Longuet-Higgins, J. C. P. Miller, Uniform polyhedra, Phil. Trans. 1954 (Tables 6-8)
- Cromwell, P.; Polyhedra, CUP, Hbk. 1997, ISBN 0-521-66432-2. Pbk. (1999), ISBN 0-521-66405-5.
- Grünbaum, B. and G. C. Shephard; Tilings and Patterns, New York: W. H. Freeman & Co., (1987), ISBN 0-7167-1193-1.
- Grünbaum, B.; Polyhedra with Hollow Faces, Proc of NATO-ASI Conference on Polytopes ... etc. (Toronto 1993), ed T. Bisztriczky et al, Kluwer Academic (1994) pp. 43-70.
- John H. Conway, Heidi Burgiel, Chaim Goodman-Strass, The Symmetries of Things 2008, ISBN 978-1-56881-220-5 (Chapter 26. pp. 404: Regular star-polytopes Dimension 2)
External links
- Weisstein, Eric W., "Star polygon" from MathWorld.
- Weisstein, Eric W., "Star polygon" from MathWorld.