Sage Pubs

For any integer n > 0, there exists an Apollonian gasket defined by the following curvatures:

Web Ani — Wed, 10 Mar 2010 17:59:24 +0000

For any integer n > 0, there exists an Apollonian gasket defined by the following curvatures:
(-n, n+1, (n)(n+1), (n)(n+1)+1).
For example, the gaskets defined by (-2,3,6,7), (-3,4,12,13), (-8,9,72,73), and (-9,10,90,91) all follow this pattern. Because every interior circle that is defined by n+1 can become the bounding circle (defined by -n) in another gasket, these gaskets can be nested. This is demonstrated in the figure at right, which contains these sequential gaskets with n running from 2 through 20.

-wiki

Apollonian gaskets

Web Ani — Wed, 10 Mar 2010 17:58:47 +0000

The figure at left is an Apollonian gasket that appears to have D₃ symmetry. The same figure is displayed at right, with labels indicating the curvatures of the interior circles, illustrating that the gasket actually possesses only the D₁ symmetry common to many other integral Apollonian gaskets.

The following table lists more of these almost-D₃ integral Apollonian gaskets. The sequence has some interesting properties, and the table lists a factorization of the curvatures, along with the multiplier needed to go from the previous set to the current one. The factorizations demonstrate an interesting chain of factors throughout the sequence of gaskets, and the multiplier appears to be converging toward

-wiki

Symmetry of integral Apollonian circle packings

Web Ani — Wed, 10 Mar 2010 17:57:48 +0000

Whenever two of the largest five circles in the gasket have the same curvature, that gasket will have D₁ symmetry, which corresponds to a reflection along a diameter of the bounding circle, with no rotational symmetry.

If two different curvatures are repeated within the first five, the gasket will have D₂ symmetry; such a symmetry consists of two reflections (perpendicular to each other) along diameters of the bounding circle, with a two-fold rotational symmetry of 180°. The gasket described by curvatures (-1,2,2,3) is the only Apollonian gasket (up to a scaling factor) to possess D₂ symmetry.

If none of the curvatures are repeated within the first five, the gasket contains no symmetry, which is represented by symmetry group C₁; the gasket described by curvatures (−10,18,23,27) is an example.

If the three circles with smallest positive curvature have the same curvature, the gasket will have D₃ symmetry, which corresponds to three reflections along diameters of the bounding circle (spaced 120° apart), along with three-fold rotational symmetry of 120°. In this case the ratio of the curvature of the bounding circle to the three inner circles is . As this ratio is not rational, no integral Apollonian circle packings possess this D₃ symmetry, although many packings come close.

-wkii

If any four mutually tangent …

Web Ani — Wed, 10 Mar 2010 17:57:01 +0000

If any four mutually tangent circles in an Apollonian gasket all have integer curvature (where the curvature of a circle is defined to be the inverse of its radius) then all circles in the gasket will have integer curvature.^[1] The first few of these integral Apollonian gaskets are listed in the following table. The table lists the curvatures of the largest circles in the gasket; a negative curvature indicates that all other circles are tangent to the interior of that circle (that is, it is the bounding circle), while a positive curvature indicates that all other circles are tangent to the exterior of that circle (these are the interior circles). Only the first three curvatures (of the five displayed in the table) are needed to completely describe each gasket – all other curvatures can be derived from these three.

-wiki

Links with hyperbolic geometry

Web Ani — Wed, 10 Mar 2010 17:56:12 +0000

The three generating circles, and hence the entire construction, are determined by the location of the three points where they are tangent to one another. Since there is a Möbius transformation which maps any three given points in the plane to any other three points, and since Möbius transformations preserve circles, then there is a Möbius transformation which maps any two Apollonian gaskets to one another.

Möbius transformations are also isometries of the hyperbolic plane, so in hyperbolic geometry all Apollonian gaskets are congruent. In a sense, there is therefore only one Apollonian gasket, which can be thought of as a tessellation of the hyperbolic plane by circles and hyperbolic triangles.

The Apollonian gasket is the limit set of a group of Möbius transformations known as a Kleinian group.

Symmetries

Web Ani — Wed, 10 Mar 2010 17:55:25 +0000

If two of the original generating circles have the same radius and the third circle has a radius that is two-thirds of this, then the Apollonian gasket has two lines of reflective symmetry; one line is the line joining the centres of the equal circles; the other is their mutual tangent, which passes through the centre of the third circle. These lines are perpendicular to one another, so the Apollonian gasket also has rotational symmetry of degree 2; the symmetry group of this gasket is D₂.

If all three of the original generating circles have the same radius then the Apollonian gasket has three lines of reflective symmetry; these lines are the mutual tangents of each pair of circles. Each mutual tangent also passes through the centre of the third circle and the common centre of the first two Apollonian circles. These lines of symmetry are at angles of 60 degrees to one another, so the Apollonian gasket also has rotational symmetry of degree 3; the symmetry group of this gasket is D₃.

-from wiki

An Apollonian gasket can also be constructed by …

Web Ani — Wed, 10 Mar 2010 17:54:34 +0000

An Apollonian gasket can also be constructed by replacing one of the generating circles by a straight line, which can be regarded as a circle passing through the point at infinity.

Alternatively, two of the generating circles may be replaced by parallel straight lines, which can be regarded as being tangent to one another at infinity. In this construction, the circles that are tangent to one of the two straight lines form a family of Ford circles.

The three-dimensional equivalent of the Apollonian gasket is the Apollonian sphere packing.

An Apollonian gasket can be constructed ….

Web Ani — Wed, 10 Mar 2010 17:53:44 +0000

An Apollonian gasket can be constructed as follows. Start with three circles C1, C2 and C3, each one of which is tangent to the other two (in the general construction, these three circles can be any size, as long as they have common tangents). Apollonius discovered that there are two other non-intersecting circles, C4 and C5, which have the property that they are tangent to all three of the original circles – these are called Apollonian circles (see Descartes’ theorem). Adding the two Apollonian circles to the original three, we now have five circles.

Take one of the two Apollonian circles – say C4. It is tangent to C1 and C2, so the triplet of circles C4, C1 and C2 has its own two Apollonian circles. We already know one of these – it is C3 – but the other is a new circle C6.

In a similar way we can construct another new circle C7 that is tangent to C4, C2 and C3, and another circle C8 from C4, C3 and C1. This gives us 3 new circles. We can construct another three new circles from C5, giving six new circles altogether. Together with the circles C1 to C5, this gives a total of 11 circles.

Continuing the construction stage by stage in this way, we can add 2·3ⁿ new circles at stage n, giving a total of 3ⁿ⁺¹ + 2 circles after n stages. In the limit, this set of circles is an Apollonian gasket.

The Apollonian gasket has a Hausdorff dimension of about 1.3057

Apollonian gasket

Web Ani — Wed, 10 Mar 2010 17:52:53 +0000

In mathematics, an Apollonian gasket or Apollonian net is a fractal generated from triples of circles, where any circle is tangent to two others. It is named after Greek mathematician Apollonius of Perga.

from wiki

Conversion from two parametric equations to a single equation

Web Ani — Tue, 09 Feb 2010 07:11:04 +0000

Conversion from two parametric equations to a single equation

Converting a set of parametric equations to a single equation involves eliminating the variable t from the simultaneous equations . If one of these equations can be solved for t, the expression obtained can be substituted into the other equation to obtain an equation involving x and y only. If x(t) and y(t) are rational functions then the techniques of the theory of equations such as resultants can be used to eliminate t. In some cases there is no single equation in closed form that is equivalent to the parametric equations.^[1]

To take the example of the circle of radius a above, the parametric equations

can be simply expressed in terms of x and y by way of the Pythagorean trigonometric identity:

which is easily identifiable as a type of conic section (in this case, a circle).

http://en.wikipedia.org/wiki/Parametric_equation