Ford circles are a special case of mutually tangent circles; the base line can be thought of as a circle with infinite radius. Systems of mutually tangent circles were studied by Apollonius of Perga, after whom the problem of Apollonius and the Apollonian gasket are named. In the 17th centuryRené Descartes discovered Descartes' theorem, a relationship between the reciprocals of the radii of mutually tangent circles. Ford circles also appear in the Sangaku of Japanese mathematics. A typical problem, which is presented on an 1824 tablet in the Gunma Prefecture, covers the relationship of three touching circles with a common tangent. Given the size of the two outer large circles, what is the size of the small circle between them? The answer is equivalent to a Ford circle: Ford circles are named after the American mathematician Lester R. Ford, Sr., who wrote about them in 1938.
Properties
The Ford circle associated with the fraction is denoted by or There is a Ford circle associated with every rational number. In addition, the line is counted as a Ford circle – it can be thought of as the Ford circle associated with infinity, which is the case Two different Ford circles are either disjoint or tangent to one another. No two interiors of Ford circles intersect, even though there is a Ford circle tangent to the x-axis at each point on it with rational coordinates. If is between 0 and 1, the Ford circles that are tangent to can be described variously as
the circles where
the circles associated with the fractions that are the neighbors of in some Farey sequence, or
the circles where is the next larger or the next smaller ancestor to in the Stern–Brocot tree or where is the next larger or next smaller ancestor to.
If and are two tangent Ford circles, then the circle through and and that is perpendicular to the -axis also passes through the point where the two circles are tangent to one another. Ford circles can also be thought of as curves in the complex plane. The modular group of transformations of the complex plane maps Ford circles to other Ford circles. Ford circles are a sub-set of the circles in the Apollonian gasket generated by the lines and and the circle By interpreting the upper half of the complex plane as a model of the hyperbolic plane, Ford circles can be interpreted as horocycles. In hyperbolic geometry any two horocycles are congruent. When these horocycles are circumscribed by apeirogons they tile the hyperbolic plane with an order-3 apeirogonal tiling. The 2015A AMC exam's last question is to find the sum of the reciprocals of the circumferences of Ford circles.
Total area of Ford circles
There is a link between the area of Ford circles, Euler's totient function the Riemann zeta function and Apéry's constant As no two Ford circles intersect, it follows immediately that the total area of the Ford circles is less than 1. In fact the total area of these Ford circles is given by a convergent sum, which can be evaluated. From the definition, the area is Simplifying this expression gives where the last equality reflects the Dirichlet generating function for Euler's totient function Since this finally becomes Note that as a matter of convention, the previous calculations excluded the circle of radius corresponding to the fraction. It includes the complete circle for, half of which lies outside the unit interval, hence the sum is still the fraction of the unit square covered by Ford circles.
Ford spheres (3D)
The concept of Ford circles can be generalized from the rational numbers to the Gaussian rationals, giving Ford spheres. In this construction, the complex numbers are embedded as a plane in a three-dimensional Euclidean space, and for each Gaussian rational point in this plane one constructs a sphere tangent to the plane at that point. For a Gaussian rational represented in lowest terms as, the radius of this sphere should be where represents the complex conjugate of. The resulting spheres are tangent for pairs of Gaussian rationals and with, and otherwise they do not intersect each other.
