In mathematics, a Ford circle is a circle with centre at (p/q, 1/(2q^{ 2})) and radius 1/(2q^{ 2}), where p/q is an irreducible fraction, i.e. p and q are coprime integers. Each Ford circle is tangent to the horizontal axis y = 0.
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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 century René Descartes discovered Descartes' theorem, a relationship between the reciprocals of the radii of mutually tangent circles.
Ford circles also appear in the Sangaku (geometrical puzzles) 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 then? The answer is equivalent to a Ford circle:
Ford circles are named after American mathematician Lester R. Ford, Sr., who described them in an article in American Mathematical Monthly in 1938, volume 45, number 9, pages 586–601.
The Ford circle associated with the fraction p/q is denoted by C[p/q] or C[p, q]. There is a Ford circle associated with every rational number. In addition, the line y = 1 is counted as a Ford circle – it can be thought of as the Ford circle associated with infinity, which is the case p = 1, q = 0.
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 xaxis at each point on it with rational coordinates. If p/q is between 0 and 1, the Ford circles that are tangent to C[p/q] can be described variously as
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.
By interpreting the upper half of the complex plane as a model of the hyperbolic plane (the Poincaré halfplane model) Ford circles can also be interpreted as a tiling of the hyperbolic plane by horocycles. Any two Ford circles are congruent in hyperbolic geometry. If C[p/q] and C[r/s] are tangent Ford circles, then the halfcircle joining (p/q, 0) and (r/s, 0) that is perpendicular to the xaxis is a hyperbolic line that also passes through the point where the two circles are tangent to one another.
Ford circles are a subset of the circles in the Apollonian gasket generated by the lines y = 0 and y = 1 and the circle C[0/1].
There is a link between the area of Ford circles, Euler's totient function φ and the Riemann zeta function.
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 of φ(q) as documented on the page for Euler's totient function. Since ζ(4) = π^{ 4}/90, this finally becomes
This sum was discussed on es.ciencia.matematicas.^{[1]}.
