Table of contents 

History
Ford circles are named after American mathematician Lester R. Ford, Sr, who described them in an article in American Mathematical Monthly in 1938.
Properties
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 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] are precisely those associated with the fractions that are the neighbours of p/q in some Farey sequence.
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. 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.
See also
External links
 http://www.josleys.com/creatures41.htm  art and graphics based on Ford circles