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Ford circles for q from 1 to 20. Circles with q ≤ 10 are labelled as p/q and color-coded according to q. Each circle is tangent to the base line and its neighboring circles. Irreducible fractions with the same denominator have circles of the same size.

In mathematics, a Ford circle is a circle with center at and radius where is an irreducible fraction, i.e. and are coprime integers. Each Ford circle is tangent to the horizontal axis and any two Ford circles are either tangent or disjoint from each other.[1]



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.[2] In the 17th century René Descartes discovered Descartes' theorem, a relationship between the reciprocals of the radii of mutually tangent circles.[2]

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 them? The answer is equivalent to a Ford circle:[3]


Ford circles are named after the American mathematician Lester R. Ford, Sr., who wrote about them in 1938.[1]


Comparison of Ford circles and a Farey diagram with circular arcs for n from 1 to 9. Note that each arc intersects its corresponding circles at right angles. In the SVG image, hover over a circle or curve to highlight it and its terms.

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

  1. the circles   where  [1]
  2. the circles associated with the fractions   that are the neighbors of   in some Farey sequence,[1] or
  3. 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  .[1]

If   and   are two tangent Ford circles, then the circle through   and   (the x-coordinates of the centers of the Ford circles) and that is perpendicular to the  -axis (whose center is on the x-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.[1]

Ford circles are a sub-set of the circles in the Apollonian gasket generated by the lines   and   and the circle  [4]

By interpreting the upper half of the complex plane as a model of the hyperbolic plane (the Poincaré half-plane model) 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.[5]

Total area of Ford circlesEdit

There is a link between the area of Ford circles, Euler's totient function   the Riemann zeta function   and Apéry's constant  [6] 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.

See alsoEdit


  1. ^ a b c d e f Ford, L. R. (1938), "Fractions", The American Mathematical Monthly, 45 (9): 586–601, doi:10.2307/2302799, JSTOR 2302799, MR 1524411.
  2. ^ a b Coxeter, H. S. M. (1968), "The problem of Apollonius", The American Mathematical Monthly, 75: 5–15, doi:10.2307/2315097, MR 0230204.
  3. ^ Fukagawa, Hidetosi; Pedoe, Dan (1989), Japanese temple geometry problems, Winnipeg, MB: Charles Babbage Research Centre, ISBN 0-919611-21-4, MR 1044556.
  4. ^ Graham, Ronald L.; Lagarias, Jeffrey C.; Mallows, Colin L.; Wilks, Allan R.; Yan, Catherine H. (2003), "Apollonian circle packings: number theory", Journal of Number Theory, 100 (1): 1–45, arXiv:math.NT/0009113, doi:10.1016/S0022-314X(03)00015-5, MR 1971245.
  5. ^ "Art of Problem Solving". Retrieved 2019-01-24.
  6. ^ Marszalek, Wieslaw (2012), "Circuits with oscillatory hierarchical Farey sequences and fractal properties", Circuits, Systems and Signal Processing, 31 (4): 1279–1296, doi:10.1007/s00034-012-9392-3.

External linksEdit