Ford circle

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$Ford circles. A circle rests upon each fraction in lowest terms. The darker circles shown are for the fractions 0/1, 1/1, 1/2, 1/3, 2/3, 1/4, 3/4, 1/5, 2/5, 3/5 and 4/5. Each circle is tangential to the base line and its neighboring circles. Fractions with the same denominator have circles of the same size.$

Ford circles. A circle rests upon each fraction in lowest terms. The darker circles shown are for the fractions 0/1, 1/1, 1/2, 1/3, 2/3, 1/4, 3/4, 1/5, 2/5, 3/5 and 4/5. Each circle is tangential to the base line and its neighboring circles. Fractions with the same denominator have circles of the same size.

In mathematics, a Ford circle is a circle with centre at (p/q, 1/(2q²)) and radius 1/(2q²), where p/q is an irreducible fraction — a fraction in its lowest terms — where p and q are coprime integers).

1 History
2 Properties
3 Total area of Ford circles
4 See also
5 References
6 External links

[edit] History

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.

[edit] 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 interiors of Ford circles intersect - even though there is a Ford circle tangent to the x-axis at each point on it with rational co-ordinates. 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 half-plane 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 half-circle joining (p/q, 0) and (r/s, 0) that is perpendicular to the x-axis is a hyperbolic line that also passes through the point where the two circles are tangent to one another.

Ford circles are a sub-set of the circles in the Apollonian gasket generated by the lines y = 0 and y = 1 and the circle C[0/1].

[edit] Total area of Ford circles

There is a link between the area of Ford circles, Euler's totient function $\varphi(q)$ and the Riemann zeta function.

As no two Ford circles intersect, it follows immediately that the total area of the Ford circles $\{C[p,q]: 0 \le \frac{p}{q} < 1\}$ 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 $A$ is

$A = \sum_{q\ge 1} \sum_{ (p, q)=1 \atop 1 \le p < q } \pi \left( \frac{1}{2 q^2} \right)^2.$

Simplifing this expression gives

$A = \frac{\pi}{4} \sum_{q\ge 1} \frac{1}{q^4} \sum_{ (p, q)=1 \atop 1 \le p < q } 1 = \frac{\pi}{4} \sum_{q\ge 1} \frac{\varphi(q)}{q^4} = \frac{\pi}{4} \frac{\zeta(3)}{\zeta(4)},$

where the last equality reflects the Dirichlet generating function of $\varphi(q)$ as documented on the page for Euler's totient function. Since $ζ(4) = π 4 / 90,$ this finally becomes

$A = \frac{45}{2} \frac{\zeta(3)}{\pi^3}\approx 0.872284041.$