The Farey sequence Fn contains all fractions p/q with 0 ≤ p/q ≤ 1 and q ≤ n, in ascending order.
Ford circles: Each fraction p/q has a circle of radius 1/(2q²) tangent to the x-axis at x = p/q.
Mediant: a/b ⊕ c/d = (a+c)/(b+d)
Adjacent Ford circles are always tangent to each other!