The Farey sequence, discovered in the early nineteenth century, is now an important object in number theory, geometry, and homogeneous dynamics. For example, a conjecture concerning the distribution of Farey sequences is equivalent to the Riemann hypothesis, one of the most famous outstanding problems in number theory and, indeed, all of mathematics. It also plays a key role in approximation in number theory through its connections with objects called continued fractions and an important role in geometry and homogenous dynamics through its connections with the dynamics of horocycles, which are important curves in hyperbolic geometry. This leads to an elegant theory in number theory, geometry, and homogeneous dynamics whose link is the Farey sequence, whose applications are found in, for example, mathematical physics and applied dynamics, and whose influence reach out towards physics and biology.
The Farey sequence is easy to describe. Let n be a natural number. The Farey sequence of order n is a sequence of rational numbers in lowest terms between 0 and 1 with denominators less than or equal to n, ordered by increasing size. For example, the Farey sequence of order 1 is the sequence {0, 1} and the Farey sequence of order 2 is the sequence {0, 1/2, 1}. These sequences give rise, via hyperbolic geometry, to continued fractions which yield the best approximations of a given real number. Such approximations are a central concern in the subfield of number theory called Diophantine approximation. These sequences also are important in hyperbolic geometry itself because they help us understand horocycles and, in particular, how horocycles distribute under the dynamics of a natural flow, namely the geodesic flow. In this way, Farey sequences provide a deep link between number theory, homogeneous dynamics, and geometry, a link which should be generalised and deepened further to the benefit of all three fields.
This proposal aims to generalise and deepen this link, and the expected results will belong to three fields, multiplying their benefit. We will study the analog of the Farey sequence in very general settings such as spaces coming from locally compact Hausdorff (topological) groups and their discrete subgroups. These can be large and complicated spaces. Topological groups are spaces for which we have a notion of multiplication, namely any two elements multiplied together yields another element in the group, and which satisfy some sensible rules. Locally compact and Hausdorff are two topological notions and many interesting spaces, such as the plane, threedimensional space or, more generally, manifolds, have these properties.
We will then use these generalized Farey sequences in two ways. The first is to study their numbertheoretic properties in analogy with the classical Farey sequences and the rich number theory coming from them. The second is to use these sequences to study dynamics on these large and complicated spaces again in analogy with the classical Farey sequences and horocycles. Some of the tools that we will use are the mixing property coming from ergodic theory, Ratner's theorems coming from homogenous dynamics, Eisenstein series coming from analytic number theory, and harmonic analysis, which is a field of mathematics concerned with decomposing functions into the infinite sum of "wavelike functions.''
By generalising the elegant theory of which the Farey sequence is the link, we will also expand upon the applications and influences of the theory.
