A density theorem on even Farey fractions

Abstract

Let $F_Q$ be the Farey sequence of order $Q$ and let $F_{Q,o}$ and $F_{Q,e}$ be the set of those Farey fractions of order $Q$ with odd, respectively even denominators. A fundamental property of $F_Q$ says that the sum of denominators of any pair of neighbor fractions is always greater than $Q$. This property fails for $F_{Q,o}$ and for $F_{Q,e}$. The local density, as $Q\to\infty$, of the normalized pairs $(q'/Q,q''/Q)$, where $q',q''$ are denominators of consecutive fractions in $F_{Q,o}$, was computed previously. The density increases over a series of quadrilateral steps ascending in a harmonic series towards the point $(1,1)$. Numerical computations for small values of $Q$ suggest that such a result should rather occur in the even case, while in the odd case the distribution of the corresponding points appears to be more uniform. Reconciling with the numerical experiments, in this paper we show that, as $Q\to\infty$, the local densities in the odd and even case coincide.