arXiv subjects

Rie Natsui

Publications and source records attributed to Rie Natsui.

4 recordsLinked to original sources

arXiv2026

Farey graphs and geodesic expansions of complex continued fractions

We discuss complex Farey graphs for the Euclidean imaginary quadratic number fields $\mathbb Q(\sqrt{-d})$, $d\in\{1, 2, 3, 7, 11\}$. We study hyperbolic versions of A. Schmidt's Farey polygons living in $3$-dimensional hyperbolic space $\mathbb{H}^3$. Using these Farey polygons we recover tessellations of the hyperbolic plane $\mathbb{H}^2$ that are defined by the action of the Hecke groups $H_4$ and $H_6$ and have been studied earlier by I. Short and M. Walker. Moreover, hyperbolic Farey polygons allow us to define polyhedra that induce Farey tessellations of $\mathbb{H}^3$ by the action of certain Bianchi groups. Using complex Farey graphs we consider geodesic complex continued fraction expansions. Our method provides a different and more general approach as the one from the discussion by M. Hockman.

math.NT↗

arXiv2024

A new class of $α$-Farey maps and an application to normal numbers

We define two types of the $α$-Farey maps $F_α$ and $F_{α, \flat}$ for $0 < α< \tfrac{1}{2}$, which were previously defined only for $\tfrac{1}{2} \le α\le 1$ by R.~Natsui (2004). Then, for each $0 < α< \tfrac{1}{2}$, we construct the natural extension maps on the plane and show that the natural extension of $F_{α, \flat}$ is metrically isomorphic to the natural extension of the original Farey map. As an application, we show that the set of normal numbers associted with $α$-continued fractions does not vary by the choice of $α$, $0 < α< 1$. This extends the result by C.~Kraaikamp and H.~Nakada (2000).

math.DS↗

arXiv2014

Farey map, Diophantine approximation and Bruhat-Tits tree

Based on Broise-Alamichel and Paulin's work on the Gauss map corresponding to the principal convergents, we continue the study of the Gauss map via Farey maps to contain all the intermediate convergents. We define the geometric Farey map, which is given by time-1 map of the geodesic flow. We also define algebraic Farey maps, better suited for arithmetic properties, which produce all the intermediate convergents. Then we obtain the ergodic invariant measures for the Farey maps and the convergent speed.

math.DS↗

arXiv2007

Generalized Brjuno functions associated to $α$-continued fractions

For αin the interval [0,1], we consider the one-parameter family of α-continued fraction maps, which include the Gauss map (α=1) and the nearest integer (α=1/2) and by-excess (α=0) continued fraction maps. To each of these expansions, and to each choice of a positive function u on the interval I_α=(0,max(α,1-α)) we associate a generalized Brjuno function B_(α,u)(x). For α=1/2 or α=1, and u(x)=-\log(x), these functions were introduced by Yoccoz in his work on the linearization of holomorphic maps. Their regularity properties, including BMO regularity and their extension to the complex plane, have been thoroughly investigated. We compare the functions obtained with different values of αand we prove that the set of (α,u)-Brjuno numbers does not depend on the choice of αprovided that α>0. We then consider the case α=0, u(x)=-\log(x) and we prove that x is a Brjuno number (for α> 0) if and only if both x and -x are Brjuno numbers for α=0.

math.DS↗