Gaussian Behavior of Quadratic Irrationals

TL;DR

This paper investigates the probabilistic distribution of continued fraction expansions of quadratic irrationals, demonstrating Gaussian limit laws through advanced dynamical systems, combinatorics, and number theory techniques.

Contribution

It introduces a comprehensive analysis of weighted continued fractions of quadratic irrationals, extending previous methods with deeper functional analysis and dynamical systems tools.

Findings

Proves asymptotic Gaussian distribution laws for weighted continued fractions.

Establishes optimal convergence speeds using analytic combinatorics and transfer operators.

Connects dynamical systems with number theory to analyze quadratic irrationals.

Abstract

We study the probabilistic behaviour of the continued fraction expansion of a quadratic irrational number, when weighted by some "additive" cost. We prove asymptotic Gaussian limit laws, with an optimal speed of convergence. We deal with the underlying dynamical system associated with the Gauss map, and its weighted periodic trajectories. We work with analytic combinatorics methods, and mainly with bivariate Dirichlet generating functions; we use various tools, from number theory (the Landau Theorem), from probability (the Quasi-Powers Theorem), or from dynamical systems: our main object of study is the (weighted) transfer operator, that we relate with the generating functions of interest. The present paper exhibits a strong parallelism with the methods which have been previously introduced by Baladi and Vall\'ee in the study of rational trajectories. However, the present study is more involved and uses a deeper functional analysis framework.