Greedy approximations by signed harmonic sums and the Thue--Morse sequence

TL;DR

This paper investigates how well real numbers can be approximated by signed harmonic sums chosen greedily, revealing their limit behavior, decay rates, and a surprising link to the Thue--Morse sequence.

Contribution

It provides a detailed analysis of the limit points and decay rates of greedy signed harmonic sum approximations and uncovers a novel connection with the Thue--Morse sequence.

Findings

Characterization of limit points of the approximation sequence

Decay rate analysis of the approximation error

Identification of the sequence of signs with the Thue--Morse sequence

Abstract

Given a real number $\tau$, we study the approximation of $\tau$ by signed harmonic sums $\sigma_N(\tau) := \sum_{n \leq N}{s_n(\tau)}/n$, where the sequence of signs $(s_N(\tau))_{N \in\mathbb{N}}$ is defined "greedily" by setting $s_{N+1}(\tau) := +1$ if $\sigma_N(\tau) \leq \tau$, and $s_{N+1}(\tau) := -1$ otherwise. Precisely, we compute the limit points and the decay rate of the sequence $(\sigma_N(\tau)-\tau)_{N \in \mathbb{N}}$. Moreover, we give an accurate description of the behavior of the sequence of signs $(s_N(\tau))_{N\in\mathbb{N}}$, highlighting a surprising connection with the Thue--Morse sequence.