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Alternating geometric progressions modulo one and Sturmian words

Qing Lu and Weizhe Zheng

Vol. 15 (2026), No. 3, 227–238
Abstract

Let b ≥ 2 be an integer. Using Sturmian words we describe all irrational real numbers ξ such that the image in ℝ∕ℤ of the sequence (ξ(−b)n)n≥0 is contained in an interval of length b−1 + b−2 − b−3. In previous work we showed that the image cannot be contained in a shorter interval.

Keywords
distribution modulo one, Sturmian word, geometric progression, negative base
Mathematical Subject Classification
Primary: 11J54
Secondary: 11A63, 11J71, 68R15
Milestones
Received: 21 May 2026
Revised: 23 July 2026
Accepted: 6 August 2026
Published: 16 October 2026
Authors
Qing Lu
School of Mathematical Sciences
Beijing Normal University
Beijing
China
Weizhe Zheng
Morningside Center of Mathematics
Academy of Mathematics and Systems Science
Chinese Academy of Sciences
Beijing
China
University of Chinese Academy of Sciences
Beijing
China