Reduced Random Walks in the Hyperbolic Plane$\hspace{1pt}!\hspace{-3.8pt}?$

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Hauptverfasser: Defant, Colin, Lee, Mitchell
Format: Preprint
Veröffentlicht: 2025
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author Defant, Colin
Lee, Mitchell
author_facet Defant, Colin
Lee, Mitchell
contents We study Lam's reduced random walk in a hyperbolic triangle group, which we view as a random walk in the upper half-plane. We prove that this walk converges almost surely to a point on the extended real line. We devote special attention to the reduced random walk in $PGL_2(\mathbb{Z})$ (i.e., the $(2,3,\infty)$ triangle group). In this case, we provide an explicit formula for the cumulative distribution function of the limit. This formula is written in terms of the interrobang function, a new function $!\hspace{-3.8pt}?\colon[0,1]\to\mathbb{R}$ that shares several of the remarkable analytic and arithmetic properties of Minkowski's question-mark function.
format Preprint
id arxiv_https___arxiv_org_abs_2504_19367
institution arXiv
publishDate 2025
record_format arxiv
spellingShingle Reduced Random Walks in the Hyperbolic Plane$\hspace{1pt}!\hspace{-3.8pt}?$
Defant, Colin
Lee, Mitchell
Probability
Combinatorics
Group Theory
20F55 (Primary) 05C81, 20F67, 60B15, 60J10, 11F06 (Secondary)
We study Lam's reduced random walk in a hyperbolic triangle group, which we view as a random walk in the upper half-plane. We prove that this walk converges almost surely to a point on the extended real line. We devote special attention to the reduced random walk in $PGL_2(\mathbb{Z})$ (i.e., the $(2,3,\infty)$ triangle group). In this case, we provide an explicit formula for the cumulative distribution function of the limit. This formula is written in terms of the interrobang function, a new function $!\hspace{-3.8pt}?\colon[0,1]\to\mathbb{R}$ that shares several of the remarkable analytic and arithmetic properties of Minkowski's question-mark function.
title Reduced Random Walks in the Hyperbolic Plane$\hspace{1pt}!\hspace{-3.8pt}?$
topic Probability
Combinatorics
Group Theory
20F55 (Primary) 05C81, 20F67, 60B15, 60J10, 11F06 (Secondary)
url https://arxiv.org/abs/2504.19367